A C T A
T H E R I O L O G I C A
VOL. XII. 29: 407— 444.
BIAŁOW IEŻA
Z bigniew K R A S I Ń S K I
30.XII.1967
& Jan R A C Z Y Ń S K I
The Reproduction Biology of European Bison
Living in Reserves and in Freedom
Bisoniana XXV
[With 7 Tables & 5 Figs.]
T he investigations w e re based on docum entation from P o lish b re e d ­
ing reserv es for th e period 1954—1965 and on observ atio n s of a fre e living bison h erd in th e B iałow ieża P rim ae v al F o rest fo r th e period
1958—1966. T he ru ttin g period reaches a peak in th e m o n th of A ugust,
S eptem ber and O ctober, w hile copulation w as observed over th e g rea ter
p a r t of th e year w ith th e exception of th e ano estru s p erio d : A pril —
Ju n e. C alving ta k es place from A p ril to D ecem ber, th e g re a te s t num bers
of calves being born d u rin g th e period M ay—Ju ly (70% in reserv es and
80% in th e free-liv in g herd). P regnancy lasts from 254 to 272 days
(x = 264.3, n = 132). In rese rv e s fem ale bison usu ally calv e for th e firs t
tim e in th e 4th year of life (x = 47.8 m onths), and ea rliest in th e 3rd
y ear (33 m onths of life). M anifestations of oestrus u su ally la st 1—3 days,
th e period of rep e titio n of oestrus is 18—22 days (x = 20.6 days). T he
calving in te rv a l is 10—13 m onths for th e m ost fertile cov/s (x = 14.4).
T he reproduction p o ten tial does not a lte r over th e w hole period of th e
fem ale’s fe rtile life. T he y ea rly calving ra te in rese rv e s is stable,
observed ran g e is 63—91%; ste rility periods m ost fre q u en tly la st one
year only. D uring th e se x u al ac tiv ity period each fem ale bison produces
an av erag e of 9 calves. N atality ra te of th e rese rv e and free-liv in g
population does not d iffer significantly, 77 and 70% resp ectiv ely of th e
m a tu re cows calving annually. In th e free-liv in g h e rd th e ra te of
in crease in young w as high (22.4%) w ith m inim um m o rta lity am ong
calves (2.3%). T he sex ratio , w hich w as 1 :1 a t b irth , changes w ith age
a re su lt of th e h igher m o rta lity ra te am ong m ales. Food is a factor
controlling, and n o rm al behav io u r — stim u latin g th e rep ro d u ctio n in
n a tu ra l populations.
I.
II.
III.
IV.
V.
VI.
V II.
I n t r o d u c t i o n .......................................................................................................................408
M aterial and m e t h o d s .................................................................................................409
B reeding s e a s o n ............................................................................................................... 409
S ex u al m a t u r a t i o n ........................................................................................................ 413
T he oestrous c y c l e ........................................................................................................ 415
D uration of p r e g n a n c y ................................................................................................. 417
E valu atio n of th e fe rtility of fem ale E uropean b i s o n .........................................419
1. Age of fem ales a t firs t c a l v i n g ............................................................................. 420
2. N um ber of young b o r n ........................................................................................... 421
3. F e rtility period and freq u en cy of c a l v i n g .......................................................... 421
4. F e rtility lim iting f a c t o r s ........................................................
425
5. T otal increase from a fem ale E uropean b i s o n ....................................................427
[407]
408
VIII.
IX.
X.
X I.
X II.
Z. K rasiński & J. Raczyński
E valuation of fe rtility and increase in th e herd
Sex r a t i o ......................................................................
Sexual behaviour
D uscussion
Conclusions
References
Streszczenie
428
432
434
436
441
442
443
I INTRODUCTION
R elatively little is know n of th e reproduction of th e E uropean bison (or w isent),
Bison bonasus ( L i n n a e u s , 1758)') u nder n a tu ra l conditions. T he oldest f irs t­
hand inform ation on th e biology of this anim al, encountered in 19th cen tu ry lite r a tu ­
re, is fra g m en tary and not alw ays reliable. L ater m ongraphs on th e B iałow ieża and
C aucasus bison (K a r c o v, 1903; K u l a g i n , 1919; W r ó b l e w s k i , 1927; B a ś k i r o v, 1939) re fe r to populations confined and isolated to an even g re a te r ex ten t,
living under conditions w idely diverging from n a tu ra l ones. T he effect of hum an
in terv ention on the E uropean bison w as evident in Poland fa irly early on (K r ys i a k, 1967) and led in various w ays to changes in th e biology of th ese anim als. The
g reatest effect w as ex erted by tre a tin g th e bison as a gam e an im al and by the
breeding operations connected w ith this. T he exterm in atio n of the E u ro p ean bison
in th e Białow ieża P rim ae v al F orest p u t an to the first stage of rese arch on n atu ra l
population.
The la ter period of w ell ab o u t fifty years of breeding in enclosures, ca rrie d out
w ith considerable intensification of inbreeding under th e a rtific ia l conditions
created in zoological gardens and enclosed reserves, d iffered fu n d am en tally from
th e orginal w ay of life of this anim al w hen it w as a fre e-liv in g species. Its re te n ­
tion in captivity and supervised feeding undoubtedly ex e rted a g rea t influence on
th e an im al’s physiology, including th e course tak en by rep ro d u ctiv e processes
( Gi l l , 1967).
M any observations on reproduction in th e E uropean bison u n d er re se rv e breeding
conditions w ere m ade durin g th e research w o rk ca rrie d out on elab o ratio n of
ratio n al m ethods for th e restitu tio n of th is species ( Z a b ł o c k i , 1939: 1957).
T he first study en tirely devoted to reproduction in th e E uro p ean bison (J ac z e w s k i , 1958), is based on data from Polish reserv es and in fo rm atio n from
p edigree books, w hich covers docum entation for th e w hole period of b reed in g in
reserv es up to 1953 inclusive.
T he 12-year period of breeding w hich has elapsed since th e com pletion of those
studies seem ed to us to be sufficiently long to p erm it of rep e atin g th e analysis
of rep roduction in bison u nder th e conditions in Polish reserv es. In ad d itio n 1952
is th e year of in troduction of a new breeding ex p erim en t — th e releasin g of th e
h erd of E uropean bison, in th e Białow ieża P rim ae v al F orest. It is of course obvious
th a t free-liv in g bison live under conditions closer to n a tu ra l ones, both as reg ard s
th e effect of th e ex tern al h a b ita t and of population rela tio n s connected w ith th e
creation of a spontaneous social stru c tu re of h erds ( K r a s i ń s k i , 1963; 1967)i In
this w ay th e o pportunity w as created for th e firs t tim e since th e successful r e s ti­
tu tio n of th ese anim als in P oland of com paring th e process of rep ro d u ctio n am ong
E uropean bison living in enclosures w ith th a t in free-liv in g bison. Such co m p ari­
sons w ould ap p ear to be req u ired now in connection w ith th e new attem p ts u n d erJ) T he com mon nam e »bison* is used th ro u g h o u t th e te x t of this p ap er in th e
m eaning »European bison« (or w isent).
T he reproduction biology
tak en at breeding the bison
ev alu ating these exp erim en ts
bonasus.
409
under free-liv in g conditions and th e necessity for
from th e asp ect of restitu tio n th e species, Bison
II. M ATERIAL AND METHODS
Use w as m ade in th e investigations of data on the rep ro d u ctio n of various cow
bison in th e four chief breeding centres — at Białow ieża, Niepołomice, Pszczyna
and B orki, obtained from : (1) the P edig ree book of th e E uropean bison 1955—1961,
(2) b reeding records kept in the various reserv es and, (3) notes in observation
logbooks k ept by assistants.
In prin cip le only th e 12-year period from 1954 to 1965 w as covered, tak in g into
consideration all th e m atings and calvings occurring d uring th is tim e. A ll th e cow
bison w hich reach ed m a tu rity during th e above period w ere tak en into co n sid era­
tion w hen assessing fertility . Only in th e case of cow bison cu rren tly living in th e
reserve, th e fe rtility season of w hich extended to earlier years, w ere all th eir
p revious calvings included in th e analysis, in order to obtain a full p ic tu re of th e
fertility of th e given cow bison.
D ocum entation and notes on the bison h erd living in freedom in th e B iałow ie­
ża P rim ae v al F o rest (BPF) w ere used as m a teria l for studies of rep ro d u ctio n
am ong fre e-liv in g bison. A lthoug bison w ere released as early as 1952 th e free r e ­
production of th is herd dates from 1958 ( K r a s i ń s k i , 1967). O bservations of th e
free-liv in g h erd w ere continued up to 1966 inclusive. T he only inform ation n ere
w as: th e com position of th e herd in consecutive years as reg a rd s sex and age
(annual in crease in young) and ap p ro x im ate dates of b irth s and th e sex of th e
calves born. It w as im possible to obtain d ata on m atings and occurrence of
ru ttin g.
III. BREEDING SEASON
T h e seasonal rh y th m of sexual functions in th e E uropean bison u n d er n atu ra l
conditions is not know n. In older studies ( K a r c o v , 1903; K u l a g i n , 1919:
W r ó b l e w s k i , 1927) the au th o rs w ere unanim ous in em phasing th a t the n a tu ra l
seasonal c h a ra c te r of reproduction had been »upset« as th e re su lt of w in ter
su p p lem en tary feeding. U nder th e living conditions in reserv es th e an im al had
a su fficient am ount of food th ro u g h o u t th e w hole year. J a c z e w s k i (1958)
considers as th e cause of d isturbance of th e n a tu ra l periodic c h a racter of r e ­
p ro duction th e effect of the h u nger facto r to w hich bison w ere su b ject a t certain
tim es of th e y ea r due to th e insufficient n a tu ra l food supply and irre g u la rity of
su p p lem en tary feeding. As a resu lt th e reproductio n of bison u n d er reserv e
conditions m ost certainly differs from th e m odel ch a racteristic of free-liv in g
populations.
The rutting season in reserves takes place during the m onths from
A u gu st to D ecem ber (Table 1) but m atings have been recorded in Ju ly
and also in January. February and March. The largest num ber of
m atings occurs in August, Septem ber and October. During this period
72.9% of all m atings, w hich included 70.9% of all successful m atings,
w ere observed. As from N ovem ber the num ber of m atings decreases
gradually, the cases of m ating in January, February and March being
410
Z. K rasiń sk i & J. Raczyński
m erely sporadic (form ing join tly about 7% of the cases), but th ey m ay
h ow ever be successful. A characteristic phenom enon is the com plete
absence of oestrus sym ptom s in cow bison from A pril to June. S tart of
pregnancy during this period w ould lead to the young being born at
the m ost unfavourable period of the w inter, from January to March.
An analogical list of effectiv e m atings calculated from dates of birth
in pedigree books is given b y J a c z e w s k i (1958). The sporadic cases
of fertilization during the period from A pril—June given there most
certain ly occurred in zoological gardens and areas situ ated in different
clim atic zones. U nder conditions in P olish reserves, as is evid en t from
dates of calving, su ccessful m ating did not take place during this period
(cf. J a c z e w s k i , 1958, Tables 7, 8, 9).
10
%
-
4 .1
18. 9
3 0 .2
2 3 .8
10. 5
48
57
60
19
20. 6
24. 5
2 5 .8
6. 2
35
7 .2 .
19
8 .2
May
2 .9
51
9
5
_
.
487
3 .7
1 .8
1 .0
-
-
100
8
7
5
_
_
233
3 .4
3 .0
2 .1
-
-
100
Jan.
116
Dec.
147
Nov.
S e p t.
92
A p r il
-
14
M arch
%
o
o
F eb.
Number o f
e ffe c tiv e
c o p u la tio n s
n
Aug.
Number o f
c o p u la tio n s
o b s e rv e d
Ju n e
Month
J u ly
Table 1.
Intensity of oestrus m an ifestations in fem ale bison in reserves.
18
T o ta l
1 9 5 4 -1 9 6 5
B irths in reserves are recorded from A pril to D ecem ber, but the main
calvin g period is in May, June and J u ly (Fig. 1), during w hich time
70,9% of all the calves w ere born. As from October all the later calvings
are sporadic in character, but cases w ere encountered in all the reserves.
J a c z e w s k i (1958) gives sim ilar results. The d ifferen ces con sist in
the slig h tly d ifferen t proportions of calvin gs in d ifferen t m onths: J a­
c z e w s k i (Z. c.) observed a preponderance of birts in May, w hereas in
the m aterial w e exam ined calvin gs w ere distributed fa irly e v e n ly over
the three peak m onths. We did not observe any m arked d ifferen ces be­
tw een the data for the various reserves, and in p articular the distribut­
ion of births at P szczyna did not in an y w a y d iffer from the average
picture. J a c z e w s k i (Z. c.) exp lained the sh ift in births to a later
period at Pszczyna b y the greater degree of inbreeding of th is breeding
line or by food differences. The fact th at these m an ifestations disappear­
ed after introducing fresh blood into the reserve follow in g the foot-and ­
-m outh epizootic in 1954 argues in favou r of the first assum ption.
