POPULATION DENSITY AND ANNUAL VARIATION IN BIRTH
MASS OF GUANACOS IN SOUTHERN CHILE
RONALD
J.
SARNO AND WILLIAM
L.
FRANKLIN
Department of Animal Ecology and Program in Ecology and Evolutionary Biology, 124 Science II,
Iowa State University, Ames, IA 50011-3221
We investigated the influence of population density and meteorological conditions on annual birth mass of guanacos in Torres del Paine National Park, Chile, from 1987 to 1996.
Between 1987 and 1990, density of guanacos on the study area nearly tripled from 16 to
43 animals/km2 • Mean birth mass was significantly different across years, and there was a
strong negative correlation between mean yearly birth mass and population density. There
was no correlation between mean yearly birth mass and mean temperature or total precipitation in either winter or spring during this period. Since 1990, density of guanacos has
decreased, which we suspect is the result of degraded range conditions, partly due to
overgrazing. Population censuses from other sectors of the park and the adjacent sheep
ranch revealed increasing numbers of guanacos, and the movement of tagged animals out
of the study area into surroundings with lower guanaco density.
Key words:
Lama guanicoe, guanaco, Chile, birth mass, population density
Little is known about the causal factors
influencing mass at birth in wild ungulates,
in part because they are difficult to capture
and weigh (Albon et aI., 1983). Maternal
condition of domestic and captive wild
mammals influences birth mass (Berger,
1979; Robinson, 1977; Verme, 1965; Willis
and Wilson, 1974), and we assume that the
same may occur in wild, free-ranging individuals.
Maternal condition likely is influenced
by many factors, including population density. Under some conditions, birth mass
may be correlated with population density
and may be one of the responses hypothesized to occur in large mammals when population levels are near carrying capacity
(Fowler, 1981a, 1981b). Some density-independent factors, such as temperature and
precipitation, also affect mass at birth of
newborn ungulates (Albon et aI., 1983) and
thus possibly influence maternal condition.
We document variation in annual birth
mass over a lO-year period in a population
of wild, free-ranging guanacos (Lama
guanicoe) in southern Chile and ascribe a
Journal of Mammalogy, 80(4):1158-1162, 1999
1158
possible cause to this variation. Numbers of
guanacos in southern Chile have increased
over the past 2 decades because of successful conservation programs (Franklin et aI.,
1997). In Torres del Paine National Park,
numbers of guanacos increased from 175 in
1975 to 3,000 in 1993. Juveniles have been
hand-captured, tagged, and weighed since
1987 as part of a long-term project studying
the ecology and life history of this population. Our objective was to determine if
birth mass of guanacos was correlated with
population density or meteorological conditions.
MATERIALS AND METHODS
Our study was conducted in Torres del Paine
National Park (51 °3/S, n055/W), an International Man and Biosphere Reserve located in the
eastern foothills of the Andean mountain range
of southern Chile. The park encompassed 2,400
km2 and provided almost undisturbed habitat for
wildlife. The 40-km2 study area ranged from
200-400 m in elevation and was bordered by
large lakes to the south, north, and west and the
Goic sheep ranch to the east. The landscape was
open with rolling hills, vegetation was rarely> 1
November 1999
SARNO AND FRANKLIN-BIRTH MASS VARIATION IN GUANACOS
m high, and animals were observed easily.
Grasses (Festuca gracillana, Anarthrophyllum
patagonium) and shrubs (Mulinum spinosum,
Senecio patagonicus, and Berberis buxifolia)
dominated the pre-Andean steppe (Pisano,
1974).
The region was characterized by two distinctive climatic periods. Summer (December-February) was marked by strong westerly winds,
warm temperatures (mean maximum 19.7°C)
and relatively high precipitation (X = 102 mm).
Winter (June-August) was cold (X = -S.3°C),
with less precipitation (X = 21 mm), and little
or no wind (Johnson et al., 1990; Ortega and
Franklin, 1988).
From 1987 to 1996, chulengos (newborn guanacos) were hand-captured (n = 810), marked,
weighed, and reunited with their mothers
(Franklin and Johnson, 1994). Fifty percent of
the chulengos were captured within 1 h after we
observed their birth. The rest were < 1 day old
when captured, which was apparent by their wet
appearance, uncoordinated locomotion (stumbling or falling), and close proximity to their
mothers. We considered the mass of any captured chulengo :524 h old as its birth mass (A1bon et al., 1983).
We conducted population censuses during
spring, autumn, and winter of 1987-1996. However, we used only the spring (September-November) counts for our analysis because there
was little movement of guanacos on the study
area. In spring, adult males vigorously defend
territories, and adult females move among a restricted number of those territories (Ortega and
Franklin, 1988). Consequently, there were no
long-distance movements of animals on the
study area or ingress of guanacos from adjacent
sectors, where there was less suitable habitat.
