Unexpected hard-object feeding in Western lowland gorillas.

OBJECTIVES: 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. MATERIALS AND METHODS: 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. RESULTS: 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. DISCUSSION: 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.

Niche differentiation and dietary seasonality among sympatric gorillas and chimpanzees in Loango National Park (Gabon) revealed by stable isotope analysis.

The feeding ecology of sympatric great ape species yields valuable information for palaeodietary reconstructions in sympatric early hominin species. However, no isotopic references on sympatrically living apes and their feeding ecology are currently available. Here we present the first isotopic study on sympatric great apes, namely western lowland gorillas (Gorilla gorilla gorilla) and central chimpanzees (Pan troglodytes troglodytes) from Loango National Park, Gabon. We successfully analyzed the stable carbon and nitrogen isotope ratios in a selection of food plants (n = 31) and hair samples (n = 30) retrieved from sleeping nests to test whether niche partitioning among sympatric chimpanzees and gorillas is detectable using isotope analysis of hair. Ape hair strands with roots were sectioned into sequential segments (n = 100) to investigate temporal isotopic variation related to seasonal variations in food resources. We found significant δ(13)C differences between herbaceous plants and fruits, most likely due to canopy effects. While the δ(13)C values of chimpanzees indicate the consumption of fruit, the low δ(13)C values in gorilla hair indicate folivory, most likely the consumption of (13)C-depleted herbaceous vegetation. Our isotopic data also confirmed dietary overlap between chimpanzees and gorillas, which varied by season. Gorillas showed significant variation in δ(13)C values in response to season due to shifting proportions of herbaceous plants versus fruits. In chimpanzees, significant seasonal variation in δ(15)N was likely related to the seasonal availability of fruit species with particularly high δ(15)N values. In summary, we found isotopic evidence for niche partitioning and seasonal dietary variation among sympatric great apes at Loango. These findings provide a valuable reference for palaeodietary research on fossil hominins using δ(13)C analyses, particularly for studies focusing on sympatric taxa and on temporal isotopic variation within incremental tissues such as tooth enamel.

A novel food processing technique by a wild mountain gorilla (Gorilla beringei beringei).

Innovation, the invention of new behavior, has been observed in wild primates only infrequently. The processing of thistle (Cardus nyassanus) has previously been described as being one of the most complex food processing techniques used by mountain gorillas (Gorilla beringei beringei). We report a case of innovation in thistle leaf processing by a subadult female mountain gorilla in Bwindi Impenetrable National Park, Uganda. This technique involved rolling the thistle leaves into a ball between her palms prior to putting them in her mouth, as opposed to the standard method of folding leaves. All other weaned individuals (n = 13) were observed to use the standard method to process thistle leaves as described in Byrne et al. While the subadult female emigrated out of the research group 6 months after she had first been observed using the innovative technique, preventing observations of possible transmission within the group, it adds to the debate of whether food processing techniques used by gorillas are socially learned or not.