In the light of results obtained from reserves the course taken by
calvings among free-livin g bison in B PF is interesting. In this connection
411
The repro d u ctio n biology
only the birth of young follow in g m atings betw een free-livin g anim als
(Table 2) w ere taken into consideration. The calving season begins in
May, and by Ju ly in clu sively over 80% of the cow bison had calved.
The rem aining cow s calved in A ugust and Septem ber. The calving
season in free-liv in g bison was therefore shorter and was lim ited to the
25
20
Enclosed reserves
N=233 (1954-1965)
- 0
CD
E
x> 0 ■
QJ
VI
IX
XI
Freedom
N=96 (1958-1966)
20
25
30
Fig. 1. C om parison of th e calving season am ong fem ale E uropean bison
in reserv es and those living in freedom .
period from M ay to S eptem ber (Fig. 1). The tw o isolated cases of calves
being born in N ovem ber and D ecem ber (2% of all births) do not affect
the gen eral rule.
The m ating period in the free-liv in g population, determ ined on the
basis of calving, occurs from A u gu st to D ecem ber (not including the
412
Z. K rasiń sk i & J. Raczyński
exceptional cases of m ating in February and March). In relation to data
from the reserves there is thus a sh ift in the start of the m ating season
from J u ly to A ugust, as calving among free-livin g cow s began every
year in May. According to earlier authors ( B r i n e k e n , 1829; K a r c o v , 1903; W r ó b l e w s k i , 1927; S t e c h o w, 1929) rutting among
European bison in B PF occurred in A u gu st and Septem ber, w h ile the
calves w ere born at a period favourable to the species — in M ay and
June. The rhythm w as later disturbed as the result of supplem entary
feeding. K u l a g i n (1919) observed calvin g in BPF during the period
from M ay to Novem ber, but says nothing of any births in the w inter.
1957
>
o
Dec.
O ct.
S e p t.
Aug.
July
s
Ju n e
A p r il
March
Yeax
Jan.
I
Table 2.
C alving period am ong fem ale bison living in freedom . * B irth s am ong cows covered
in enclosures before being released into th e forest.
1*
T o ta l
-
1958
2
1959
1
-
3
1960
1
2
3
1961
2
2
4
1
2
1962
5
1
-
1
1
8
1963
5
3
2
2
1
13
1964
5
2
4
3
2
1965
4
4
5
1
-
1966
6
8
T o ta l
«
•
2
1
5
1*
6
11
16
-
1
15
1
31
22
26
9
6
3 2 .3
2 2 .9
27. 1
9. 4
6. 2
-
1
1
-
1 .0
1 .0
20
96
100
D ifferences in duration of the breeding season b etw een a free-livin g
population and bison livin g in reserves leads to the assum ption that
th ey depend on the d ifferen ces in the an im als’ livin g conditions. As
regards m ovem ent and food conditions the ex ten siv e forest enclosures
of the B iałow ieża reserve create conditions sim ilar to those in freedom.
D uring the sum m er, for instance, the bison often fail to v isit the feeding
places for a long period, since th ey find enough food in the large en clo­
sures (10— 80 ha). On the other hand, the free-livin g herd is in ten sively
fed during the w in ter and during the m ost d ifficu lt period visit the
feed in g places up to the end of April, w ith ou t being subject to the
h unger factor, w hich m ost certain ly affected w ild European bison. Under
such circum stances it w ould appear that the fact of natural organization
413
The reproduction biology
of the herd is d ecisive here. Apart from the rutting period the adult
m ales form a separate group, and on ly join the herd during the m ating
period ( K r a s i ń s k i , 1967). The sexu al selection fr eely made at that
tim e increases the effectiven ess of m ating. In this way, by reduction in
the num ber of repeat sexual cycles in the cow s, the duration of the
m ating season in the w hole population is sim u ltaneously shortened.
IV. SEXUAL MATURATION
Breeding records from the reserves show that the tim e of sexual
m aturation of cow bison can be established on the basis of the dates of
the first calvings and concluding the age of the first effectiv e m ating
from them . The earliest calving in our m aterial w as observed in a cow
at the age of 33 m onths (»Puczaja« 1300, N iepołom ice, born A ugust 17th
1960, first calving M ay 30th 1963). Cow bison begin calving for the first
tim e far more often at the age of 35— 38 m onths, i. e. at the turning
point betw een tne third and fourth year of live. Thus appropriately for
the given periods of calving the dates of the first effective m ating come
b etw een the 26th— 29th m onth of life. The cow »Puczaja« w as e ffec tiv ­
e ly covered at the age 24 m onths, that is, on com pletion of the second
year of live. J a c z e w s k i (1958) gives as an extrem e case the calving
of the cow »Borgia« at the age of 32 m onths; effectiv e covering m ust
therefore have taken place w hen the cow w as 23 m onths old.
Table 3.
Age of fem ales a t firs t calving in breeding reserves.
Hear
of li f e
3rd
Month
of l i f e
33
n
1
Per
cent
34
-
1. 9 -
4 th
35
1
36
3
1. 9 5. 8
9 .6
37
38
4
3
39
-
7. 7 5. 8 -
40
1
41
2
42
1
5 th
43
3
44
2
1
45
46
47
1
5
5
48
1. 9 3. 8 1. 9 5. 8 3. 8 1. 9 1. 9 9 . 6 9 . 6
5 3 .9
6 th
T o ta l
4 9 -6 0 6 1 -7 2
13
6
2 5 .0
1 1 .5
52
100
In our m aterial, out of 52 cases half the cow bison dropped their first
calf in the fourth year of live (Table 3), b ut about 1A of the cow s drop
th eir first calf in their 5th or 6th year of life. The mean age of first
calving, calculated for the w hole m aterial, is 47.8 m onths (n — 52). No
d istin ct d ifferen ces w ere observed in the tim e of sexual m aturation of
cow bison in d ifferen t reserves. This indicates that the of attaining
m a tu rity is probably an individual characteristic. The distribution of
fe r tility of cow bison w hich began reproduction early does not confirm
414
Z. K rasiń sk i & J. Raczyński
W r ó b 1 e w s k i’s (1927) assum ption that early calving has a harm ful
e ffec t on fertility .
There are no system atic observations as to the age at w hich a cow
bison becom es steril. W r ó b l e w s k i (1927) assum es that the B iało­
w ieża bison becom e exh au sted early on, and only produce from 7— 8
calves during their lifetim e. D ata from reserves include the interesting
case of the cow »Beste« 524, w hich calving regularly produced its last
calf at the age of 19, after w h ich it w as steril u n til it died at the age
of 24 years. The cows »Podwika« 736 and »Poświata« 740, althoug 17
years old, w ere observed to lead calves in 1965. Bearing in m ind h o w ­
ever that conditions are more favourable in reserves it m ay be assum ed
that the fertility period of cow bison is shorter under natural conditions.
It is d ifficu lt to define the age at w hich m ale bison attain sexu al
m aturity since m ales are as a rule allow ed to cover cow s at an age of
over 3 years. The m ale bison »Pokorny« used for cross-breeding w ith
dom estic cattle, su ccessfu lly covered a cow at the age of 31 m onths.
Cases of early su ccessful m ating given by Z a b ł o c k i (1949) and J a ­
c z e w s k i (1958) point to the possib ility of the m ales attaining repro­
d uctive capacity at the age of 15— 20 m onths of life. K a r c o v (1903)
d efin es this m om ent at the 3— 4 year of life, W r ó b l e w s k i (1927)
gives the figure of 5 years. H istological exam ination of testes of 2 and
2 V 2 year old m ale B iałow ieża bison revealed the absence of sperm a­
tozoa. The late sexu al m aturity of B iałow ieża m ale bison, in W r ó fo­
l e w s k i’s opinion, is connected w ith the poorer food conditions in the
Forest; the Caucasus bison m ature earlier according to him. A fter attain ­
ing sexu al m aturity, how ever, the m ale bison are capable of reproduct­
ion u ntil very old, sperm atogenesis being absent on ly in old m ales e x ­
hausted by disease.
U nder natural conditions the tim e at w hich the m ale b egins reproduct­
ion is not of course determ ined by the m om ent of physiological m aturity,
but by intrapopulation factors connected w ith the social hierarchy of
the herd. In a free-livin g population the adult m ales over the age of
4— 5 u su a lly live alone throughout the greater part of the year. During
the rutting period th ey join the herd of fem ales and you ng bison and
prevent the younger m ales from covering fem ales ( K r a s i ń s k i , 1967).
The oldest bulls are also prevented from m ating. K u l a g i n (1919)
observed w eakening of the sexu al functions in m ales as early as the
10th year of life, w hen some of them left the herds and continued living
alone. O bservations in reserves show that bull bison continue to cover
cow s efectiv e ly up to the age of 15 years. Old m ales do not m anifest
su xual excitem ent.
The reproduction biology
415
V. THE OESTRUS CYCLE
M anifestations of oestrus in a fem ale European bison, p articularly at
the initial phase, are not characteristic. P ractically speaking oestrus is
discovered on ly on the basis of the an im als’ behaviour in the presence
of the bull. During the initial period the m ale »attends« the fem ale,
m ating taking place in the second phase (cf. Section X). According to
our data taken from keepers’ observations (ch iefly in the N iepołom ice
reserve) v isib le m an ifestations of heat occurred m ost often from 1 to 3
days (N — 54, x = 2.6 days), w h ile longer periods w ere sporadically
recorded (up to 7 days). J a c z e w s k i (1958) states that the period of
»attendance« m ost often lasts one day. Copulation w as observed at
various tim es of the day, and often take place tw ice or m ore in a day.
The phase of the oestrus during w hich th e fem ale allow s the bull to
cover it m ay last 2— 3 days.
The oestrus cy cle in fem ale European bison takes place in a w ay
sim ilar in m any respects to the w ell-k n ow n relations in dom estic cattle.
A cow w hich is not fertilized repeats the oestrus u ntil it becom es
pregnant. The successive appearances of oestrus in fem ale bison are easy
to grasp am ong anim als kept in an enclosed area, since during the
rutting season the breeding herd alw ays includes a sex u a lly active male,
w hich is roused by the occurrence of oestrus in a fem ale to cover it. It
¿s w orthy of note that from A pril to June o n ly one case of heat and m a­
ting w a s recorded in P olish reserves, in the reserve at Gorce (the fem ale
»Pupilka« on April 11th 1952), w here the breeding conditions in fact
d iffered from those in other P olish reserves (cf. J a c z e w s k i , 1958 —
Tab. 10). The data presented prove that a period of anoestrus in April,
M ay and June, occurs during the reproductive cycle of fem ale bison.
Under natural conditions anoestrus w as probably even more d istin ctly
m arked and m ost probably included the w h ole w in ter and spring period,
as is show n by the low percentage of m atin gs in January, February and
March. J a c z e w s k i ’s opinion (1958) th at oestrus in fem ale bison m ay
be repeated throughout the w hole y ea r is inaccurate in relation to
anim als bred in Polish reserves, that is, in the clim atic zone proper to
this species. This does not apply to m ales, which are probably
d istinguished by constant reproductive capacity. For instance, the m ale
bison »Pokorny« used for cross-breeding w ith dom estic cattle covered
cow s of the Low land B lack-and-W hite and Polish Red breeds several
tim es in April, M ay and June. The fact th at none of th ese m atings w ere
e ffec tiv e is alm ost certain ly connected w ith the d ifficu lt conception in
dom estic cow s covered b y a m ale European bison ( K r a s i ń s k a , 1963).
416
Z. K rasiń sk i & J. Raczyński
Sporadic cases m ay occur of copulation during pregnancy, u sually
during the first period after fertilisation, and in such cases the date of
effectiv e covering can be determ ined e x a c tly by know ing the tim e of
calving and average duration of pregnancy. A certain degree of anom aly
w as found in tw o fem ale bison in the N iepołom ice reserve. The cow bison
»Pulka« and »Puźnianka« perm itted copulation up to 3— 4 days before
calving. Considerable variation is observed in the length of the oestrus
in fem ale bison, due partly to the d eficiencies in observation m ethods,
but probably also p artly due to the season of the year, food conditions,
the anim al’s condition etc. In order to obtain reliable data on ly m aterial
relating to a period of 10— 30 days w as taken into consideration. Shorter
repeats of oestrus were on ly sporadically recorded, but longer periods
occurred more often. In som e cases the interval b etw een tw o consecutive
appearances of oestrus w as fairly ex a c tly equal to the m u ltiple of the
N
12
13
14 ' t5
16
17
18
19
20
21
22
23
24
25
26
27
28
Doqs
Fig. 2. F requency of rep etitio n of oestrus in fem ale bison.
normal sexu al cycle, w hich indicates that one or more oestral periods
w ere om itted from the records. Som e of the cases m ight h ave been due
to recording disturbances in the sexu al cycle. M aterial originating ch iefly
from the reserve at N iepołom ice is set out in Fig. 2. W ith a range of
variation from 12— 28 days alm ost 80% of the cases com e w ith in lim its
of 18 to 22 days, w h ile the average len gth of the cycle is 20.6 days.
