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Seasonal variations in the bed group size of the chimpanzee (Pan troglodytes) in the Petit Loango Reserve, Gabon.

Yuji Takenoshita & Shigeru Suzuki

     Chimpanzee grouping pattern is analyzed in relation to fruit food availability in the Petit Loango Reserve, Gabon. The Reserve is located on the southwestern coast of Gabon. The forest in this reserve is characterized by (1) scarce terrestrial herbaceous vegetation (THV), (2) clumped distribution of large tree species, and (3) seasonal change in the fruit distribution (i.e. clumped or dispersed) which does not necessarily correspond to the seasonal change in the density of fruit food patches.
     It has been suggested that party size of the chimpanzee is positively correlated with food patch density, as higher patch density reduces foraging costs to allow chimpanzees form larger parties (Wrangham et al., 1993, Chapman et al., 1995). In Petit Loango, however, mean monthly bed group size of the chimpanzee was not correlated with fruit food patch density. On the other hand, mean monthly bed group size was significantly correlated with fruit food distribution. That is, bed group size was larger when food tree distribution was clumped, and smaller when it was dispersed, regardless of food density.
     Assuming that the bed group size of the chimpanzee reflects their foraging party size, the chimpanzee of Petit Loango appeared to gather into large parties to share the large clumps of patches when the patches were clumped, whereas they disperse into small parties when fruit patches were dispersed even with overall high patch density. Thus, fruit patch distribution might be more important than patch density, as a factor affecting party size of Petit Loango chimpanzees. Impact of patch distribution on the chimpanzee's party size might be enhanced by the scarcity of THV i.e. the lack of small patches, which lie between the clumps of fruit patches.

Chapman CA, Wrangham RW, and Chapman LJ (1995) Ecological constraints on group size: an analysis of spider monkey and chimpanzee subgroups. Behavioral Ecology and Sociobiology 36:59-70.
Wrangham RW, Gittleman JL, and Chapman CA (1993) Constrains on group size in primates and carnivores: population density and day-range as assays of exploitation competition. Behavioral ecology and sociobiology 32:199-209.