Monitoring of marked animals during that period also suggested that guanaco movement was
minimal. Therefore, we were confident that our
counts accurately reflected number of guanacos
on the study area.
Each census was conducted with four observers and lasted 3 days. Observers followed standardized routes and collected data on group size,
group composition Guvenile or adult), sex of
adults, and identification of tagged individuals.
On day 1 of the census, we began at the eastern
end of the study area, and each successive day
we followed routes further west. We never ob-
1159
served a marked animal in a different route on
the same day, or on successive days.
We used standard correlation and regression
techniques (SAS Institute Inc., 1990) to analyze
the relationship between mean cohort birth mass
and population density, mean seasonal temperature (summer and winter), and total seasonal
precipitation (we obtained daily precipitation
and temperature data from park weather stations). We were unable to acquire meteorological data for 1996. We used meteorological data
for winter because low temperatures during the
coldest season can reduce fetal development
(Albon et aI., 1983; Mitchell, 1971) and for
spring because nutrition during the later stages
of pregnancy has the greatest effect on fetal
growth and birth mass (Albon et al., 1983; Robinson, 1977; Sadleir, 1969; Skogland, 1984).
RESULTS
Mean annual birth mass of guanacos was
12.6 kg ± 0.06 (range = 7.0-17.0 kg, n =
810). There was no difference between
males (12.6 kg ± 0.08, n = 398) and females (12.6 kg ± 0.08, n = 412; t = 0.15,
d.f. = 808, P = 0.881; Table 1), but birth
mass differed among years (F = 12.23; d.f.
= 9, 808; P < 0.001).
Between 1987 and 1990, the number of
guanacos on the study area increased from
623 to 1,709 animals or from 16 to 43 animalslkm2 , then declined by 1996 (Fig. 1).
Mean cohort birth mass was correlated negatively with population density during this
period (r = -0.62, n = 10, P = 0.056; Fig.
2). The slope of the fitted regression line (Y
= 13.79-O.00089X) suggested that for an
increase in population density of 5 animals/
km2 (throughout the range of observed population densities), mean birth mass decreased by ca. 0.3 kg. We found no correlation between mean annual birth mass and
mean temperature or total precipitation, respectively, in either winter or spring during
the same period.
DISCUSSION
Our study was conducted during a period
when population size of guanacos increased
rapidly during the late 1980's, peaked in the
1750.-----------------,
i.-Mean birth mass (n = 810) by sex
and cohort of juvenile guanacos born in Torres
del Paine National Park, Chile, in 1987-1996.
TABLE
Year
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
Vol. 80, No.4
JOURNAL OF MAMMALOGY
1160
Sex
Females
Males
Cohorta
Females
Males
Cohort
Females
Males
Cohort
Females
Males
Cohort
Females
Males
Cohort
Females
Males
Cohort
Females
Males
Cohort
Females
Males
Cohort
Females
Males
Cohort
Females
Males
Cohort
Mean
mass (kg)
SE
n
l3.3
l3.6
l3.4
12.8
13.0
12.9
12.9
12.6
12.7
12.5
12.7
12.6
13.0
13.1
13.1
12.2
12.4
12.3
12.9
12.6
12.7
11.5
11.8
11.7
11.9
11.6
11.7
l3.3
l3.3
l3.3
0.32
0.29
0.22
0.21
0.24
0.16
0.26
0.26
0.l7
0.26
0.24
0.18
0.26
0.28
0.19
0.24
0.26
0.18
0.25
0.26
0.18
0.22
0.24
0.16
0.25
0.24
0.17
0.24
0.25
0.l7
27
20
47
39
46
85
42
51
93
44
54
98
53
45
98
44
47
91
48
41
89
38
30
68
32
28
60
45
35
80
1550
II 1350
N
'iii
C 1150
.S!
~
:::l
c..
950
o
a.
750
550+--r-~--.-.-~--.--.-r-~
1987198819891990199119921993199419951996
Year
FIG. I.-Number of guanacos on the study
area in Torres del Paine National Park, Chile, in
1987-1996.
population density was so high that it
masked any climatic influences on birth
mass.
Since 1990, the number of guanacos on
the study area generally has decreased (Fig.
1), which we suspect to be the result (in
part) of overgrazing and degraded range
conditions. Population censuses from other
sectors of the park and the adjacent sheep
ranch revealed increasing numbers of guanacos, and the movement of tagged animals
out of the study area into surroundings with
lower density of guanacos. For example,
"Overall mean birth mass of females and males.
