Unexpected hard-object feeding in Western lowland gorillas.

Abstract

Gorilla diets are characterized by large amounts of fruit and tough fibrous plant material. Hard-object feeding is not generally associated with this genus as the high crests on their molar teeth would be at risk of damage from the mechanically challenging woody endocarp. This study aims to demonstrate that at least one population of western lowland gorillas are seasonal hard-object feeders, orally processing the seeds of Coula edulis. Feeding behavior of habituated western lowland gorillas and phenology of fruiting trees was observed over a 4-year period to determine the extent they exploited the seeds of C. edulis. Additionally, the endocarps of C. edulis were subjected to testing to determine their mechanical properties. Our results demonstrate that during the fruiting season (January, February, and December) gorillas consistently opened the seeds of C. edulis using their postcanine dentition. The protective endocarp is composed of a very stiff material, presenting a substantial mechanical challenge to a gorilla. However, the high ratio between elastic modulus and toughness will facilitate brittle, cataclysmic fracture of the seed shell given a high enough load. Although a rich energy source, C. edulis likely tax gorilla dentitions to their upper limit. The rarity of such behavior at sites where it could be observed may indicate a degree of social learning or culture driving its occurrence. This shows a greater breadth of gorilla diets than previously described and suggests gorillas may be a useful model for interpreting the dietary mechanics that necessitated robust craniodental morphology in australopiths.