According to J a c z e w s k i ’s data (I. c.) fem ale bison m ost often repeat
oestrus at intervals lasting about 20 days, although this period is often
variable in d ifferen t cows.
The reproduction biology
417
The observation that the oestral cycle exhibited considerable constancy
in the group of cow s w hich conceived in the given year m erits emphasis.
In the group of sex u a lly m ature fem ale bison w hich ended the rutting
season sterile, the sexu al cycles m ost often took a som ew hat irregular
course and there w ere long intervals in it.
Occurrence of first oestrus follow in g parturition form s one of the
factors regulating .the seasonal character of reproduction. In our m aterial
w e found that the shortest tim e which elapsed b etw een calving and
occurrence of post-parturition oestrus w as 13 days. This occurred in the
bison cow »Pociecha« 909, w hich calved on J u ly 9th 1961, and w as
covered on J u ly 22nd 1961. A v er y short period of 23 days w as also
found in this sam e fem ale in 1960. The gaps in docum entation m ade it
im possible to check up on th is in all of the fem ales. It is only on the
basis of duration of periods b etw een d ifferen t calvin gs that it is possible
to draw conclusions ind irectly as to the tim e at w hich post-parturitional
oestrus takes place in the reserve population. B earing in mind the fact
that the first copulation rarely p roves effective, it m ay be assum ed
that oestrus appears rela tiv ely soon after giving birth in a high percen t­
age of fem ales. E ffective covering took place in relation to 6.2% of the
fem ales during the first m onth after calving (the interval betw een
con secutive calvings did not then exceed 10 m onths). A s m an y as 15.6%
of the fem ales w ere covered during the period of tw o m onths after
parturition. In all 45% of the fem ale bison becom e pregnant during
the first three m onths after calving.
In older literature it is o n ly K a r c o v (1903) w h o m entions that in
the B iałow ieża P rim aeval Forest oestrus occurs in fem ale bison from
4— 5 m onths after calving. J a c z e w s k i (I. c.) gives the periods of 22
and 25 days after giving birth as the earliest cases of occurrence of
post-parturitional oestrus, copulation proving effective in the first case.
He also found that under reserve conditions oestrus occurs in fem ales
m ost often w ithin 40— 80 days after calving. In theory, in order that
the cyclic character of reproduction m ay be m aintained, p ost-parturit­
ional oestrus should appear about 100 days after giving birth. A tendency
to earlier appearance of post-parturitional oestrus m ay, in our opinion,
be treated as a visible m anifestation of dom estication. This phenom enon
is certain ly connected w ith the effect exerted on the fem ale bison’s
organism by food factors.
VI. DURATION OF PREGNANCY
In older lite ra tu re on the E uropean bison th e du ratio n of p regnancy w as alm ost
un an im ously given by d iffe ren t auth o rs as about 9 m onths ( B r i n e k e n , 1826;
418
Z. K rasiń sk i & J. Raczyński
K a r c o v , 1903; K u l a g i n , 1919; W r ó b l e w s k i , 1924; B a â k i r o v , 1939) both
for th e Białow ieża (Lithuanian) and C aucasian subspecies. It only becam e possible
to obtain accu rate data un d er enclosure conditions by m eans of recording co p u lat­
ions and calvings. K a l u g i n (1958) defined th e d u ratio n of pregnancy in a h erd
of cross-bred E uropean and A m erican bison in th e Caucasus in this w ay as 261—
283 days. Z a b ł o c k i (after G e p t n e r et al., 1961), on th e oth er h an d , gives
this period as 262—267 days.
°/
Fig. 3.
A t top — a u to rs’ own data,
J a c z e w s k i (1958), from
distrib u tio n of frequency
D uration of pregnancy.
at bottom — d ata ta k en from th e stu d y by
tables X —X II. T he cu rv e illu stra te s th e
for th e w hole of th e m a te ria l (N = 158).
In the present in vestigation s it w as ch iefly data from N iepołom ice
reserve w hich w ere used to define duration of pregnancy. Calculation
of duration of pregnancy w as m ade on the basis of th e differen ce in
tim e betw een the last registered (effective) covering of the fem ale bison
and date of calving. In addition to the authors’ own m aterial the data
given by J a c z e w s k i (1958) w ere also used, assum ing that these
observations are in principle uniform (docum entation of reserves). B ear­
ing in mind the p ossib ility that the final copulation m ay not be observed
The reproduction biology
419
it is n ecessary to take into consideration the p ossib ility that a certain
percentage of the data in th e m aterial m ay be incorrectly high. It is
therefore n ecessary to take a critical view of the m aterial.
The range of duration of pregnancy obtained from 254 to 284 (30 days)
would appear im probably w id e for a w ild anim al (Fig. 3). It is sufficient
for purposes of com parison to m ention that even for breeds of cattle
varying greatly p hysiologically duration of pregnancy is from 278— 290
days (A s d e 11, 1946), and on ly in a few breeds lasts as long as 311 days
(S t u d i e n c o v, 1956). In this case how ever the difference betw een
extrem e v a lu es is not more than 20 days. The distribution of the cases
investigated d eflects sharply in the 272-day class of pregnancy durat­
ion, and it m ay therefore be assum ed that the lim its of m axim um
duration of pregnancy in the European bison runs close to this value.
Longer periods up to the m axim um are caused by the failure to record
the fin al copulation. Both the sporadic and irregular character of occur­
rence of such cases points to this, and also the fact that th ey are separat­
ed from the low est valu es b y an interval of 17— 20, days that is, a period
com ing w ith in the lim its of cycle lenght in the fem ale bison. The
p ossib ility cannot be ruled out, h ow ever, on the basis of the character
of distribution of num bers (cf. Fig. 3). that both erroneous data due to
failure to observe the fin al covering, and real extrem e durations of
pregnancy m ay be contained w ith in the class of 271— 275 days. An
agreed lim it of 272 days w as accepted for further calculations as the
m axim um value.
Duration of pregnancy in fem ale bison therefore varies from 254 to
272 days w ith average valu e of 264.29 + 4.20 (confidence interval 0.72,
N = 132). This result introduces a correction on the basis of a greater
am ount of m aterial, to the data given by J a c z e w s k i (1958), w ho
stated th at duration of pregnancy in the fem ale bison varies from 260
to 270 days (average 265 days). [N. b. for w h ole range of data quoted by
J a c z e w s k i (256— 280 days) the average calculated is 265.8 days].
VII. EVALUATION OF THE F E R T IL IT Y OF FEM ALE EUROPEAN BISON
T he fe rtility of th e fem ales is a p artic u la rly im p o rta n t elem ent in th e re s titu t­
ion of th e E uropean bison, yet it has n ever form ed th e su b ject of special analysis.
T h e reason for th is state of affairs is m ost certain ly th a t in none of th e w ork so
fa r c a rrie d out w ith E uropean bison is th e re any noticeable tendence to selection
based on basic breeding observations. An additio n al facto r h e re is form ed by th e
d ifficulties of a m ethodical n a tu re connected w ith th e sem i-w ild ch a racter of b re e d ­
ing E uropean bison.
T he age at w hich the fem ale first gives b irth and la te r reg u la rity of calving in
successive years a re th e m ost im p o rta n t facto rs affecting th e fe rtility of fem ale
bison. Z a b ł o c k i (1957) takes th e se factors into consideration, defining th e fertility
420
Z. K rasinski & J. Raczynski
of d iffe ren t cows in percentages: cows w hich calved every y ears from th e fo u rth
y ear of th e ir lives u n til th e ir d eath h ave 100% fertility . T he p ercen tag e of fertility
m ay even exceed 100, if the firs t calving occurs ea rlier. F or p rac tica l purposes
th is m ethod is not convenient, as it can only be defined e x post in co m p arativ e
evaluations, and in addition only for fem ales w hich lived for, a t least, th e av erage
life span of th ese anim als.
S e v e r c o v (1940; 1941) takes significant facto rs affecting th e fe rtility of
fem ales into consideration by introducing the so-called rep ro d u ctiv e rate. W hen
estim ating th e fe rtility of a population S e v e r c o v (1941) includes, for instance,
th e age at w hich th e fem ale firs t gives b irth (age of m a tu rity ), th e n um ber of
calves born during a year and th e period elapsing betw een tw o b irth s. For a com ­
plete analysis of effective fe rtility it w ould only be necessary to add h ere an
evalu ation of progeny from th e aspect of h ealth in ess and capacity for survival.
T he p ic tu re of fe rtility of fem ale E uropean bison given below refers to anim als
living in Polish breeding reserves over th e period of th e la st 10 years under
rela tiv e ly stab le conditions.
1. Age of first calving
In this question w e are concerned w ith the estim ation of two values
in the population aspect: (1) the age at w hich the fem ale bison can
produce progeny for the first tim e and (2) the tim e from w hich m ost of
the fem ale bison in a population can be treated as »breeding«. Both
these v alu es m ust be expressed in calendar years of the fem ale bison’s
life on account of the convenience of such treatm ent in breeding
practice.
Table 4.
R atio of fem ale bison w hich h av e calved to th e to ta l n u m b er capable
<f reproduction depending on calen d ar age.
P e r c e n t o f f e m a le s :
/n = 5 l/
c a l v i n g f o r th e f i r s t
c o n s id e r e d c a p a b le o f
re p ro d u c tio n
tim e
2
C a le n d a r y e a r o f l i f e
3
4
5
6
-
27
44
21
2
-
27
72
98
l 00
D efinition of the second m om ent presents a problem , as the only
criterion of attain m ent by the fem ale of sex u a l m atu rity is its first
calving. For this reason all cow bison w hich had given birth to their
first calf w ere considered capable of reproduction, regardless of w hether
th ey w ere in calf at the tim e of exam ination or sterile (Table 4).
It m ay be assum ed that as from the fifth calendar years of life all the
cow s in a breeding population should participate in reproduction, but for
all calculations of an estim ative character the m ean age at w hich the
first calving takes place, during the first calendar year of th e fem ales’
life, should be used (x = 47.8 m onths) (Table 3).
The reproduction biology
421
2. Number of young born
N o case of tw in pregnancy occurred during the study period, neither
w ere cases of tw in s encountered in the Białow ieża Prim aeval Forest by
either B r i n c k e n (1826) or K a r c o v (1909). According to W r ó b ­
l e w s k i (1927) and P f i t z e n m a y e r (1929) twin calves w ere occas­
io n a lly born both at B iałow ieża and am ong the Caucasian bison: W r ó b ­
l e w s k i describes a case w hich he h im self saw of the birth of twin
sister calves. A nother case confirm ed by docum entation is the birth of
still-b orn tw in calves at Pszczyna reported by M o h r (1952). More
recen tly another case of tw in calves being born occurred at Sierpuchow
cn J u ly 1st 1960, to the cow »Molnija« 838 (Pedigree book 1961) and
tw in pregnancy in the herd livin g in the Caucasus ( K a l u g i n , in lilt.).
The birth of tw in calves need not therefore be taken into account as
a factor a ffectin g fe rtility in European bison.
3. Fertility period and frequency od calving
T he fe r tility period in the life of the fem ale bison, beginning in the
3rd— 5th years of life, lasts u ntil the anim al reaches late old age. Its
len gth is determ ined to a great degree by the lon gevity of the fem ale.
In the series of cow bison exam ined all the anim als in the 17 year old
class (n = 6) w ere still fu lly fertile (Fig. 4). The cow »Beste« 524 gave
birth to her last calf at the age of 19, then rem ained sterile until her
death at the age of 24. The cow »Kauka« described by M o h r (1952) as
v er y fecund, calved for the first tim e at the age of 4 and then produced
20 ca lv es over a period of 21 years (1925— 1945), thus being sterile in
one y ea r only. Her last calf w as born w hen she w as 24. In the breeding
practice of the Central European B ison R eserve of the U SSR the age at
w h ich the fin al calving takes place (end of the fertile period) w as taken
as 19 yea rs ( Z a b ł o c k i , 1957) on the basis of m any years exp erience in
p lan n ing herd increase, exclu d ing cow s aged 20 years or over from the
reproducing group.
It rem ains to explain the characteristics of fertility in fem ale bison
over the w h ole fecund period. According to our data (Fig. 4), as from
th e 5th year of life, w hen p ractically speaking all the fem ales enter the
reproducing group, the percentage of calving fem ales rem ains steady
w ith in lim its of 72—85%. From the tim e the cow bison attains sexual
m a tu rity its reproductive p otential is therefore as a rule m aintained on
an unvarying lev el until the reproductive functions cease. The age factor
in fem a les should not therefore be taken into consideration w hen
estim atin g the freq uency of calving during the fecund period. The age
422
Z. K rasiński & J. R aczyński
of the fem ale does not affect the q u ality of its progeny, since no d if­
ferences w ere found in the m ortality of calves depending on the age of
the fem ale bison (Z a b 1 o c k i, 1957).
In Z a b 1 o c k i ’s opinion (1957) the num ber of calves obtained from
a fem ale show a peak at the age of 4 years and decreases w ith age. This
view , how ever, is based on erroneous interpretation of results, the
diagram referred to in support of this opinion only con stitu tin g a picture
of te age structure of reproducing fem ales.
