Feeding in Forest Chimpanzees: Do Food Type and Canopy Location Predict Positional Behavior?

Distinctive aspects of great ape anatomy and positional behavior, including upright torso stability and suspension, are hypothesized to have evolved to facilitate ripe fruit-eating in the terminal zone of tree canopies at large body size. Fossil discoveries challenge this view, and we test this perspective by investigating relationships among feeding posture, food type, and canopy zone in chimpanzees.

Focal follow data were collected over 10 months on 103 chimpanzees from Ngogo, Kibale Forest, Uganda. Generalized Linear Mixed Modeling was used to examine the effects of food, canopy location, and age on arboreal feeding posture versatility. Branch size and number, and tree size and species were also examined.

When considering all trees, consuming young leaves versus ripe fruit positively affected versatility. When examining only large-canopy trees, eating leaves versus ripe fruit had no effect, but unripe versus ripe fruit had a negative effect. Terminal zone feeding was associated with an increase in versatility, and in this zone, consuming leaves versus ripe fruit had a negative effect. Versatility was inversely correlated with age, varied across tree species, and was higher on small branches and in small trees.

A positive "terminal zone effect" on versatility was detected in large trees, with behavioral impact varying by food type. Within the forest as a whole, consuming leaves over ripe fruit increased versatility. Thus, food type and availability may, in different combinations, lead to equifinality in great ape positional versatility. Hominoid morphofunctional specializations could thus evolve in many ecological contexts.