SUCKLING BEHAVIOR OF PRAIRIE VOLES
(MICROTUS OCHROGASTER)
BETTY MCGUIRE
Department of Biological Sciences, Smith College, Northampton, MA 01063
In some mammals, young exhibit obvious preferences for anterior or posterior teats. In
addition, individuals within litters may suckle consistently from the same teat or pair of
teats-patterns of behavior termed teat fidelity and teat-pair fidelity, respectively. I examined if young prairie voles (Microtus ochrogaster) preferred particular locations of teats
and exhibited fidelity to a particular teat or teat-pair. I marked individual young and scored
location of teats suckled every other day from day 4 to day 16 postpartum. Young preferred
the hind pair of teats over the middle teats and least preferred the front teats. Fidelity in
choice of teat and teat-pair varied with litter size, being more pronounced in small litters.
Advantages of suckling from hind teats are unknown. Enhanced teat fidelity and .teat-pair
fidelity in small litters probably reflect reduced competition for preferred locations of teats
and the need for consistent stimulation of a teat to ensure adequate milk production.
Key words:
Microtus ochrogaster, prairie voles, suckling, teat preference, teat fidelity
bank, 1964; Felis silvestris-Ewer, 1959;
Macacafuscata-Ota et al" 1991; Tanaka,
1989; Macaca mulatta-Deets and Harlow,
1970; Pan troglodytes-Van-Lawick Goodall, 1967). In species with multiple young,
fidelity in choice of teat or teat-pair generally has been interpreted as a mechanism to
reduce fighting among littennates and to
hasten attachment to teats (Ewer, 1959;
Hartsock and Graves, 1976; McBride,
1963).
Fidelity in choice of teat or teat-pair has
not been reported, to my know ledge, for
any rodent. However, young of some species in the family Muridae exhibit tenacious
nipple attachment-a behavior most recently interpreted as having evolved in rodents
in response to high levels of sibling competition for teats and milk (Gilbert, 1995).
Whether fidelity in choice of teat or teatpair occurs in addition to tenacious nipple
attachment in these species is unknown.
Young prairie voles (Microtus ochrogaster) display tenacious nipple attachment
(Kruckenberg et al.,1973; Salo et al., 1994;
Thomas and Birney, 1979). Early observations of field-caught lactating females also
In some mammals, young prefer to suckIe from particular locations of teats; some
species prefer anterior teats while others
prefer posterior teats. For example, young
pigs (Sus sero/a-Fraser and Thompson,
1986; Hartsock et al., 1977; Wyeth and
McBride, 1964) and young Norway rats
(Rattus norvegieus~Tsai, 1931) prefer to
suckle anterior teats, but young cats (Felis
silvestris) prefer posterior teats (Ewer,
1959). Such preferences could reflect differences among teats in yield or composition of milk, ease of colostrum extraction,
accessibility, likelihood of being dislodged
or trampled, or proximity to the mother's
calls (Donald, 1937; Ewer, 1959; Fraser and
Lin, 1984; Hafez and Signoret, 1969; Jeppesen, 1982; McBride, 1963; Tsai, 1931).
In most cases, however, position preferences remain unexplained.
In addition to anterior or posterior preferences, individuals within litters sometimes suckle the same teat or pair of teatsphenomena known as teat fidelity and teatpair fidelity, respectively (Sus sero/aDonald, 1937; Hemsworth et al., 1976; Wyeth and McBride, 1964; Ovis aries-EwJournal of Mamma[ogy, 79(4):1184-1190, 1998
1184
Novemb er 1998
McGUI RE-SUC KLING BEHAV IOR OF VOLES
indicat ed that posteri or teats were used
more frequen tly than anterio r teats (Fitch,
1957; Jameso n, 1947), sugges ting that
young of this species also display positio n
prefere nces related to sucklin g. Howev er,
no further analysi s of prefere nce for particular teat locatio ns or any assessm ent of teat
fidelity or teat-pa ir fidelity is availab le for
prairie voles.