N
51
51
50
47
43
39
35
27
24
50
25
11
Age in years
Fig. 4. F e rtility index in fem ale bison depending on age.
In theory a fem ale bison is capable of calving each year throughout
the w hole period of fertility. In rea lity recorded facts of sim ilar
fecu n d ity are on ly those in the cases of the cow »Kauka« or »Plakette«
(M o h r, 1952). In breeding reserves periodical occurrence of sterile years
is the rule am ong the m ajority of fem ale bison. U nder natural conditions
sterile periods occurred more in ten sively. Fem ale bison from natural
populations in B P F w ere held to calve once every 2— 3 years at m ost
( K a r c o v , 1909; K u l a g i n , 1919; W r ó b l e w s k i , 1927).
Considerable freq u en cy and regu larity of calving form s a characteristic
feature of the m ost fecund cow bison. U tilization of these facts for
estim ating fe r tility is possible thanks to the use as index of the mean
calving interval. A lm ost h alf the births (45.8%) take place su ccessively
The repro d u ctio n biology
423
at in tervals of 10— 12 m onths. L onger periods point to an absence of
regularity in calving, although intervals of one year can still be consi­
dered regular. It is the longer periods, from 18— 19 m onths, which
indicate a lack of calving in one or tw o consecutive calendar years. In the
group of fem ale bison (excluding cases of perm anent sterility) the mean
interval betw een calvin gs w as 14.4 m onths (Fig. 5), but is m ost often
from 11— 13 m onths (55%).
Months
Fig. 5. D uration of calving in te rv als in fem ale bison in reserves.
T able 5 gives a com parative estim ate of the fe r tility of all the fem ale
bison studied. The individual m ean calvin g in tervals w ere calculated
h ere for older cows, w hich had reproduced longer (over 5 calvings), and
w ere found to vary b etw een 11.8 and 22.5 m onths. O ne-year periods
(calendar) predom inate am ong the various intervals b etw een calvings,
and form 16.4% of the total potential calvings. T w o-year intervals in
reproduction occurred on ly 4 tim es (2.4% of p oten tial calvings). Periods
of m ore than tw o years of sterility are connected w ith perm anent
ste r ility of the fem ale.
819
823
827
831
832
859
903
909
911
912
935
937
939
941
973
975
9 77
979
1021
1030
1070
1131
1136
1141
1143
1204
1205
1206
1232
1284
1293
1300
1303
1356
1357
1357
PUDLARKA
PULONKA
POGOŃ
POZŁOTA
POLATUCHA
POBUDKA
PUSTELNIA
POCIECHA
PUŹNIANKA
KAMIONKA
PUNITA
PUSZKA
POLANKA
PU5Ł0NKA
PULEŚHA
POKUSA
POWAGA
PURELLA
PUSKAŁKA
PUCUŁKA
PUTNIA
KANIA
POCHMURNA
PUJANKA
PORĘCZNA
PLISZKA I I
PUSTAŁKA
POTĘGA
PUZONKA
PLENNA
PUCERTA
PUCZAJA
PURORA
PLAGA
PLATYNA
PLANETA
1949
1949
1950
1950
1950
+
1957
1958
1958
1958
1958
1959
1959
1959
1959
1960
1960
1960
196«
1961
1961
1962
4
4-
4-
4-
4
4
- 1/ - 1/ M
ZOO
4
-
+
♦
•f
+
4
4
4
4
4
4
4-
4
4
-
4
-
.1 /
4-
4-
4
4
4
4
4
4
4
u
+
+
4
4
4
4
4
4
4
4
4
4
zoo
+
4*
4
-
4
4
4
4
+
-
+
4
4
4
4
4
M
+
4-
4-
-
4
4
+
4
4
4
4
-
+
-
4
4
ZOO
♦
4
4
200
+
4
R
+
-
4
4
4
ZOO
4“
4
4
4
4
4
zoo
R
4
4
-
4
4
4
4
R
4
4
+
+
4“
+
4-
4♦
+
44-
+3 /
4
4
-
4
4
-
4
4
4
ZOO
-
4
4
4
4
4
4
4
ZOO
4
4
4
4
4
4
-
-
4
4 ^
4
4
,
<•0
u
4
4
C a lv in g
In te rv a l
1966
1.962
1960
-
1961
4-
1959
_
+
ITS
y£>
""
+
+
♦
+
1951
1951
1951
1951
1951
1951
1952
1953
1953
1953
1953
1954
1954
1954
1954
195 5
1955
1955
1955
1956
1956
1958
1947
1947
1947
1948
1948
1948
1948
_
♦
+
1939
1942
rs
yo
1957
BBSTE
PUTKA
PUŁOMKA
PUSZYNKA
PUBORKA
PODWIKA
PLAMKA I I
POŚWIATA
PUUIANKA
PCZORKA
POPIELICA
PUSZCZANKA
PULKA
PURTANKA
PORĘBA
of
b irth
1956
524
596
723
724
725
736
737
740
748
760
770
790
791
795
812
Y ear
1955
P e d e g re e No
A Name
1954
Table 5.
Table of fe rtility of fem ale bison in b reeding reserv es. T h e fe rtility ( + ) and
ste rility (—) of fem ale bison is given in th e ta b le for th e stu d y period, fro m th e
tim e of firs t calving u n til th e end of observations, due to: d eath of th e fem ale (M),
its relase into th e fo re st (R) or tra n sfe r to a zoo (ZOO). M ean calving in te rv als
calculated only for older cows, ta k in g into consideratio n th e w hole fe rtility period
of th e cow. l)i — not covered, 2) — died w ith foetus in u teru s, 3) — abortion 4) —
verv fat.
-
4
/
R
+
♦
8
9
8
8
13
9
11
12
6
11
1 5 .0
1 4 .3
1 2 .1
1 4 .1
1 2 .2
7
9
4
6
-
-
Avg.
-
•f
-
4
4
4
-
-
4
4
4
4
4
«*•
4
♦
4
-
-
R
♦
4
-
4
4
<
4
4
4
-
4
-
4
4
4
4
M
+
4
♦
4
4
-
4
3
7
5
1 3 .3
15. 3
1 6 .8
+
-
7
1 5 .3
-
5
1 4 .2
-
4
4
20. 0
22. 5
1 1 .8
-
4
+
4
R
♦
♦
♦
+
+
-
6
4
15. 5
-
6
6
1 1 .8
1 2 .0
1 1 .8
12. 5
♦
+
+
+
♦
5
4
♦
-
-
♦
*
-
♦
4-
+
-
+
+
+
-
♦
+
♦
R
-
R
4
15. 5
14. 6
1 3 .8
1 3 .8
12. 5
1 5 .5
-
R
*
♦
+
■f
+
14. 3
13. 4
12. 4
-
4/
4
4
r2
+
+
N
4+
.1 /
♦
+
R
♦
ZOO
+
+
-
425
The repro d u ctio n biology ,
The follow in g three factors: tim e of first calving, num ber of calves
born in relation to the num ber of fertile years and regularity of calving
(length of calving interval) m ust be taken into consideration in order
to determ ine w hich fem ale has m axim um fertility. It is prim arily older
fem ale bison w hich have already produced a large num ber of calves
w hich should be allocated to this group, i.e.: »Puborka« 725, »Poswiata«
740, »Plam ka II« 737, »Puzorka« 760, »Puszczanka« 790 and »Pulomka«
723. Of the younger cow s »Kania« 1131, »Puslonka« 941, »Puskalka«
1021 and »Puculka« 1030 calve very regularly (x period b etw een calvings
11.8 — 12.0), and prom ise to prove good breeding m aterial.
A characteristic feature of h igh ly fertile fem ales is the fact that they
attain sexu al m aturity relatively early (they u su a lly calve as early as
the 3rd calendar year of life) and are then distinguished b y considerable
regularity and norm al course of the reproductive processes. A ll the
fem ale bison w hich produced larger num bers of calves exh ibit a low
valu e for the calving interval. T hese observations m ay form some con­
firm ation of the opinion th at the fe rtility of these fem ales m ay constitute
their individual characteristic.
4. F e rtility lim iting factors
The phenom enon of periodical
the fem ale bison. Under reserve
that about 20% of the fem ales in
not calve. There are som e causes
sterility is a more or less normal in
conditions it m ay be taken as a rule
the group of fu lly m ature anim als w ill
of this state.
Table 6.
E ffect of th e tim e of calving on ste rility of fem ale bison th e follow ing
breeding season.
Month o f c a l v i n g
S e p t.
O ct.
Nov.
D ec.
No. o f f e m a le s w hich c a lv e d
o u t s i d e th e m ain c a l v i n g p e r i o d ,
16
8
4
5
7
5
4
5
and
w ere s t e r i l e th e f o ll o w i n g s e a s o n
The capacity of the fem ale bison to conceive probably decreases
togeth er w ith the sh ift of the final calving outside the main calving
period. Am ong the fem ale bison analysed w hich calved during the period
from Septem ber to D ecem ber the m ajority w ere sterile during the
follow in g season (Table 6).
B earing in m ind that the principal period of oestru s in European bison
occurs during the m onths from A ugust— October (w hich corresponds to
calvin g from M ay to Ju ly) belated calvin gs in N ovem ber and Decem ber
426
Z. K rasiriski & J. Raczyriski
are the result of copulation taking place outside the true rutting season.
The fact that such copulation does not then result in fertilisation m ay
be due prim arily to the failure of post-partum oestrus to appear in the
fem ale, and as a result the cow is not covered u ntil the n ext ru ttin g
season, and thus there is a pause in reproduction for one year.
The death of the calf m ay be a cause of transistory in fertility in the
fem ale. This fact, observed in reserves, w as confirm ed by us in the case
of available fem ale bisons w hich either gave birth to dead calves, or
w ere deprived of their calf a short tim e after calving, during the period
of in ten sive lactation. Am ong 19 such cow s — 8 (42%) becam e sterile
the follow in g year, but in the group of adult fem ales (from 5th year of
life) w hose calves lived through the lactation period, ste rility occurred
in only 17% of the cases out of 281 possible calvings. This com parison
dem onstrates the im portant effect that the ca lf’s death has on sterility
the follow in g year, although such a result is by no m eans the rule. The
m echanism of this phenom enon is u ndoubtedly based on disturbances in
the processes of horm onal regulation in the lactating cow.
A separate group is form ed by those cases of in fertility w hich cannot
be exp lained as due to a d efin ite cause, althought th ey occur w ith con­
siderable inten sivity. In breeding enclosures perm anent ste rility in a bull
is easy to perceive, as the sam e m ale u su ally covers several cow s, often
at short intervals. In our m aterial it w as on ly once that w e m et w ith
a case of a fem ale being sterile as the result of using a sterile m ale. It
m ust be assum ed that the total num ber of cases of in fertility include
cases of sterility due to loss of the em bryo during the early stages of
em brionic d evelopm ent (cf. data for cattle — N a l b a n d o v , 1958).
It w as found that freq u en tly prolonged sterility, cases of abortion or
dropping lo w -v ita lity calves occur more often in som e fem ales than in
other. Such fem ales can easily be distinguished in table 5, e. g. »Polatucha« 832 is distinguished by particularly low reproduction: it aborted
tw ice, dropped stillborn calves tw ice and one of its calves died soon
a fter birth. Out of all the calves produced by this fem ale it is on ly its
first — »Pokusa« 975, w hich su rvived and the tw o calves born to the
latter died soon after birth due to their low v ita lity . Several other
fem ales exh ib it low fe r tility as the result of prolonged periods of
sterility, and irregularity of calving connected w ith this, and also long
intervals betw een calving. Such fem ales include: »Puszka« 937 (Avg.
calving interval — 20.0), »Polanka« 939 (22.5), »Purella« 979 (intervals
betw een consecutive calvings resp ectively 16, 22 and 33 m onths), and
»Punita« 935 (intervals of 25 and 36 m onths b etw een the first three
calvings). Cases of sudden loss of fe rtility after a period of normal high
fertility are also encountered, as in the case of »Pociecha« 909, which
T he reproduction biology
427
began producing calves in its third year of life, produced 5 su ccessive
calves at regular intervals, then from that tim e onwards w as perm a­
n en tly sterile (in the breeders’ opinion due to grow ing v er y fat). S im i­
larly, although w ithout an y visible cause, »Pulesna« 973 becam e sterile
although it had prom ised to prove v ery fertile.
In the m ajority of cases the ten d en cy w hich can be perceived by
m eans of carefu l observations of rutting and reproduction processes in
d ifferen t fem ales is confirm ed during its later life. It is not d ifficu lt to
discover both h ig h ly fertile fem ales or poor reproducers on the basis of
accurate breeding records. The case described here of th e low fe rtility
of »Polatucha« and its daughter »Pokusa« unm istakably point to the
h ereditary basis of such tendencies. On the other hand the occurrence
in som e fem ales of decidedly high fe rtility argues in favour of recognis­
ing this feature as a hereditary property. The practical conclusions
arising from the foregoing m ay prove of considerable im portance to
breeding practice in reserves.
