Examining dietary composition and overlap between species living within the same environment is important for improving our understanding of their distribution and abundance, as well as answering questions about community ecology, and the processes of speciation and adaptive radiation. Most species that occur sympatrically and have a similar diet are assumed to have evolved species-specific ecological adaptations that decrease the competition between themselves and other species living in the same habitat, and make co-existence possible (Ec­card & Ylönen 2003).

Dietary overlap of sympatric apes is complex and understudied. Chimpanzees and gorillas occur sympatrically across a wide variety of ecological habitats, including the East African mountains of Bwindi Impenetrable National Park in Uganda, the Kahuzi-Biega National Park in the Democratic Republic of the Congo, and the Ebo forest of Cameroon on the west coast of the continent. Ecological differences cause variation in the degree of dietary overlap and niche separation between sympatric species, making it difficult to generalize from a single study site to a broader area of sympatry. In addition, large inter-annual variation in fruit production in rainforest habitats means that short-term studies only provide a snapshot of interspecies dynamics, which can vary substantially both within and between years.

In 2005 we set up a new field site in Loango National Park, Gabon, with the aim of habituating sympatric western gorillas (Gorilla gorilla gorilla) and central chimpanzees (Pan troglodytes troglodytes) in order to answer questions about feeding ecology, interspecies competition, ranging behaviour and social structure. Loango National Park, located on the coast of Gabon, contains highly heterogeneous habitat that includes mature and secondary forest, mangroves, swamps, savannah and coastal forest, making it an interesting location to study interspecies dynamics of closely related species. The 100 km² study area where our project is based is also notable in that terrestrial herbs (considered to be a staple gorilla food in other field sites) are extremely sparse. We wanted to know whether living in such an environment might lead to an increase in frugivory by gorillas as well as increased dietary overlap between them and the more frugivorous chimpanzees compared to other sites. Would chimpanzees remain more frugivorous than gorillas despite these ecological constraints, and would gorillas find other ways of maintaining their intake of dietary fibre through an increased consumption of bark or leaves? Another question was whether the density of gorillas in Loango was lower than at sites where terrestrial herbs are abundant, and whether this location would be unable to support a high density of gorillas. If gorillas occur at very low densities in Loango then competition between them and the chimpanzees would be reduced even if dietary overlap was high.

From 2005 to 2008 we studied food availability and dietary composition of the chimpanzees and gorillas using a variety of methods, but largely through faecal analysis and trail signs since the apes were still unhabituated to human presence and we were therefore unable to observe them directly for long periods of time. We tried to collect faecal samples every day from under night nests, on trails and after contacts with the apes, and then brought them back to our research camp for washing. After washing away all the faecal matter through a sieve we then identified and counted all the seeds found within, in order to measure both the diversity and quantity of fruit consumed by both ape species. By separately weighing whole fruits of species that we found in the faeces, we were also able to measure the volume of fruit consumed, and estimate how many grams of each faecal sample were fruit remains.

We monitored changes in fruit abundance from 750 trees from 57 species known to be consumed by chimpanzees or gorillas on a monthly basis, and we used data on the presence of fruits, flowers and young leaves to create a monthly fruit abundance index for each ape species. We then measured overlap in fruit consumption between the two species through faecal analysis, in addition to looking at seasonal variation in fruit consumption and its relation to overall fruiting patterns in the forest.

Our results showed that Loango National Park has a very low herb density compared to other locations where gorillas have been studied and a different seasonal fruiting pattern than reported for other ape habitats, illustrating that there can be high variability between sites. Mean dietary overlap for fruit between chimpanzees and gorillas was 27.5% but varied greatly seasonally, ranging between 0.3% and 69%. More fruit was available for chimpanzees than gorillas throughout the study, and chimpanzees consistently consumed a greater number of fruit species than did gorillas. Chimpanzees were also significantly more frugivorous and their faeces contained more fruit than gorilla faeces 90% of the time.

We found a positive correlation between the seasonal consumption of fruit and the availability of fruit in the forest for both chimpanzees and gorillas, indicating that both ape species respond to fluctuations in fruit availability by consuming more as it becomes increasingly available. A very low availability of terrestrial herbs did not lead to increased frugivory by gorillas nor increased overlap between the two ape species compared to other field sites. Instead, our results suggest that Loango gorillas may eat more tree leaves to make up for the lack of terrestrial herbs. Only direct observations once the apes are fully habituated to human presence will confirm this theory, but the number of species of tree leaves eaten by gorillas in Loango is higher than that found in any other site where gorillas are studied, and so we await direct observations to confirm if the quantity of leaves eaten is also higher.

We also found that gorillas avoided fatty and oily fruits that were high in crude lipids, but these fruits were regularly eaten by the chimpanzees, and were the main cause of differences in fruit consumption between the two ape species. In each month of the study, fruits from one or more of these fatty species were available, indicating that in Loango chimpanzees have a more consistent supply of fruits throughout the year that gorillas do not compete for, and it is possible that this dietary differentiation allows the two ape species to coexist without too much competition. In addition, genetic analysis confirmed that the density of gorillas in Loango was comparable to that found in other gorilla study sites (Arandjelovic et al. 2010), indicating that the lack of terrestrial herbs does not appear to have led to a reduced density of gorillas in Loango. Our results therefore support the idea that forests with minimal terrestrial herbs can support healthy western gorilla populations.

Our study showed that chimpanzees and gorillas in Loango have a similar pattern of niche differentiation as that found in other locations, with chimpanzees being more persistent frugivores and gorillas more generalist folivore-frugivores. This suggests that each species has evolved a certain range of dietary flexibility as a response to variation in food availability and the presence of a potential competitor. It is possible that greater digestive flexibility leads to the more variable dietary patterns of gorillas compared to chimpanzees and reduces interspecies competition between them. In conclusion, our study highlighted the fact that forest composition, fruit availability and dietary variability of sympatric species can vary greatly between locations, and that chimpanzees and gorillas can adapt to heterogeneous forest with few terrestrial herbs where they concentrate their diet on fruit and leaves.

Josephine Head, Christophe Boesch, Loïc Makaga and Martha Robbins

References
_{Arandjelovic, M. et al. (2010): Effective non-invasive genetic monitoring of multiple wild western gorilla groups. Biological Conservation 143, 1780–1791
Eccard, J. A., & Ylönen, H. (2003): Interspecific competition in small rodents: from populations to individuals. Evolutionary Ecology 17, 423–440
Original article: Head, J., Boesch, C., Makaga, L. & Robbins, M. M. (2011): Sympatric chimpanzees and gorillas in Loango National Park, Gabon: Dietary composition, seasonal changes and inter-site comparisons. International Journal of Primatology 32, 755–775}