I examin ed if individ ual prairie voles exhibit prefere nce for a particu lar locatio n of
teat or teat or teat-pa ir fidelity . Measu res of
prefere nce and fidelity were studied relative
to litter size. Oi ven observ ations of lactating prairie voles by Fitch (1957) and Jamesl?n (1947) , I predict ed that young would
prefer posteri or teats to anterio r teats and
that anterio r teats would be used by only
young in large litters. Further , becaus e competitio n for teats and milk is probab ly high
in prairie voles, as sugges ted by the tenacious clingin g to teats by young, I predict ed
that young would show fidelity to a teat and
teat-pair. Finally, I predicted that teat fidel-
ity and teat-pa ir fidelity would be greater in
small litters of prairie voles; this pattern has
been reporte d for pigs, the only species for
which data present ly are availab le (Donal d,
1937; Hemsw orth et a1., 1976; Winfie ld et
aI., 1974).
MATER IALS AND METHO DS
Prairie voles were descend ants of animals
trapped in 1987 and 1994 in Illinois. Voles were
housed as breedin g pairs with young in plastic
tubs (47 by 25 by 20 cm) that contain ed a substrate of peat moss and pine shaving s and a covering of hay. Rabbit chow, oats, sunflow er seeds,
cracked com, and water were provide d ad lib.
and apples and lettuce were supplie d at least
twice a week. The light cycle was 15L:9D
(lights on at 0600 h), and the ambien t tempera ture was ca. 20"C.
I checked daily for litters and recorde d date
of birth and litter size. When young were 4 days
old, I marked each on the dorsal surface with
black Nyanzo l dye. Data collecti on began on
day 4 of life and continued every other day until
day 16 postpar tum (prairie vole young begin to
ingest solid food at 14 days of age and stop
IIB5
sucklin g at ca. 20 days of age-M cGuire and
NOVak, 1984). On each day of data collection, 1
gently picked up the mother and litter and scored
the particul ar teat to which each pup was attached. Prairie voles have six teats arrange d in
three pairs; one pair is pectora l and the remaining two pairs are abdomi nal. Moving from anterior to posterio r on the female, I considered
the first pair of teats the front pair, the next pair
of teats the middle pair, and the last pair of teats
the hind pair. I scored the teat-pai r and side (e.g.,
hind left) for each attached young. Young that
were not attached to a teat were scored as "off."
Data were collecte d from March 1995 through
August 1997 and represent observa tions on 45
litters.
To detenni ne if prairie- vole young display a
prefere nce for anterior or posterio r teats, 1 calculated the number of checks out of a possible
seven that each teat-pair was occupie d. I considered a teat-pai r occupied when a young prairie
vole was attached to at least one of the teats of
the pair. To examin e how pattern of occupation
change d over time, I plotted the proport ion of
litters using each teat-pair in relation to age of
the young; I used a X2 analysis to compar e such
proport ions on day 16 for litters with five or
more young.
To address the question of whethe r prairie
vole young show fidelity in their choice of teat
or teat-pair, I measured teat fidelity (proportion
of checks in which a young vole used the teat
on which it was most commo nly seen), teat-pair
fidelity (propor tion of checks in which a young
vole used the teat-pai r on which it was most
commo nly seen), and teat-pai r consiste ncy. Teatpair consiste ncy was describe d by Van Loen and
Molena ar (1967) as C; = l:K;/I[! K;jF where C,
was the consiste ncy score for the ith young and
ith young
Kij was the frequen cy with which the
teat-pair
both
e
Becaus
.
teat-pair
suckled the jth
in the
appear
ncy
consiste
r
teat-pai
and
fidelity
literatu re as measures of the frequen cy with
which young pigs return to the same teat-pair, I
used both measure s here.