5. Total increase from a fem ale European bison
C alculation of increase in young on the basis of the assum ption that
a fem ale should calve ev ery year from the tim e it attains sexu al m a­
tu rity is artificial. This kind of »potential fertility« can never be attain ­
ed. F ertility over a fem a le’s lifetim e can on ly be defined on the basis of
calcu lated average duration of calving interval, and in order to assess
total production of young it is essen tial to take into consideration in
addition the m ean length of a cow ’s period of fertility.
The duration of the fertile period can be defined on the basis of the
m ean length of life of a fem ale bison, but the m ajority of authors
d eterm ine this index in exactly, m ost often giving the extrem e values.
The v a lu es given in older literature are alm ost alw ays too high (cf.
K a r c o v , 1903; B a s k i r o v , 1940). The on ly reliable figu res calcu lat­
ed from a group of anim als more than 5 eyars old are those given b y
J a c z e w s k i (I. c.), i. e. 13 years. If it is assum ed that the first three
years of a fem ale bison’s life are in fertile (first calving in the fourth
year of life) a 10 year period of fe rtility rem ains. The calculation based
on an index accepted in th is w ay is too high already, as the oldest cow s
over 20 years old, the m ajority of w hich m ust have been sterile, also
contribute to this figure. It m ay, how ever, be assum ed (detailed ca l­
culations w ere not made), th at under the present standarised conditions
in reserve breeding the average lifespan of European bison m ay be
longer than w as the case at the start of the breeding operations in
reserves and various zoological gardens.
428
Z. K rasiń sk i & J. Raczyński
Taking the above data as the basis for calculating total increase it is
found that an average fem ale produces a m ean num ber of 8.83 (<=-■ 9)
calves in a reserve (10 years — 120 m onths w ith average freq uency of
calving every 14.4 m onths). This is an approxim ate value as it includes
stillborn calves and those which died soon after birth. We shall leave
this error as we have no exact k now ledge of the causes of death of all
calves which w ould m ake it possible te distinguish b etw een cases of
deaths due to abnormal em bryonic developm ent and com plications
during birth and death due to accidents etc. (crushing b y a fallin g tree,
drowning, being tram pled to death by adult anim als, injury and
killing of a calf by a m ale etc.). According to Z a b ł o c k i (1957)
m ortality among calves during the first year of life w as about 17%
under reserve conditions.
VIII. EVALUATION OF F ER T IL IT Y AND INCREASE IN THE HERD
1. Fertility
The concept of fertility of a herd applies prim arily to natural p opu l­
ations of anim als. The total increase of young in European bison p opu l­
ations is affected by such factors as the fe rtility of fem ales, the age
structure of herds (participation in the herd of the fertile group of
fem ales), sex ratio and v ita lity of the young anim als. F ertility of fem ales
in enclosed reserves m ay be used, w ith in certain lim its, for com parative
investigations of the fe rtility of »breeding herds« esp ecially in v iew of
the fact that in the breeding of the European bison so far carried out
efforts have been directed at m aking the fu llest possible use of the
reproductive capacity of th ese anim als.
No visible differences w ere perceived in the reproduction of European
bison in P olish reserves. In our opinion the v ery sim ilar food conditions
under whiph the anim als are kept in reserves, the sim ilarity of their
m anagem ent, care, phrophylaxis etc., should com e w ith in the lim its of
variation tolerated b y the bison’s organism. D ifferences b etw een the
B iałow ieża line bison bred in P olish reserves (i. e. all the bison at B iało­
w ieża and som e at Pszczyna) and bison in w hich there is som e degree
of Caucasian blood (all the rem aining), should not, p ractically speaking,
p lay any im portant part in reproduction processes, even if on ly on
account of the relatively negligib le percentage of the Caucasian blood
in this latter group of bison. In addition both forms exh ibited under
natural conditions greater sim ilarity than is now observed in relation to
the reserve »herds« in breeding centres geographically m ore distant and
differing as to conditions (Cf. Z a b ł o c k i , 1949). The m aterial we ela ­
borated from the reserves can therefore be treated as hom ogeneous.
429
T he reproduction biology
F ertility of bison in reserves expressed by the percentage of fertile
fem ales in th e sex u a lly m ature group (from the 4th year of life) is on
the average 76%, w ith variations from 63— 91% in different years
(Table 7). C hanges in fe rtility over a period of years does not in this
case represent the influence exerted b y d efin ite factors on the reproduc­
tion of the herd, if on ly on account of the fact of joint treatm ent of
m aterials from d ifferen t breeding centres and the low degree of repre­
sen tativen ess of the series. A rrangem ent of: the m aterial in years is the
only one in agreem ent w ith the natural p eriodicity of reproduction in the
European bison and m ay thus con stitu te a scale for m easuring the range
of variations and ex ten t of flu ctu ation s in in ten sivity of reproduction
under reserve conditions.
Table 7.
P er cent of calvings in breeding reserv es and in freedom in relatio n
to the num ber of cows capable of rep ro d u ctio n (N).
Y ear
1954
1955
1956
1957
19 58
1960
1959
1961
1962
1963
1964
1965
30
28
28
27
29
26
-
315
242
1966
T o ta l
H V 3 E R V E 3
N
C alv ­
in g s
14
21
23
25
31
33
n
12
16
21
17
24
30
19
21
20
18
23
21
-
%
0 5 .7
7 6 .2
9 1 .3
6 8 .0
7 7 .4
9 0 .9
6 3 .3
7 5 .0
7 1 .4
6 6 .7
79. 3
9 1 .3
-
7 6 .8 2
7 R E E I) 0 M
H
C alv ­
in g s
-
-
-
-
3
7
8
11
15
16
18
28
32
n
-
-
-
-
2
6
6
11
8
13
16
15
20
97
*
-
-
-
-
53
81
89
54
62
7 0 .3
86
138
The flu ctu ation s in the num ber of calvin gs among free-livin g fem ales
(53 to 89%) and also the m ean valu e for the 9-year period of free
reproduction in the B iałow ieża Forest (70.3%) are very sim ilar to the
corresponding v alu es for anim als livin g in enclosures (Table 7). D iffer­
ences b etw een average indices are not sta tistica lly significant. In order
to exp lain this phenom enon it is necessary to take into consideration the
sp ecific character of a free-liv in g herd due both to its current com oosition and to the conditions to which it is subject ( K r a s i ń s k i . 1967).
The cause of high fe rtility in cow s in a free-livin g herd is undoubtedly
in ten sive w in ter supplem entary feeding, probably com pletely satisfyin g
the q u an titative and q u alitative food requirem ents of bison during this
period. U nder natural conditions the anim als w ere u su ally exposed to
hunger during this period. B efore the extin ction of the natural herds of
European bison in BP Forest, at the turning point of the 19th century.
430
Z. K rasiński & J. Raczyński
w hen the forest w as unable to provide enough food for the num erous
herds of big game, the fe rtility of the bison d istin ctly decreased and
sterile periods of 2— 3 years occurred. D uring this tim e fem ale bison in
B iałow ieża enclosures m ost often calved ev ery year (K a r c o v, 1903,
K u l a g i n , 1919; W r ó b l e w s k i , 1927). A sim ilar fact of fertility
increase is given by Z a b ł o c k i (1949) using as exam ple the herds of
crossbred European and A m erican bison after -the anim als w ere m oved
in 1941 from the poor steppe areas of Askania Nova to the Caucasus,
w here th ey had better pastures and w ere given regular supplem entary
feeding. T he absence of significant d ifferen ces in fe rtility b etw een
a free-liv in g and enclosed anim als indicate that a free-liv in g bison
herds are not now adays subject to the effect of lim iting factors.
2. Population increase
A fu ller description of the developm ent dynam ics of: a free-liv in g herd
o f^ u r o p e a n bison is obtained by assessing the indices of real quantita ­
tive population increase. This m akes it possible on the one hand to grasp
the flu ctu ation s in the population dynam ics of bison under sem i-natural
conditions, and on the other perm its of evalu atin g the degree to which
such breeding is u sefu l and effective in fu rther restitution of the species.
Q uan titative indices of th e renascent herd of European bison in BP
Forest can be com pared w ith analogical data obtained before com plete
extin ction of the bison in this Forest ( K a r c o v , 1903).
The list given by K a r c o v (I. c.) contains the results of the annual
estim ations of European bison population made by the forest adm i­
nistration service from 1809 to 1902. The author h im self questions the
relab ility of these official data, n everth eless as unique m aterial th ey
should be used for stu d ies on the population dynam ics of the European
bison ( K u l a g i n , 1919; S e v e r c o v , 1940). The period from 1888 is
to be taken as the m ost authoritative.
A nalogical data on the d evelopm ent and dynam ics of the present
free-liv in g herd in the B iałow ieża Forest are not u n fortu n ately free of
error. E stim ate of num bers is m ade ex trem ely carefu lly in this case, but
calculation is seriou sly com plicated by the fact that fu rther anim als
w ere released in successive years (either sin gle anim als or groups) w ith
d ifferen t proportions of sex and age. Each year has th erefore been
treated in d ivid u ally in calculating reproductive rate on th e basis of full
records.
T otal increase of the free-livin g herd flu ctu ation s w ith in lim its of
13— 33% (increase in relation to the in itial num bers of th e herd in
a given year). The m ean value for a 9-year period (1958— 1966) is
The reproduction biology
431
m aintained on a high lev el of 19.9%, to w hich resu lt both the consider­
able increase of young (average 22.4%) and very low m ortality contribu­
te. A ccording to W r ó b l e w s k i (1927) from 1889— 1902 increase in
young in the B iałow ieża bison population w as on an average no more
than 14%, varying from 11— 20% in different years. D uring th is sam e
period losses form ed from 6— 8% of the head, reducing total increase
in the herd b y n early half. These losses w ere due to variou s causes
(including shooting, both legal and poaching), but the official statistics
state that the m ajority w ere due to deaths from disease or old age
( K a r c o v , 1903). In W r o b l e w s k i ’s opinion, how ever, the chief
cause of low population increase w as the high m ortality am ong the
young, form ing 49.3% of the w h ole annual increase. In the present
free-livin g herd, on the other hand, m ortality am ong calves up to 1 year
old does not exceed 2.29% of th e increase in young ( K r a s i ń s k i ,
1967). In relation to herd num bers for the w hole period during w hich
the anim als have lived in freedom this is only 0.5%. The h igh rate of
population increase in free-livin g herds is due to several causes. The
excep tio n a lly sm all losses in th e group of adult anim als (on ly 9 anim als
died during the w hole period of free-liv in g breeding) m ay be explained
by the current age structure of the herd. At the present tim e young
anim als form 42% of the herd ( K r a s i ń s k i , 1967) w hich, differs
g reatly from the m odel of natural age structure in a population of
anim als w ith sim ilar reproductive p oten tial and longevity. T his situation,
although abnorm al from the point of view of natural population pro­
cesses, is ex trem ely favourable from the aspect of herd increase in the
near future.
The extrem ely low m ortality rate am ong young anim als d eserves
special em phasis. It is at present d ifficu lt to state to w h at d egree this is
connected w ith the present age structure of the herd. In v ie w of the
frequent cases observed of calves dying as the result of in ju ry b y adults
and old m ales it m ay be assum ed that this factor p lays a certain part.
W r ó b l e w s k i (1927) considered that the ch ief cause of high
m ortality rate am ong young anim als w as due to unfavourable living
conditions. If these conditions consisted m erely in the food factor, then
it is true that a v ery great im provem ent has taken place in the B iało­
w ieża Forest in this respect (rich undergrow th, reduced com petition
from red deer), w hile the bison now receive better care in the w inter.
There is no doubt that the phenom enon of particu larly in ten sive herd
increase is to some degree transistory and m ay be exp ected to drop after
reaching optim um num bers and herd structure. If this m om ent can be
grasped, it w ill undoubtedly be of great practical im portance to free-
432
Z. K rasiński & J. Raczyński
living breeding of bison, since the param eters of habitat and intra-popul­
ation factors are known.
IX. SEX RATIO
A large num ber of observations and num erical d ata have been accu m u lated for
a long tim e in lite ra tu re on th e subject of sex ratio in th e E uropean bison, despite
w hicn this question has not as yet been closed. M ention is encountered in th e
oldest sources of unequal sex ratio in w ild bison populations. The m a jo rity of the
au th o rs speak of th e clear predom inance of m ales, both in th e case of B iałow ieża
and C aucasian bison ( B r i n e k e n , 1826; K a r c o v , 1903; B a ä k i r o v , 1939).
It is possible th a t th is view has been adhered to for so long as a re su lt of th e
opinion established by U s o v and B ü c h n e r (cited a fte r B a s k i r o v , 1939)
th a t th e fact of a g re a te r num ber of m ales th a n fem ales being born proves
a tendency to n a tu ra l extinction of th e E uropean bison.
