Abstract Among primates, group size is highly variable.
The standard ecological model assumes that better
predation avoidance as group size increases favours living
in larger groups, whereas increased travel costs and
reduced net food intake due to within-group competition
for resources set the upper limit. Folivorous primates,
however, tend to defy this generalisation in that some
live in small groups despite low costs of feeding competition.
To resolve this folivore paradox , it has been suggested
that folivore group size is limited by social factors
such as male harassment or infanticide, or that females
can disperse more easily and thus maintain group size
near optimum levels. In this paper, we examine the effects
of group size on home range size, day-journey
length, activity budget and diet in wild Thomas's langurs
(Presbytis thomasi), which live in one-male multi-female
groups with a limited life cycle. We examined only data
from the stable middle tenure phase when factors such as
the strength of the breeding male or the way in which
groups were formed did not influence ranging and activities.
During this phase, group size affected day-journey
length and home range size, and had a minor effect on
diet, but did not influence time spent feeding or resting,
allogrooming or birth rates. Hence the upper limit to
group size during the middle tenure phase in Thomas s
langurs is not set by feeding competition. The folivore
paradox is not due to frequent female dispersal in
Thomas s langurs. The timing of female dispersal is not
as expected if it serves to keep group sizes near the ecological
optimum, and groups seem to be below this optimum.
Instead, female reproductive success is presumpresumably
maximised in small to mid-sized groups because
larger groups show a clear trend to experience higher
risk of take-over, often accompanied by infanticide. Because
females can redistribute themselves among nearby
groups when groups reorganise each time a new male
starts up a new group, females can keep the group small.
Thus, a social factor, risk of infanticide, seems to provide
the selective advantage to small group size in
Thomas s langurs