I grouped litters of 1-2 young (n = 8), 3-4
young (n = IS), 5-6 young (n = 17), and 7-8
young (n = 5). Of the 45 litters, 15 were born
to primipa rous females and 30 to multiparous
females . As reported for wild-ca ught female
prairie voles (Rose and Gaines, 1978), litter
sizes of primipa rous and multipa rous females
were similar. For example, proport ions of litters
JOURNAL OF MAMMALOGY
1186
Vol. 79. No.4
TABLE I.-Use oftea~-pairs in relation to litter size in prairie voles. Values represent mean number
of checks out of a possIble seven x SE. Checks occurred on alternate days between days 4 and 16
of age.
Litter size
Teat-pair
Front
Middle
Hind
5-6
I 2
3 4
(n = 8)
(n = 15)
(n -
0.0 + 0.0
0.0::'::: 0.0
6.2 ::'::: 0.3
0.1 + 0.1
5.0 ::'::: 0.3
6.6 ;. 0.1
2.5 ~ 0.4
5.8 ~ 0.2
6.4 + 0.2
born to primiparous females did not differ
among categories of litter size (1-2 young,
37.5%; 3-4 young, 40.0%; 5-6 young, 35.3%;
7-8 young, 0%; X2 = 2.89, df. = 3, P = 0.41).
1 used mean values per litter for all measures of
fidelity and consistency, and analyzed data using
ANOVA with litter size as the between-subjects
variable.
RESULTS
Of the 198 young born during the study,
188 (95%) survived to weaning. Mortality
of young occurred in six of the 45 litters
and tended to be associated with large litter
size; 22.7% (5122) of litters with more than
four young experienced mortality compared
with 4.3% (1123) of litters with four or fewer young (X' ~ 1.89, dJ. ~ I, P ~ 0.17).
Four of the 10 deaths occurred after day 4;
data from those young were not included in
analyses.
Prairie-vole young preferred hind teats to
middle teats which they in turn preferred to
front teats (Table 1). More specifically,
young in litters of one or two always occupied the hind pair of teats. In litters of
three or four young, the hind teat-pair was
occupied somewhat more than was the middle teat-pair, and the front teat-pair was almost never occupied (I observed occupation of the front teats for one litter of four
during three of seven checks). In litters of
five or six young, a situation in which routine occupation of the front teats would be
expected if all pups were to attach at the
same time, the front teats were occupied
less frequently than the other pairs. In four
of the 17 litters in this category of litter
17)
7-8
(n
5)
5.6 ~ 0.2
6.4::'::: 0.2
6.8 ;. 0.2
size, young were never observed attached
to the front teats; in the remaining 13 litters,
occupation of the front teats was observed
sporadically and usually on about three of
seven checks (range, from one to five
checks). In summary, it was not uncommon
for some individuals in litters with five or
six young to remain unattached to a teat
rather than to attach to a front teat. Finally,
in litters of seven and eight young, a situation in which there are more young than
teats, I observed the front teats to be occupied with a frequency similar to that of
the middle and hind teats (Table 1). Use of
the front teats occurred in all five litters in
this category of litter size and t)'1- :cally was
observed on about four of seven checks
(range, from three to six checks). To test
the statistical significance of these overall
patterns, I assigned a value of 1.0 to the
front teat-pair, 2.0 to the middle teat-pair,
and 3.0 to the hind teat-pair. I calculated a
mean teat-pair number for each litter and
analyzed those data using ANDV A with litter size as the between-subjects variable.
Mean teat-pair number was greater in
smaller litters than in larger litters (F =
82.96, dJ. ~ 3,41, P < 0.0001; 1-2 young,
3:00 ± 0.00; 3-4 young, 2.67 ± 0.04; 5-6
young, 2.37 ± 0.03; 7-8 young, 2.10 ±
0.03).
Use of hind and middle teat-pairs was
high until about day 12 postpartum after
which it began to decline (Fig. 1). Use of
the front teats in litters of five and six, and
seven and eight young, declined sooner
than use of the hind or middle pairs of teats.