In m ore recent studies Z a b ł o c k i (1957), basing his findings on a large
am ount of m a teria l from enclosure breeding, dem o n strated th a t cow bison p red o ­
m inate. In
addition he m ade a detailed analysis of v aria tio n s in th e ra tio of
m ales to fem ales in d ifferen t years, both in populations stocking reserv es and in
th e annual increase in young, and found considerable v ariatio n s. D uring th e sam e
period J a c z e w s k i (1958) obtained on th e basis of reco rd s in p ed ig ree books of
th e sex of calves born, ratio s sim ilar to th e 1 :1 ratio in resp ect of a very
considerable am ount of m a terial.
.
T hese ap p a ren tly co ntradictory data on th e sex ra tio in th e E u ro p ean bison can
be logically
explained, if th e possibility isaccepted th a t sex ra tio in a population
m ay differ
in d ifferen t age groups of th e anim als. A n o th er reserv atio n
of
a m ethodical n a tu re is req u ired w hen evaluating flu ctu atio n s in sex ra tio in d iff­
ere n t years. T his index belongs to th e phenom ena w h ich a re rev ealed statistically
w hen the num ber of observations is sufficiently large. T his im poses lim its on all
conclusions and speculations based on m a teria l w hich is in sufficiently re p re ­
sen tative.
i.
S e x r a t i o a t b i r t h — this can be treated as being closest
to the prim ary sex ratio established at the m om ent of fertilisation. Up
to the m om ent of birth it can be m odified on ly by selectiv e action of
intra-uterine m ortality. In the m aterial exam ined, out of 249 births of
live and dead calves in reserves during the period from 1954— 1965 the
ratio of m ales to fem ales w as 138 : 111 (1 : 0.30), w hich does not differ
sta tistica lly sig n ifican tly from the ratio 1 : 1 (test y 2). A balanced sex
ratio also occurs in J a c z e w s k i ’s m aterial (I. c.) and also in the
com parison of sexes of calves born in d ifferen t breeding reserves in the
world, as given b y Z a b ł o c k i (1957). The ratio of 233 : 233 is obtained
from the data given in table 8 (p. 35) of th e stu d y b y the author
referred to.
The sex ratio of calves bcrn in the free-liv in g bison population in the
B iałow ieża P rim aeval Forest for the period 1958— 1966 is 52 : 46 (1 : 0.89),
which does not d iffer sign ifican tly from the theoretical ratio of 1 : 1
'test y 2).
The reproduction biology
433
2.
S e x r a t i o i n t h e h e r d — a significant predom inance of
iem a les is u sually revealed. This phenom enon w as observed b y Z ab l o c k i (1957) in the head of bison in B PF given by W r ó b l e w s k i
(1927) in 1909 (256 : 329) and also in his own statistics covering all the
European bison livin g in the w orld during the period 1921— 1951. We
obtain the follow in g proportions from his data: for B iałow ieża bison —
557 : 734, for bisons of the Caucasus and B iałow ieża lines join tly —
1273 : 1389. A ll the proportions given above differ sign ifican tly from the
1 : 1 ratio in favour of fem ales.
The facts presented prove that sex ratio in a bison population is
subject to variations, th e causes of w hich should be sought for in the
d ifferen t m ortality of the tw o sex es in postnatal life. A study w as made
of the sex of all calves w hich died in reserves during the first year of
life. The ratio of 22:5 obtained reveals the decided predom inance of
m ales and exclu d es the p ossib ility that the result is fortuitous. Z ab l o c k i (1939) also found m arked predom inance of m ortality among
m ales in herds of crossbred European and Am erican bison in Askania
Nova during the period from 1907— 1938, w h ere out of a total num ber
of 78 stillborn calves 21% w ere m ales and 10.3% fem ales. In the B iało­
w ieża reserve alone during the period from 1954— 1965 abortions and
m ortality among calves caused 18% losses of the total increase of young.
W r ó b l e w s k i (I. c.) m ention s losses of as much as half the increase
in young in a year. If the proportions of this m ortality am ong m ales and
fem ales are borne in m ind the change in sex ratio on the group of adult
bison becom es easy to understand. The relations observed in the reserves
do not appear to from a special case. The lists of losses in the B iałow ie­
ża Olson population given by W r ó b l e w s k i (I. c.) from 1889— 1907
show the proportions for adult anim als are 427 m ales and 308 fem ales,
sig n ifica n tly differing from the ratio 1 : 1 . A lthough varied causes of
losses in head of bison during th is period are involved, the m ajority of
the anim als died from causes not d irectly due to hum en intervention
(diseases, old age, accidents, results of fights betw een anim als, depred­
ations of w olves and bears etc.).
The exam ples given sh ow that the m ales in a bison population have
less v ital force than the fem ales. T hey are subject not on ly to higher
m ortality rate at birth and during the ju ven ile period, but also during
their later free life in the herd. U nder natural conditions it is possible
that behavioural factors also influence the higher m ortality rate in adult
m ales (fighting betw een m ales or leading a lone life aw ay from the
herd), but in relation to m ales in general it is prim arily factors of
a p hysiological nature, connected w ith low er resistance to unfavourable
434
Z. K rasiń sk i & J. Raczyński
conditions, infection s and diseases, w hich exert the greatest influence,
resulting in the life span of m ales being sta tistica lly shorter than that
of fem ales ( Z a b ł o c k i , 1957; J a c z e w s k i , 1958).
It w ould appear that the change in sex ratio in favou r of fem ales in
successive age groups of bison form s a rule of population d evelopm ent
under natural conditions. This does not m ean, how ever, that operations
aim ed at artificial form ation of the sex ratio in favou r of fem ales in
enclosed populations controlled by man serve an y u sefu l purpose.
Consideration should also be given as to w h eth er the use of a v ery
lim ited num ber of m ale reproducers in breeding practice in reserves
serves a u sefu l purpose. The sex ratio in enclosed breeding proposed by
Z a b ł o c k i (1957), of 1 m ale to 10 fem ales, w ould not appear desirable
during the phase of restitu ting the European bison, if on ly for genetic
reasons.
X. SEXUAL BEHAVIOUR
T he behaviour of E uropean bison has not so fa r form ed th e su b ject of a se p arate
study. M any ethological observations re fe rrin g both to th e b eh av io u r of a herd
and of individual anim als a re to be found in m onographic elab o ratio n s of the
species (K a r c o v, 1903; W r ó b l e w s k i ,
1927)i. J a c z e w s k i
(1958) has
devoted a considerable am ount of space to th e b eh av io u r of bison in connection
w ith reproduction.
D uring the rutting period m ales exh ibit special excitem ent. W hile
placid for the greater part of the year, th ey becom e aggressive during
this period, driving other m ales aw ay, and they m ay even attack
hum ans. It is during this tim e that cases occur of calves being injured
or even k illed w hen th ey attem pt to suck their m others. Som e bulls
drive the calves away fairly gen tly »Podbipięta«, w h ile others attack
them violen tly , e. g. »Karpacz«. This latter bull has already killed
several ca lv es in reserve and injured 3. A case of a calf being killed by
a bull w as also recorded in 1961 among free-livin g herd. The b ulls also
work off their aggressiveness and excitem en t in figh ts w ith other males.
In the reserves tw o b ulls livin g in adjacent enclosures often break
through the strong wooden fencing and start figh ts w hich u su ally last
a long tim e, but on ly rarely cause the an tagon ists’ death. In one case
a figh t of this kind ended in the bull »Pokaz« being fa ta lly injured.
A typical m anifestation of the aggressive behaviour of m ales towards
other b ulls is breaking young trees, paw ing the ground w ith their fore
feet and rolling on the ground.
The on ly m anifestations of oestrus in fem ales under norm al circum ­
stances is their allow ing the m ale to cover them , and not v ery marked
sym ptom s of excitem ent. In the experim ental reserve of the M ammals
T he rep ro d u ctio n biology
435
Research In stitu te tw o fem ale bison kept w ithout a m ale during oestrus
kept attem p tin g to jump up on each other. Sim ilar m an ifestations w ere
encountered only tw ice in fem ale bison in the breeding reserves at B ia­
łow ieża and Niepołom ice.
Copulation is preceded by a period w hen the bull »attends« the cow.
For 1— 2 days the m ale never leaves the cow, lick s the latter’s vu lva,
makes m ovem ents sim ulating the jump on the fem ale’s hindquarters,
and w h ile doing so utters the sound term ed »snoring«. It frequently
sm ells the co w ’s external genitalia, stretching and characteristically
wrinkling its nose. The m an ifestations of »attendance« form the best
evid en ce that oestrus is occurring in a fem ale bison. M ost often on the
third day after the first m an ifestations of attendance have been observ­
ed, the fem ale bison allow s the bull to cover it, w hich it u su ally does
once or tw ice that day, then ceases to take any interest in the cow.
J a c z e w s k i (1958) observed 4 such coverings in one day. A fter
copulation the fem ale exh ibits for several hours the so-called »m anifest­
ations of copulation« ( J a c z e w s k i , 1958), expressed by hum ping its
w h ole body and holding its tail out stiffly .
In free-liv in g populations of European bison the start of the rutting
period is signalled by the adult b u lls joining the herds, w hich are
com posed ch iefly of fem ale and you ng anim als ( K r a s i ń s k i , 1967). In
the B iałow ieża Forest this takes place tow ards the end of July. The
strongest m ales then drive off the you ng bulls, w hich u ntil then had
lived w ith the cows, and during the rutting period keep at som e distance
from the herd and do not take part in reproduction. N o such fights
b etw een bulls as w ere freq u en tly described by earlier authors (K a rc o v , 1903; K u l a g i n , 1919) have so far been observed in free-livin g
bison, but the bulls have often been observed to break you ng sprucę,
overturn stum ps etc. which is behaviour often exhibited b y excited
m ales in enclosures. There are no exact data available on the tim e at
w hich the m ales leave th e herds. In reserves in w hich the b u lls often
spend the w h ole year w ith the cow s in one enclosure it w as noticed that
during calving tim e the bull separates from the cows and liv es on its
own.
C alving observed in reserves is preceded by characteristic behaviour
of the fem ale, w hich ceases its v isits to the feed in g place, or only
appears for a short time, a few days before birth takes place. Som etim es
the w h ole herd livin g in the enclosure is absent at feed in g tim e. Accord­
ing to K a r c o v (1903) in natural populations also the fem ale bison
lea v es the herd from 5— 6 days before calvin g and returs w ith its calf
after the birth. The fem ale u su ally calves during the night or early
m orning, and itself eats the placenta ( W r ó b l e w s k i , 1927). The calf
436
Z. K rasiń sk i & J. Raczyński
gets to its fe e t soon after being born and a few hours later is able to
run after its m other. The fem ale never leaves the calf for several w eeks,
and even in cases in w hich the calf died the m other stood over it for up
to 3 days and allow ed no-one to approach. The fem ale does not com e to
the feeding place for the first few days after calving, although in a few
cases this took place on the day after calving. If there is little natural
food in the enclosure th e fem ale leaves its calf hidden in the forest in
the interior of the enclosure and itself approaches the feeding place on ly
in order to seize a little food, and then at once returns to its calf.
There are no exact data available on the duration of lactation and
suckling in the fem ale bison. The calf continues to suck its m other for
a long time, even w hen it has begun feed in g independently. W r ó b l e w ­
s k i’s observations (1927) are confirm ed, both in reserves and in freelivin g populations, that w ellgrow n anim als born the previous year
readily suck the m other side by side w ith young calves.
X I. DISCU SSIO N
At the present tim e, w hen it is as y e t too early to cross the European
bison off the list of species threatened w ith extin ction , one of the ch ief
tasks of its protection is the rational breeding of this anim al, enabling
m axim um advantage to be taken of its biological capacities w ith the
m inim um disturbance on the part of man. This condition is p articularly
d ifficu lt to m eet w ith regard to a process of such fundam ental im port­
ance to building up the head of bison as is reproduction. M any p heno­
m ena m ay h ave an influence on reproduction which is not alw ays easy
to identify, beginning w ith abiotic habitat factors, then food conditions,
and ending w ith influences connected w ith behaviour properties. The
exp erience gained over a long period of breeding has supplied know ledge
on m any basic factors conditioning in ten siv ity of reproduction in the
European bison and q uan titative increase in the head of these anim als.
A p articularly valuab le source supplem enting this k now ledge is form ed
by observations under natural conditions. The state of k now ledge w e
now possess on reproduction am ong bison in enclosed breeding reserves,
com bined w ith the inform ation contained in older studies on natural
populations and the first results of keeping free-liv in g herds in the B ia­
łow ieża Forest, enable a critical review to be m ade of the w hole group
of natural and artificial factors w hich affect the reproduction and popu l­
ation increase of the European bison.
A ccording to Z a b ł o c k i (1949) under good conditions a bison p op u l­
ation is characterised by: (1) a d istin ctly defined and correctly tim ed
calving season, (2) considerable regularity of annual calving in fem ales,
The reproduction biology
437
(3) low m ortality rate am ong young anim als. There are m any proofs
available that deviations of any one of these indices signals the existence
of phenom ena unfavourable to the population, w h ile their rem oval
brings about visible im provem ent.
