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Primates (2005) 46:19DOI 10.1007/s10329-004-0093-9ORIGINAL ARTICLEColin A. Chapman Mary S. M. PavelkaGroup size in folivorous primates: ecological constraints
and the possible influence of social factorsReceived: 16 May 2003 / Accepted: 23 April 2004 / Published online: 10 June 2004
Japan Monkey Centre and Springer-Verlag 2004Abstract The ecological-constraints model assumes that
food items occur in depletable patches and proposes that
an increase in group size leads to increased day range
due to more rapid patch depletion. Smaller groups become advantageous when an increase in travel costs is
not repaid by an increase in energy gained or some other
tness advantage. On the other hand, we also know that
group size can be inuenced by social factors. Here we
contrast the diet and group size of red colobus (Procolobus badius) and black-and-white colobus (Colobus
guereza) in Kibale National Park, Uganda to consider
how ecological and social factors are aecting their
group sizes. Subsequently, we examine whether the insights gained from this detailed comparison can provide
an understanding of why the social organization and
group size of mantled howlers (Alouatta palliata) and
black howlers (A. pigra) dier. Two groups of red colobus and two groups of black-and-white colobus were
studied over 10 months. Red colobus groups were larger
(48 and 24) than black-and-white colobus groups (9 and6). The two groups of red colobus overlap home ranges
with the two groups of black-and-white colobus; 75%
and 95% of their home ranges were within red colobuss
home range. There was a great deal of similarity in the
plant parts eaten by the two species and both species fed
primarily on young leaves (red colobus 70%, black-andwhite colobus 76%). In terms of the actual species
consumed, again there was a great deal of similarity
between species. The average dietary overlap among
months for the two neighboring groups of red colobus
was 37.3%, while the dietary overlap between the red
colobus and the black-and-white colobus group that had
its home range almost entirely within the home range of
the red colobus groups averaged 43.2% among months.
If ecological conditions were responsible for the dierence in group size between the two colobine species, one
would expect the density of food trees to be lower in the
home ranges of the black-and-white colobus monkeys,
since...