November 1998
McGUlRE-SUCKLING BEHAVIOR OF VOLES
\187
... front teat-pair
•
•
100
.~
.
~
50
~
Litter Size 1-2 (0=8)
~
$
middle teat-pair
hind teat-pair
0
0>
0
.~
2
4
•
8
10
12 14
,.
• • • .
100
50
Litter Size 5-6 (0=17)
0
0
2
4
~
•
8
10
~,.
12
14
'"
m
~
'0
E 100
100
~
m
0-
50
50
Utter Size 3-4 (0=15)
Litter Size 7-8 (n=5)
0
0
0
2
4
6
8
10
12 14 16
Age of young (days)
0
2
•
Age of young (days)
4
8
10
12
14
,.
FIG. l.--Changes in use of teat-pairs relative to age of the young in prairie voles.
Indeed, in litters with five or six young, I
never observed use of the front teats on day
16 (0% oflitters used the front teats, 23.5%
used the middle teats, and 58.8% used the
hind teats; X2 = 14.96, d.! = 2, P = 0.001).
Similarly, in litters with seven or eight
young, I never observed use of the front
teats on day 16 (0% of litters used the front
teats, 40.0% used the middle teats, and
80.0% used the hind teats; X' = 6.67, elf
= 2, P = 0.036). Typically, individuals in
litters of seven or eight young were observed attached to a teat on about four or
five checks out of the possible seven (mean
± SE for each of the five litters: 4.75 ::t
0.37, 4.62 ± 0.42, 4.43 ± 0.30, 4.71 0:
0.36, 4.86 ± 0.26), and instances of being
unattached to a teat were intermittent. For
example, in 87.2% (34/39) of instances
when an individual was scored as off, it was
attached to a teat at the next check.
All measures of fidelity and consistency
were greater in litters with one or two
young than in larger litters (Table 2). In particular, litters with one or two young displayed greater teat fidelity than litters with
three or more young (F := 20.63, d.f = 3,
41, P = 0.0001). Litters with one or two
young also displayed greater teat-pair fidelity than litters of three to six young, and
those litters in tum displayed greater fidelity than litters of seven or eight young (F
= 38.97, df = 3,41, P = 0.0001). Finally,
litters with one or two young displayed
JOURNAL OF MAMMALOGY
1188
Vol. 79, No.4
TABLE 2.-Fidelity to teat and teat-pair and consistency to teat-pair in relation to litter size in
prairie voles. Values represent means:!: SE.
Litter size
1-2
3-4
5-6
7-8
Measure
(n = 8)
(n = 15)
(II = 17)
(n = 5)
Teat fidelity
Teat-pair fidelity
Teat-pair consistency
0.80 ::'::: 0.06
1.00 :t 0.00
1.00 :t 0.00
0.52 ± 0.02
0.72 :t 0.03
0.65 :t 0.03
0.48 ::t 0.02
0.65 :t 0.02
0.56 :t 0.02
0.50 :t 0.02
0.58 :t 0.01
0.47 :t 0.01
greater teat-pair consistency than litters
with three or four young, and those litters
in turn displayed greater consistency than
litters with five or more young (F = 57.89,
d.f ~ 3, 41, P ~ 0.0001).
DISCUSSION
Young prairie voles displayed a preference for teats, with preferences from posterior to anterior. In general, the front teats
were occupied only in very large litters. For
example, I observed routine occupation of
the front teats in litters of seven and eight
young, a situation in which there were more
young than teats. In litters of five and six
young, individuals often remained unattached to teats rather than occupying the
front teat-pair.