1. H a b i t a t f a c t o r s . Clim atic factors, according to Z a b ł o c k i
(1949), m ay be responsible for the differences in tim e of occurence of
oestrus in bison betw een the Polish reserves and the Askania Nova
reserve and Sw edish A vesta. Local clim atic conditions, in addition to
specific breeding conditions, m ost certainly form the cause of fem ales
calving at various tim es over the w hole year in differen t zoological
gardens over the world. The clim atic factor does not how ever p lay any
perceptible part in a region covering the original natural range of the
European bison.
W here European bison are kept in enclosures,
the size of
the enclosure
is im portant. Enclosures w hich are too sm all, such as those in w hich
bison w ere first kept at Pszczyna and B iałow ieża, have an unfavourable
effect on the course of reproduction. These sym ptom s disappeared after
increasing the area of these reserves ( Z a b ł o c k i , 1949). It w ould seem
that the food factor played an im portant part in this case. Crowding,
how ever, tends to increase the num ber of cases of young anim als being
killed by old ones, such as occurred, e. g. in the Sw edish reserve A vesta
( Z a b ł o c k i , 1957).
The direct influence of the habitat m ay be expressed in increased
m ortality am ong young and adult anim als as the result of accidents.
W r ó b l e w s k i (1927) describes num erous cases of the death or serious
injury of European bison in th e B iałow ieża Forest due to fallin g trees,
to becom ing entangled in uprooted trees, drowning in the mud of river
v a lley s etc.
2. F o o d . The food factor m an ifests its action very early in bison
breeding. The reason for this is the fact that the natural habitat of the
bison has undergone v ery great changes as the result of hum an m ana­
gem ent. Under natural conditions the forest provided the bison with
su fficien t food supplies over the fu ll y early cycle. Y oung shoots,
branches and bark form on an average h alf the food eaten by bison
( Z a b ł o c k i , 1957; K o r o ć k i n a , 1966). There can be no doubt that
during sn ow y w inters such as often occur at Białow ieża, this com ponent
m ust have alm ost com pletely m et the food requirem ents of the bison,
w h ile in sum m er it w as supplem ented by su ccu len t fodder.
During the tim e w hen the first scientific elaborations appeared the
bison in the B iałow ieża Forest encountered conditions w hich had already
been g reatly transform ed b y man. In the second h alf of the 19th century,
as the result of inten sive artificial breeding in the Forest of red deer,
438
Z. K rasiński & J. Raczyński
fallow deer and elk, the livin g conditions of bison m arkedly deteriorated.
The particularly inten sive reproduction of red deer led to alm ost
com plete exhaustion of the bison’s natural food supply at the tu rn ing
point of the 19th century. According to W r ó b l e w s k i (I. c.) this had
a distinct effect on the developm ent, reproduction and condition of the
bison, m anifested by, i nt er alia: late sexu al m aturation, low calvin g
rate and increased m ortality am ong the young anim als, w hich radically
reduced the d evelop m en t rate of the population. Inform ation on the first
calving taking place w hen the fem ales w ere 5— 6 years old ( B r i n e k e n, 1826; K u l a g i n , 1919) w ith subsequent calvin g occurring on ly
ev ery 2— 3 years com e from this period. Fem ale bison kept at that tim e
in special enclosures w ith the aim of supplem enting the natural p o p u l­
ation, and fed w ith h ay and oats, calved alm ost every year (S t e c h o w,
1929).
The conditions of contem porary enclosure breeding of bison, ensuring
high reproductive rate, are based on su pp lem entary feeding. The anim als
are given highcalorie fodder throughout the w h ole year (oats, barley)
and h ay and root crops in w inter. During th e grow ing season natural
plant food grow ing in th e forest area is the basic com ponent. In e x ­
ten siv e enclosures the bison feed in the interior for the greater part
of the day in sum m er, and often do not appear at the feeding places
for days on end. The part played by artificial feeding does not becom e
im portant until the critical period, and then form s a significant factor
d ifferen tiating the livin g conditions of the enclosed herd from those
of natural populations.
A free-liv in g bison population is able to make use of the natural food
su pp ly in the Forest throughout the w h ole year, but in fact during the
tim e that there is a deep snow cover the anim als gather round the feed ­
ing places and do not leave them u n til the sum m er grow ing season
( K r a s i ń s k i , 1967). Such conditions have proved favourable to the
bison population, as is expressed in the high birth rate and population
increase w hich does not differ from the param eters characteristic of the
herds enclosures. It m ust be em phasised that the present food supply
in the Forest m eets the bison’s requirem ents better than during the
period of m axim um d en sity of rum inants at the beginning of the 20th
century ( B o r o w s k i ,
Krasiński & Miłkowski.
1967). The
fundam ental difference, how ever, con sists in the elim ination of the effect
of hunger, w hich w as m ost certain ly a normal phenom enon in the early
spring. According to J a c z e w s k i (I. c.) the »hunger period« p resum ­
ab ly lasted about tw o m onths and form ed an im portant physiological
m om ent in the annual periodicity of the bison’s life, There is no doubt
T he reproduction biology
439
that the deterrioration in livin g conditions caused considerable m ortality
and w eakened the anim als in som e years. This regulating effect of
natural selection has now been com pletely elim inated by supplem entary
feeding, but the anim als’ good conditions over the w h ole year is to a great
ex ten t the result of th eir using the natural food supply. The ab ility of
the bison to m ove about the Forest, com bined w ith the considerable
biotopic d ifferen ces in their environm ent, enables them to m ake a free
choice of feed in g place, depending on their food preferences.
It m ust be stated th at food form s the basic lim iting factor of a bison
population. Their ab ility to feed in the forest determ ines the condition
of anim als bred in enclosures and is one of the reasons w hy the breeding
of free-liv in g bison in th e B iałow ieża Forest has proved so successful.
3.
P o p u l a t i o n f a c t o r s . The group of factors connected w ith
the bison population structure plays an im portant part in regulating the
processes of herd organisation and increase under natural conditions. The
exp erience obtained in breeding free-livin g bison over the recent period
points to the n ecessity for taking these factors into consideration.
The present bison population in the BP Forest is to a very great exten t
an artificial population. Its age and sex com position is the result of
gradually setting anim als free from the breeding reserves, the sex and
age proportions being in principle en tirely accidental. During their free
reproduction, conditions favouring the form ation of the natural organ­
isation of herds w ere artificially disturbed by releasing further anim als.
D espite th ese disadvantages, signicifant from the aspect of population
studies, the free-livin g herd in its present form provides interesting
m aterial for an alysis of reproduction and population increase.
D ifferences in reproduction betw een free-livin g bison and those kept
in reserves consist ch iefly in the form er’s selfregulation of the seasonal
rhythm of sexu al activity. In our opinion the m ain cause of the return
to the natural rutting tim e is the effec t of intrapopulation factors. Herd
organisation sim ilar to the natural one, grouping the cow s together w ith
you ng anim als and separation of active m ales, w ith the b ulls joining
the fem ales at the b eginning of the rutting season, m ost certainly
con stitu te a factor evenin g up in ten sivity of oestrus in the w h ole popul­
ation. The shortening of the rutting period observed is due to the greater
effectiv en ess of copulations. The influence of population factors on this
phenom enon m ay be analogical to that observed in some dom esticated
sp ecies, x. e. increased percentage of fertilisations w hen coverings take
place in the herd in com parison w ith controlled copulation such as in the
case of horses ( B i e l a ń s k i , 1962).
The high calving rate am ong free-livin g anim als proves that w hen
lim itin g factors are absent the reproductive potential of the European
440
Z. K rasiński & J. Raczyński
bison under sem i-natural conditions is the sam e as in the enclosures. The
excep tionally high lev el of population increase among free-liv in g anim als
is how ever undoubtedly a transitory phenom enon due to the curren tly
existin g age structure. During the last few years annual increases
flu ctu ated in free-liv in g herd w ithin lim its of 18— 33%, the average
being 20%. Comparison w ith the rate of population increase am ong bison
in the B iałow ieża Forest at the end of the 19th century (after S e v e r c o v , 1941 — from 2.7 to 4.4%) or during the period of trial and error
with breeding bison in reserves (6.4% after Z a b ł o c k i , 1957) shows
how exceptional a phenom enon this is.
W hen the free d evelopm ent of the herd proceeds w ith ou t intervention
it m ay be expected th at the natural age structure of population w ill be
attained in the future. This w ill m ake it possible for the population
developm ent rate to becom e stabilised and the progressive phase of po­
pulation developm ent w ill be at an end. It is therefore obvious that
acceptance of the present indices as a basis for forecasting free-livin g
herd increase is u njustified. It w ould appear to be one of the most
im portant problem s for the future restitution of the European bison by
m ethods of keeping a free-livin g population to establish the indices of
population increase depending on its structure and to d efin e population
potential under natural conditions.
4. G e n e t i c f a c t o r s . The detailed analysis w e m ade of the fe rti­
lity of fem ale bison in breeding reserves made it possible to establish,
that there are individuals distinguished by reduced fertility. B y analogy
w ith the relations observed in dom estic anim als it w ould seem that at
least som e of sim ilar cases are due to genetic factors. This is the more
probable in relation to the European bison in w hich the present head
w as obtained from the initial herd num bering, according to the estim ates
m ade by the International S ociety for the Protection of the European
bison, about 40 anim als capable of reproduction. Further breeding carried
out ind ep en d en tly in several centres took place under conditions of
inten sive inbreeding. M ating of closely related anim als takes place at
the present tim e also in breeding reserves. In such circum stances it is
on ly lethal characters which are elim inated by the death of the young.
Others, such as reduced fertility, can only be grad u ally elim inated in
natural populations subject to the factors of natural selection. As
increased fe rtility of fem ale bison is an im portant factor facilitatin g
quicker successes in the restitution of the European bison, the question
rises as to w h eth er certain operations of selection for fe r tility should
not be carried out in breeding practice. With the present head of bison
such operations should be lim ited to elim inating from reproduction cows
w ith tendencies to irregular calving, abortions or dropping calves with
T he repro d u ctio n biology
441
low vita lity . In our opinion there is no need to fear that this operation
would produce undesirable sid e -effec ts to the species, since it is a q uest­
ion here of the general property of biological vitality, and not of
selection aim ed at any one defined character. The only places in which
breeding work of this kind could be carried out w ith the aim of im prov­
ing n a ta lity in the herd are enclosed breeding centres. E stim ates should
be based on detailed docum entation including above all the w hole of the
fem ale bison, arranged from the aspect of purposely chosen criteria for
estim ating fertility. The opportunity of selecting by this m eans individ­
uals m ost valuable from the standpoint of population increase should be
used, particularly in fu rther attem p ts at breeding free-livin g bison.
X II. CONCLUSIONS
A t the present stage of restitution of the European bison the chief
aim of breeding is to ach ieve rapid increase in the head of this species in
the world. From this aspect research on reproduction of European bison
under the conditions form ed by the present m ethods of enclosed and
free-liv in g breeding supplied m aterial perm itting of reaching the follow ­
ing conclusions:
1. Enclosed breeding of bison, based on m ethods elaborated over years
of experim entation, now ensures that the reproductive potential of the
species is u tilized to a v ery great exten t. The first results of free-livin g
breeding in the B iałow ieża Forest prove that the fe rtility of the herd
does not change in a natural habitat w hen the anim als are given
in ten sive supplem entary feed in g in the w inter, but that a tendency,
favourable from the standpoint of this sp ecies’ biology, appears to re­
creation of the natural seasonal rhythm of reproduction. This m akes the
free-liv in g breeding of bison the m ethod of choice, as being better
ju stified from the aspect of the bison’s biology.
2. The results obtained so far from breeding bison in enclosures and
in free-liv in g herds, and observations of natural populations, ju stify the
statem en t that the basic factor lim iting bison’s fe rtility is food.
3. An im portant factor regulatin g th e reproduction dynam ics of fr ee­
-liv in g bison herds is the natural course of population processes e x ­
pressed by the spontaneous form ation of age, sex and social structure of
herds. This indicates that hum an interference in the free developm ent
of free-liv in g bison herds should be lim ited as far as possible.
4. It w ould appear desirable w ith the present head of bison to
introduce the principles of elim ination of individuals w ith low er vitality
and fe rtility in enclosed breeding centres, in order to increase the re­
productive potential of the species and im prove breeding m aterial.
442
Z. K rasiński & J. Raczyński
5.
F ree-livin g populations of European bison should alw ays be
m aintained, by releasing on ly p erfect you ng anim als in sex-ratio 1:1 and
ensuring that the natural d evelopm ent of these populations continues is
an im portant factor of further research on the biology and ecology of
such populations.
REFERENCES
1. A s d e l l S. A., 1946: P a tte rn s of m am m alian rep ro d u ctio n . Com stock P ublish
Co.: 1—437. Ithaca.
2. B a s k i r o v l . , 1939: K avkazkij zubr. In ,,K avkazkij zu b r”. G lav n o je u p rav le nie po zapovednikam , zooparkam i zoosadam : 3— 72. M oskva.
3. B i e l a ń s k i W., 1962: Rozród zw ierząt gospodarskich. P ań stw . Wyd. Roln.
Leśne: 1—546. W arszaw a.
