Available online at www.sciencedirect.com
Behavioural Processes 76 (2007) 192–197
Group size effects on foraging and vigilance in migratory Tibetan antelope
, Tongzuo Zhang
, Yifan Cao
, Jianping Su
, Simon Thirgood
Northwest Institute of Plateau Biology, The Chinese Academy of Sciences, Xining 810008, PR China
Graduate School of the Chinese Academy of Sciences, Beijing 100049, PR China
Macaulay Institute, Aberdeen, AB15 8QH, UK
Received 26 July 2006; received in revised form 26 March 2007; accepted 12 May 2007
Large group sizes have been hypothesized to decrease predation risk and increase food competition. We investigated group size effects on
vigilance and foraging behaviour during the migratory period in female Tibetan antelope Pantholops hodgsoni, in the Kekexili Nature Reserve
of Qinghai Province, China. During June to August, adult female antelope and yearling females gather in large migratory groups and cross the
Qinghai–Tibet highway to calving grounds within the Nature Reserve and return to Qumalai county after calving. Large groups of antelope
aggregate in the migratory corridor where they compete for limited food resources and attract the attention of mammalian and avian predators and
scavengers. We restricted our sampling to groups of less than 30 antelopes and thus limit our inference accordingly. Focal-animal sampling was
used to record the behaviour of the free-ranging antelope except for those with lambs. Tibetan antelope spent more time foraging in larger groups
but frequency of foraging bouts was not affected by group size. Conversely, the time spent vigilant and frequency of vigilance bouts decreased
with increased group size. We suggest that these results are best explained by competition for food and risk of predation.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Foraging; Group size; Kekexili nature reserve; Tibetan antelope; Vigilance
For many species, especially birds and mammals, vigilance
is traditionally considered to function predominantly for pro-
tection from predators (Roberts, 1996; Treves, 2000). Previous
research has focused on the inﬂuence of predation risk on
behavioural patterns at the individuallevel (Bednekoff and Lima,
1998b; Beauchamp and Ruxton, 2003). Animals typically tend
to increase time spent vigilant and decrease time spent foraging
when predation risk is high. If the predation risk is low, animals
allocate their behavioural time budget according to other fac-
tors, such as group size and nutritional requirements (Hopewell
et al., 2005).
Group-size effects in mammals and birds are most often stud-
ied by examining the trade-off on time allocated to antipredator
vigilance and foraging (Elgar, 1989; Roberts, 1996; Treves,
2000; Beauchamp, 2001; Bednekoff and Lima, 1998a, 2004).
Generally, animals in larger groups devote less time to vigi-
lance and correspondingly more to foraging than those which are
Corresponding author. Tel.: +86 971 6143748.
E-mail address: email@example.com (J. Su).
solitary (Pulliam, 1973; Caraco, 1979; Shorrocks and Cokayne,
2005). This ﬁnding can be interpreted in two different ways.
First, a dilution effect exists so animals in larger groups are
more likely to detect predators and are less likely to be killed
than animals in smaller groups. As a result, individuals in larger
groups can decrease their time spent on vigilance and increase
their time foraging (Dehn, 1990; Lazarus, 2003; Beauchamp,
2003a). Second, competition for limited food resources is greater
in large groups and thus individuals must compete by foraging
more quickly than their companions (Clark and Mangel, 1986;
Beauchamp, 2001, 2003a, 2003b).
Tibetan antelope (or chiru) Pantholops hodgsoni are endemic
to the high-altitude Qinghai-Tibet Plateau of western China
(Schaller, 1998). Historical accounts indicate that very large
populations of Tibetan antelope ranged across an area of
in the early 20th century. Illegal hunting for both
meat and wool resulted in major declines in abundance and dis-
tribution. Current population estimates of Tibetan antelope are
∼75,000 individuals and they are classiﬁed as endangered by
IUCN (2001) and listed in Appendix I of CITES. The distri-
bution of Tibetan antelope is now restricted to remote areas
in the Tibetan autonomous region (TAR), Qinghai province
and the Xinjiang autonomous region and the largest remaining
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