Sleeping Parties and Nest Distribution of Chimpanzees in the Savanna Woodland, Ugalla, Tanzania
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We conducted ecological studies of chimpanzees (Pan troglodytes) in the Ugalla area, western Tanzania. Ugalla is one of the driest habitats of chimpanzees and the Ugalla River is the eastern boundary of chimpanzee distribution. Most of Ugalla is occupied by savanna woodlands dominated by deciduous trees of Brachystegia and Julbernardia. Chimpanzees tended not to make nests in riverine forests in plains, but in small patchy forests dominated by Monopetalanthus richardsiae and valley forests dominated by Julbernardia unijugata on slopes in mountainous areas. We estimated population density of chimpanzees to be 7–9 × 10−2 individuals/km2 based on nest censuses, suggesting that 2–3 × 102 individuals inhabited the 3352 km2 area of Ugalla. The size of the largest nest cluster (n=23) suggests that 1 unit group (community) comprised 30–35 individuals. In the daytime, chimpanzees formed small feeding parties (mean 2.0 individuals), but larger ones in the evening (mean 4.8 individuals and 5.2 individuals based on fresh nest clusters). The pattern might reduce the predation risk from large nocturnal carnivores such as lions and leopards. The sleeping sites may function as both a safe sleeping site and a meeting point for chimpanzees with a huge home range that may have difficulty in finding other members of their unit group.