13.5
early 1990's, and declined thereafter. During this interval, we observed a negative
correlation between mean cohort birth mass
and population density from 1987 to 1996.
Skogland (1990) also observed decreasing
birth mass resulting from increasing population density in a population of wild reindeer (Rangifer tarandus).
Although we observed a correlation between mean annual birth mass and population density, we observed no suchcorrelation between mean annual birth mass and
weather as was demonstrated for red deer
(Cervus elaphus-Albon et al., 1983). Perhaps annual variation in climate was insufficient to influence birth mass, or possibly
Ui'
Cl
~
13.0
.
1987
1996
1991
:E
Cl
'iii
1993
12.5
1990
==
s::
1::
:c
C
III
.
1989
1988
1992
12.0
II
::!:
Y = 13.79 . 0.00089X
r = ·0.62, P = 0.056
11.5
14
19
24
1995
1994
29
34
39
44
Number of guanacos/km 2
FIG. 2.-Least squares regression of mean
birth mass of guanacos against population density in Torres del Paine National Park, Chile.
November 1999
SARNO AND FRANKLIN-BIRTH MASS VARIATION IN GUANACOS
during the severe winter of 1995, ca. 78%
(n = 3,100) of the sheep on the adjacent
ranch died (J. Goic, pers. cornm.). In the
following spring, 250-300 guanacos were
observed on the ranch compared with 50100 during previous years. We hypothesize
that decreased feeding competition with
sheep, in conjunction with the potentially
degraded range conditions on the study
area, resulted in more guanacos moving
onto the ranch. Additionally, because
ranchers drastically reduced the number of
patrols with horses and dogs in this sector,
guanacos may have been harassed less.
Guanaco numbers on the ranch and in
other sectors of the park, however, cannot
account for the nearly two-thirds reduction
in population density on the study area from
1990 to 1996. Therefore, it is conceivable
that low mean birth mass during the early
1990's contributed to lower juvenile survival, which led to a reduction in population
density by the end of the study. Indeed,
Gustafson et al. (1998) demonstrated a significant negative correlation between birth
mass and early mortality of newborn guanacos. Juvenile survival to :51 year of age
also decreased from 70% in the early
1990's (Behl, 1992) to ca. 30% by 1996
(Sarno et al., in press). In similar studies,
Fairbanks (1993) and Singer et ai. (1997)
reported that lighter neonates of pronghorn
(Antilocapra americana) and elk (Cervus
elaphus) suffered higher mortality. It also is
possible that increasing population density
elevated mortality rates of adult guanacos
although this response has not generally
been observed in other ungulates (Caughley, 1970; Clutton-Brock et aI., 1982;
McCullough, 1979; Sauer and Boyce, 1983;
Skogland, 1985, 1990).
Various density-dependent and densityindependent effects on large mammals have
been documented in several ungulate species (Albon et al., 1983; Berger, 1986;
Caughley, 1970; Clutton-Brock et aI., 1982,
1987; Sauer and Boyce, 1983; Skogland,
1984, 1985, 1990). The relative importance
of these effects, however, likely varies both
1161
spatially and temporally. Future controlled
studies therefore are needed to elucidate
how population density, climate, and even
predation interact to influence individual
survival, especially of newborns and adult
females. Studies of this type would more
directly test Hom's (1968) suggestion that
population regulation is not under the exclusive control of either density-dependent
or density-independent mechanisms.
ACKNOWLEDGMENTS
We thank: the Chilean National Forestry and
Park Service (CONAF), and the administration
at Torres del Paine National Park for their assistance and collaboration, particularly G. Santana,
J. Toro, and N. Soto. D. Cox, H. Stem, and B.
Clark provided statistical advice. The insightful
comments of B. Danielson, W. Johnson, and two
anonymous reviewers greatly enhanced the quality of the manuscript. M. Bank:, R Barrientos, B.
Behl, M. Behl, C. Bergman, G. Blundel, T. Boggen, T. Chladny, J. Cleckler, S. Daugherty, A.
Engh, G. Garay, E. Gaylord, K. Gaylord, K.
Guderian, K. Harms, R. Hunter, A. Iriarte, W.
Johnson, W. Loya, P. Heaven, K. Nielsen, I.
O'Connell, W. Prexl, J. Rathje, J. Reed, S. Shoemaker, B. Soppe, K. Stueckrath, T. Sulser, D.
Whitaker, and all Patagonia Research Expedition
participants from 1987 to 1996 provided field
assistance. This study was supported by Patagonia Research Expeditions, National Science
Foundation Grant No. BSR-9112826, and Organization of American States Grant No. 19104.
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Submitted 2 June 1998. Accepted 6 February 1999.
Associate Editor was Renn Tumlison.