Ewer (1959) reported that young of domestic cats prefer posterior teats and suggested that posterior teats have larger mammary glands and greater milk yield than do
anterior teats; however, no direct evidence
of differences among teats in structure or
milk production was provided. Young pigs
prefer anterior teats and whether these teats
produce more milk than do posterior teats
has been controversial (Barber et aI., 1955;
Donald, 1937; Hartman et aI., 1962; Hemsworth et aI., 1976). Such debate arises, in
part, because of the difficulty in determining if the anterior teats produce more milk
and are therefore preferred by young pigs,
or if the young prefer anterior teats for
some reason unrelated to milk production
and then by their vigorous suckling stimulate greater milk yield (Fraser and Jones,
1975). There also is the question of whether
young pigs, or any young mammal, can as-
sess differences among teats in milk yield
by sampling from them. Some studies have
shown, for example, that young pigs continue to use anterior teats even when these
teats provide little, if any, milk (De Passille
et aI., 1988; Jeppesen, 1982). Other explanations for preference for anterior teats in
young pigs include closeness to the
mother's calls, greater accessibility to teats,
and less likelihood of being trampled or dislodged (Hafez and Signoret, 1969; Jeppesen, 1982; McBride, 1963). Most explanations emphasize an advantage gained by
young pigs suckling anterior teats. However, Fraser (1973, 1976) found that stimulation of the anterior teats was necessary
to elicit the full nursing posture and milk
ejection in female pigs, suggesting that the
anterior preference was advantageous not
only to the anterior-suckling young but to
the entire litter.
The underlying basis for the posterior
teat preference in prairie voles is unknown.
Explanations put forth for other species that
display positional preferences, such as differences among teats in milk yield, accessibility, likelihood of being dislodged, and
effectiveness in eliciting milk ejection,
seem worth investigating for prairie voles.
It is also possible, however, that areas on
the mother's ventrum differ in aspects unrelated to milk and suckling. For example,
locations on the mother's ventrum might
differ in temperature or likelihood of receiving grooming from the mother (females
frequently groom young while they are attached), and such differences might explain,
or at least contribute to, the posterior teat
preferences in prairie voles.
Novemher 1998
McGUIRE-SUCKLING BEHAVIOR OF VOLES
Young pigs that consistently use the same
teat-pair usually gain more weight than do
those that frequently change the teat-pair
from which they suckle (De Passille et aI.,
1988; Fraser and Thompson, 1986). Because productivity of mammary glands appears to depend, at least in part, on efficient
removal of milk and vigorous stimulation
of the teat before and after milk ejection,
Fraser and Thompson (1986) suggested that
the best strategy for young pigs could be to
consistently return to the same teat or teatpair to maintain high milk production by
the gland(s). Young prairie voles also show
fidelity in their choice of teat and teat-pair,
although the relationship between such behavior and weight gain is unknown. I did
not weigh prairie voles because of the difficulty in removing young that were attached to females. Although such removal
was possible, it had the potential of damaging teats of females and influencing data
being collected.
In pigs, small litter size is associated with
greater consistency in suckling position
(Hartsock et al., 1977; Hemsworth et a1.,
1976; Winfield et aI., 1974). Similarly, the
consistency with which young prairie voles
attached to the same teat or teat-pair was
greater in litters with one or two young than
in larger litters. The greater consistency in
small litters probably reflects reduced competition for preferred teat locations (i.e., the
hind teats) and the need for consistent stimulation of a teat to ensure adequate milk
production. Finally, my finding of fidelity
in choice of teat and teat-pair in prairie
voles indicates that such behavior occurs in
rodents and can be found in combination
with tenacious nipple attachment.
ACKNOWLEDGMENTS
I thank W. Bemis for support and help with
the figure, and M. Roberts at the National Zoological Park, Washington D. c., for sending me
prairie voles to integrate into my existing colo·
ny. Smith College students E. Henyey, M. Stu·
der, C. Zampieri, and S. Sullivan helped me dye
pups and score their suckling position. These
1189
students, along with laboratory animal care technologists, D. Ewell and V. Flood, cared for the
voles. This research was supported by a grant
from the Smith College Conunittee on Faculty
Compensation and Development.
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Submitted 17 October 1997 Accepted 2 February
1998.
Associate Editor was Robert K. Rose.