Female relationships and food availability in a forest community of chimpanzees

Abstract

Our observations strongly support the view that a lethal gang attack occurred against a young adult male within a wild chimpanzee (Pan troglodytes schweinfurthii) community. This is the first report of such an occurrence. At least three adult male chimpanzees are believed to have been involved in the attack. This case is unusual because the attack was fatal, involved directed group aggression within a community and was not linked to the overthrow of the alpha male. During the period before the attack the percentage of cycling females in the community was very 10\\ (15 males : 17 females, a maximum of 2 regularly cycling). This fact, coupled with the presence of one female in full genital tumescence, may have incited and escalated the yiolence of the attack. The lethal attack is interpreted as an act of intra-communitv male sexual competition resulting in the complete exclusion of one male from estrous females.

This thesis investigates the behavioural ecology of the Sonso community of chimpanzees, Budongo Forest, Uganda; with particular reference to the social relationships of females. Within both the genus Pan and different communities of P. troglodytes there is considerable variability in female social relationships. This study quantified both forest food availability and the behavioural patterns of chimpanzees over a 16 month period.

Quantitative measures of food availability were made using a phenology trail including 26 species of known chimpanzee foods. The availability of fruit, leaves, buds and flowers was monitored. Food production was closely linked to rainfall and irradiance levels, revealing 4 distinct seasons. All phenological states were synchronised. The time of highest food production was in Marchi April, coinciding with the onset of the first annual wet season.

The chimpanzees were observed to alter their behavioural patterns in response to changes in food availability. Party size, in particular the size of all-male and sexual parties, was positively correlated with measures of food availability, but the size of all-female parties did not increase during periods of high food availability. However, the number of females in mixed parties did increase, suggesting a preference for females to associate with males. In addition, the number of oestrous females in the community was positively correlated with measures of food availability. General Linear Models were used to show that food availability was the best predictor of both party size and the number of oestrous females.

The chimpanzees' response to changes in food availability was evident also in their feeding behaviour and activity budgets. During times of resource scarcity the chimpanzees showed a flexible response: either they decreased time spent feeding and fed on lower quality food items, or they increased time spent moving to incorporate seeds and fruit in their diet. Several keystone foods were identified both from phenological data collection and feeding observations. Fruit was the major component of the diet but in contrast to other study communities. young leaves were also very important. In particular, the young leaves of Celtis mildbraedii were not only fed upon during times of low fruit availability but were also a preferred food item at other times.

A bisexual community, with both males and females occupying smaller home ranges within the community range, was revealed by the analysis of ranging behaviour. Male ranges were larger than those of females and within females the most important life history trait determining ranging patterns was their reproductive state: cycling females had larger home ranges. The Sonso community had a relatively small community range, but female core area sizes were similar to those in other communities. Females were identified as being either central or peripheral. There was no obvious relationship between female dominance and ranging behaviour, but there was a tendency for higher ranking adult females to have larger core areas. Analysis of association patterns showed overall that males were more closely bonded than females. However, some female-female dyads were found to have similar association strengths to those of males. All association indices were greater when food availability was high. The period of transfer between communities was identified as the most important period for females in terms of resource competition.