4. B o r o w s k i S., K r a s i ń s k i Z. & M i ł k o w s k i L., 1967: Food and role
of the E uropean bison in forest ecosystem s. A cta th erio l., 12, 25: 367—376.
5. B r i n e k e n J., 1826: M ém oire d escrip tif su r la fo rêt im p é riale de B iałow ieża
en L ithuanie. Im p rim e u r-L ib ra rie de L ’u n iv e rsité R oyale: 1—127. V arsovie.
6. G i l l J., 1967: T he physiological p ro p ertie s of th e E u ro p ean bison. A cta
theriol., 12, 23: 349—360.
7. H e p t n e r V. G., N a s i m o v i c A. A. & B a n n i k o v A. G., 1961: M lekopitaju śće Sovetskogo Sojuza. I. P arn o k cp y tn y e i neparn o k o p y tn y e. Gos. Izd.
„V yssaja śkola” : 1—776: M oskva.
8. J a c z e w s k i Z., 1958: R eproduction of th e E uro p ean bison, Bison bonasus
(L.), in reserves. A cta theriol., 1, 9: 333—376.
9. K a l u g i n C. G., 1958: Z ubry v estestvennyh uslo v jah K avkazkogo zapovedn ika. T r. K avkazsk. gos. zap., 4: M ajkop.
10. K a r c o v G., 1903: B elovezskaja P uśca. A rtist. Z aved. A. F. M ark sa: 1—414.
P etersburg.
11. K o r o ć k i n a L. N., 1966: K voprosu o znaéenii drevesnoj ra stite ln o sti v pitan ii zubrov Belovezskoj Puséi. Vesci AN BSRR, 1: 106—111.
12. K r a s i ń s k a M^ 1963: W eitere U ntersuchun g en über K reu zu n g en des W i­
sents, Bison bonasus ( L i n n a e u s , 1758) m it dem H au srin d , Bos ta u ru s dom.,
L i n n a e u s , 1758. A cta theriol., 7, 14: 301—310.
13. K r a s i ń s k i Z., 1963: O żubrach na w olności w Puszczy B iałow ieskiej. Chr.
P rzyr. Ojcz., 19, 5: 3—7. K raków .
14. K r a s i ń s k i Z., 1967: F ree living E u ropean bisons. A cta theriol., 12, 29: 391—
405.
15. K r y s i a k K., 1967: T he history of th e E uropean bison in th e B iałow ieża forest
and th e resu lts of its protection. A cta theriol., 12, 19: 323— 331.
16. K u l a g i n N. M., 1919: Z ubry Belovezskoj Puśći. Izd Mosk. N aucn. Inst.: 1—
166. M oskva.
17. M o h r E., 1952: D er W isent. Die n eue B rehm -B iicherei. 74: 3—75. Leipzig.
18. N a 1 b a n d o v A. V., 1958: R eproductive physiology. C o m p arativ e rep ro d u ctiv e
physiology of dom estic anim als, la b o ra to ry anim als, and m an. W. H. F reem an
and Co.: 1—271. San Francisco.
19. P edigree book of th e E uropean bison 1955— 1961.In tern atio n al Society for th e
Protection of th e E uropean bison. W arszaw a.
20. P f i z e n m a y e r E. W., 1929: B iologische und m orphologische N otizen ü b er
den K aukasusw isent. Abh. m a th .-n atu rw iss. Abt. B ayer. A kad. W iss. 11, B eitr.
443
Biologia rozrodu żubra
21.
22.
23.
24.
25.
26.
27.
28.
29.
zu N at.- u. K u ltu rg esch ich te L ith au en s u. angrenz. G ebiete: 497—504. M ün­
chen.
S e v e r c o v S. A.: 1940: V idovye k onstanty razm nożenija belovezskogo zubra
i dinam ika naselenija etogo vida. T r. Inst, evoljuc. m orfol. im. A. N. S evercova, 3, 1: 3—32.
S e v e r c o v S. A., 1941: D inam ika neselenija i p risp o so b iteln aja evoljucija
zivotnyh. Izd. AN SSSR: 1—316. M oskva—L eningrad.
S t e c h o w E., 1929: U ber die einstige H ege des W isents im U rw ald e von Bialowies. Abh. m ath .-n atu rw iss. A bt. B ayer. A kad. Wiss. Suppl., 12, B eitr. zur
N at.- u. K ulturgeschichte L ithauens u. angrenzender G ebiete: 505—507. M ün­
chen.
S t u d i e n c o w A. P., 1956: Położnictw o i ginekologia w ete ry n ary jn a. P ań stw .
W yd. Roln. Leśne, 1—703. W arszaw a.
W r ó b l e w s k i K,, 1927: Ż ubr Puszczy B iałow ieskiej. W yd. pol., 1—234+X V I.
Poznań.
Z a b ł o c k a j a L. V., 1957: P ita n ie i estestven n y e korm a zubrov. Tr. P rio k sk o-T errasn . gos. Zap., 1: 66—143. M oskva.
Z a b ł o c k i M. A., 1939: T rid c a t’ p ja t’ let razv ed en ija zubrov i ih gibridov
v zooparke A skanija-N ova. In „K avkazkij zu b r”. G lavnoje u p rav le n ije po zapovednikam zooparkam i zoosadam : 73—137. M oskva.
Z a b ł o c k i M. A., 1949: N eobhodim ost' izućenija osobennostej zubra i ego
vosstanovlenie v SSSR. N auc.-M etod. Z apiski, 13: 128—146. M oskva.
Z a b ł o c k i M. A., 1957: N ekotorye biologićeskie osobennosti zubra i ih izm enenie v uslovijah zagonnogo soderżanija. Tr. P rio k sk o -T errasn . gos. Zap., 1:
5—65. M oskva.
R eceived, M ay 30, 1967.
Białow ieża N ational P a rk
Białow ieża, Poland.
and
Polish A cadem y of Sciences,
M am m als R esearch In stitu te,
B iałow ieża, P oland.
Zbigniew K R A SIŃ SK I i Ja n RACZYŃSKI
BIO LOG IA ROZRODU ŻUBRA W REZERW ATACH I NA WOLNOŚCI
S treszczenie
A utorzy w ykorzystali do bad ań urzędow ą doku m en tację k rajo w y ch O środków
Hodow li Ż u b ra w Białow ieży, Niepołomicach,, Pszczynie i w B orkach za okres
1954— 1965, biorąc pod uw agę term in y pokryć, w ycieleń i płeć potom stw a oraz ob­
serw acje dotyczące innych przejaw ów aktyw ności płciow ej k ró w i byków , z a w ar­
te w n o ta tk ac h hodow lanych. Rozród żubrów na sw obodzie badano analizując w
kolejnych latach sezonowość w ycieleń, p rzy ro sty i rozw ój wolno bytującego na te ­
ren ie Puszczy B iałow ieskiej sta d a w okresie 1958—1966.
Stw ierdzono, że sezon godow y u żubrów w rez erw atach rozciąga się od lipca do
m arca, zaś na m iesiące kw iecień, m aj i czerw iec przy p ad a u sam ic okres anoestrus.
W iększość pokryć zachodzi w okresie: sierp ień — październik (Tabela 1). Sezon
w ycieleń rozciąga się odpow iednio od k w ietn ia do g ru d n ia w rezerw atach i od
m aja do w rześnia (sporadycznie tylko obejm u je m iesiące listopad i grudzień) na
sw obodzie (Tab. 2). W iększość w ycieleń m a m iejsce w m aju, czerw cu i lipcu, za­
rów no w rez erw a tac h (70%), ja k i na w olności (80%) — (Ryc. 1). Czas trw a n ia cią­
ży u żubra ustalono na 254—272 dni, średnio 264,3 dnia (Ryc. 3).
444
Z. K rasiński & J. R aczyński
Z ubrzyce d ojrzew ają płciow o najczęściej w trzecim roku życia. P ierw sze w y ­
cielenie dokonuje się zw ykle w czw artym roku życia (54°/o)i, średnio dla b ad an ej
p opulacji rezerw atow ej w w ieku 33 m iesięcy (Tab. 3). W piątym kalendarzow ym
ro k u życia w szystkie sam ice m ogą być już tra k to w a n e jako zdolne do rozrodu
(Tab. 4). P otencjał rozrodczy żubrzyc w chw ili p rzy stąp ien ia do rozrodu nie ulega
w iększym zm ianom aż do zakończenia okresu płodności (Ryc. 4). O bjaw y ru i u żu ­
brzyc w idoczne są zw ykle w ciągu 1—3 dni. D ługość cyklu płciow ego w ah a się
najczęściej w zakresie od 18 do 22 dni, średnio trw a 20,6 dnia (Ryc. 2). W znow ienie
cyklu płciowego po w ycieleniu zachodzi u w iększości żubrzyc w okresie do trzech
m iesięcy: najw cześniejsze skuteczne pokrycie zarejestro w an o w 13 dni po w ycie­
leniu. O kres m iędzyw ycieleniow y w ynosi u n ajb ard zie j płodnych kró w 10—13 m ie ­
sięcy, przeciętnie 14,4 m iesiąca (Tab. 5).
P rzeprow adzone szczegółową analizę płodności poszczególnych żubrzyc z re z e r­
w atów hodow lanych, uw zględniając czas osiągnięcia dojrzałości płciow ej, fre k w e n ­
cję w ycieleń w kolejnych latach oraz sezonową reg u larn o ść porodów. Pozw oliło to
na w ytypow anie grupy n ajb ard zie j płodnych krów (w czesna dojrzałość, śred n i okres
m iędzyw ycieleniow y 10—13 m iesięcy, zdrow e potom stw o) oraz w yróżnienie żubrzyc
o obniżonej płodności (częste okresy jałow ienia, poronienia, rodzenie m a rtw y c h lub
m ało żyw otnych cieląt) — (Tab. 5). W korzystnych w aru n k ac h rezerw atow ych ro ­
dzi corocznie 63—91°/o żubrzyc zdolnych do rozrodu. P rzejściow a jałow ość m a zw ią­
zek z term inem ostatniego w ycielenia, padnięciem cielęcia w okresie k arm ien ia oraz
w ynika z zespołu innych, nieznanych przyczyn. Istn ie je możliwość, że u nisko p ło d ­
nych żubrzyc zjaw isko to m a c h a ra k te r dziedziczny. P rz y jm u ją c w y n ik ający z d łu ­
gości życia 10-cio letni okres płodności żubrzycy, je d n a sam ica w rezerw acie daje
w ciągu życia przeciętnie 9 cieląt.
Płodność sta d a wolnego, w yrażona procentem żubrzyc w ycielonych w stosunku
do ogółu zdolnych do rozrodu w ynosi średnio 70,3% i nic różni się isto tn ie od p łod­
ności żubrzyc w rez erw a tac h zam kniętych 76,8% (Tab. 7). W ysokie przy ro sty ilo ­
ściowe wolnego sta d a (13—^33% rocznie, średnio 19,9%) są w ynikiem dużego p rzy ­
rostu m łodych (średnio 22,4% rocznie)i oraz m inim aln ej śm iertelności cieląt (2,3%).
Intensyw ny rozw ój sta d a na sw obodzie tłum aczy się rów nież istn iejącą ak tu aln ie
- sztuczną s tru k tu rą w iekow ą i płciow ą stada.
S tosunek płci przy urodzeniu nie odbiega od pro p o rcji 1:1 (n = 249).
W celu uchw ycenia czynników , od k tórych zależy intensyw ność i praw idłow y
przebieg rozrodu żubrów przeprow adzono porów naw czą analizę rozrodu stad żu ­
brzych w zależności od m etody (zam knięta, półw olna) i ogólnych w aru n k ó w hodo­
wli. S tw ierdzono ograniczający w pływ czynnika pokarm ow ego, zarów no od strony
zapotrzebow ania ilościowego, jak i składu jakościow ego (udział sk ład n ik ó w n a tu ­
ralnych) oraz reg u lu jące działanie stosunków w ew n ątrzp o p u lacy jn y ch , zw iązanych
z istnieniem w w olnych populacjach n a tu ra ln e j stru k tu ry socjalnej stad.
Z p u n k tu w idzenia postępów re sty tu c ji żubra au to rzy oceniają pozytyw nie m e­
todę w olnej hodow li, k tó ra w korzystnym środow isku i przy ak ty w n ej opiece czło­
w ieka zapew nia w ysoką płodność stad a i rek o n stru k c ję n atu ra ln eg o ry tm u sezo­
nowego. Ze w zględu na doniosłe znaczenie poznaw cze prow ad zen ie hodow li tą m e­
todą w inno odpow iadać zasadom eksperym entu naukow ego przy zachow aniu w a ­
ru n k ó w dla sw obodnego w zrostu populacji. O środki zam kniętej hodow li żubrów
są terenem , gdzie m ożna prow adzić p race w k ieru n k u zw iększenia poten cjału ro z­
rodczego stad a hodow lanego i elim inow ania szkodliw ych skutków ■w ieloletniego
chow u w sobnego poprzez szczegółową kontrolę i stosow anie zasad selekcji w sto ­
sunku do osobników przenoszących cechy obniżonej żyw otności i płodności.