Gorilla Journal 33, December 2006

How Insectivorous are Gorillas?

More than we think! Based on studies of mountain gorillas (e.g. Fossey & Harcourt, 1977), gorillas in general were thought to mainly eat leaves, stems, shoots and bark (> 90%). This idea of the overwhelmingly folivorous gorilla had to be changed when lowland gorillas appeared to include more fruits in their diet, these being much more available in lowland forests (Tutin & Fernandez 1985). Nor could gorillas any longer be called exclusively vegetarian, as Tutin and Fernandez (1983) published the first evidence of regular termite feeding by western lowland gorillas in Gabon. Even so, deliberate insect-eating by mountain gorillas was considered minimal to negligible (Harcourt & Harcourt 1984).
During the following years, however, more evidence of deliberate and regular insectivory was found, especially in western lowland gorillas (Nishihara 1992; Remis 1997). Gorillas seem to prefer ants and termites, which are absent, or less abundant, in mountain areas. As with fruits, gorillas seem to include ants and termites in their diet when available but, since insects only make up about 3% of the western lowland gorilla diet, not much attention was paid to this part of their behaviour. Until recently, only one study had been conducted - a comparative of insect-eating by sympatric chimpanzees and gorillas by Tutin and Fernandez (1992). These authors suggested that insect-eating by gorillas is nutritionally important, because gorillas eat ants deliberately and regularly, as chimpanzees do.
During the next 10 years, no recorded studies focused on gorilla insectivory. In contrast, insectivory by chimpanzees was studied more often; chimpanzees mostly use tools to gain ants and termites, which makes it more interesting to investigate. Recently, however, ant and termite eating by gorillas has been the subject of more attention, and it is more interesting and important than has been thought.
At the northern periphery of the Dja Biosphere Reserve in Cameroon (two sites: Ntonga and "La Belgique", 45 km apart), the insect diet of unhabituated chimpanzees and gorillas was studied in detail (Deblauwe et al. 2003; Deblauwe & Janssens, submitted). Gorillas at the Dja periphery have the highest frequency of insect-eating (percentage of faeces with insect remains) and the highest prey diversity ever recorded. Both are even higher than those for the sympatric chimpanzees. At "La Belgique", on the other hand, the estimated biomass of important prey eaten by gorillas is lower than that eaten by chimpanzees.
At both sites the most important prey species are different for gorillas and chimpanzees. The ants Oecophylla longinoda, Crematogaster spp. and Tetramorium aculeatum, and the termites Cubitermes spp. and Thoracotermes macrothorax, are the most important prey for gorillas, while for chimpanzees these are the army ants Dorylus spp. and the termites Macrotermes spp. Gorillas forage for ants and termites with their hands, while chimpanzees use tools to obtain termites and probably use their hands to catch army ants. After having investigated the accessibility of these prey species, it becomes clear that tool-use alone cannot explain the prey preferences, in contrast to what Tutin and Fernandez (1992) suggested. There are important differences in the nutritional composition of the gorilla and chimpanzee termite prey species and in their nutritional contribution to the ape diet, which might help to explain the variation in prey choice. Chimpanzees select fungus-growing termites high in protein, energy and manganese, while gorillas select soil-feeding termites high in iron and ash with possible anti-diarrhoeal characteristics. Termite eating in western lowland gorillas might therefore be a high quality alternative for geophagy. At this moment data analyses of the spatio-temporal availability of ants and termites and of the seasonality in the insect, plant and fruit diet of both apes at "La Belgique" are still ongoing.

Thoracotermes nest Oecophylla nest
Piece of a Thoracotermes nest, broken by gorillas, with many larvae   Leaf with Oecophylla ants
Photos: Isra Deblauwe

Inter-site comparisons demonstrate a similar frequency of insect-eating (percentage of faeces with insect remains) by gorillas in Gabon (30%; Tutin & Fernandez 1983, 1992) and the Congo Republic (24%; Nishihara 1992), which are mainly primary forest sites, while higher frequencies are found at the predominantly secondary forest sites in Cameroon (78-96%; Deblauwe et al. 2003; Deblauwe & Janssens, submitted) and the Central African Republic (42-73%; Remis 1997; Cipoletta et al., in press). Mountain gorilla groups in Bwindi feed more on army ants (Dorylus spp.) when ranging in secondary habitats than when ranging in open and mixed species forest (Ganas & Robbins 2004), although the availability of ant and termite prey species at these sites should be measured in a standardized way, before these differences can be attributed to quality differences between primary and secondary forest in the plants and fruits available to gorillas (Deblauwe et al. 2003; Ganas & Robbins 2004).
Insect prey choice by western lowland gorillas also differs between sites. Although Cubitermes termites are available at Lopé in Gabon, gorillas there do not eat them (Tutin & Fernandez 1992). Apart from Lopé, they are eaten to a certain extent at all other sites where insect-eating by gorillas was recorded, which indicates that local traditions may exist across gorilla populations (Tutin & Fernandez 1992; Deblauwe et al. 2003). Although Cipolletta et al. (in press) emphasize the importance of investigating ecological factors first and try to explain the difference in Cubitermes eating by the lower abundance of mounds in Marantaceae forest - Cubitermes is abundant at Lopé and preferred by local gorillas (White et al. 1995) - this still cannot really explain the lack of this termite in the gorilla diet at Lopé.
Lopé has the second highest density of herbs after Ndoki in Congo (Doran et al. 2002). Although gorillas at Ndoki prefer the swamp forest (Nishihara 1992), which also has low Cubitermes mound availability (Deblauwe, unpublished data), they still eat this termite deliberately. It seems that insectivory by gorillas is more complex than we assumed. Next to ant and termite availability, the techniques used by gorillas to forage on termites and ants should be investigated at different sites, as these might also reveal local gorilla traditions (Cipolletta et al., in press).
On a recent visit to Bwindi National Park (Uganda), I recorded the presence of Cubitermes in the forest there. It seems the abundance of mounds is very low, but it would be interesting to investigate the distribution of Cubitermes mounds in the Bwindi gorilla home ranges, as these termites have so far never been recorded in the mountain gorilla diet.
In conclusion, western lowland gorillas are as insectivorous as chimpanzees, but probably for different nutritional reasons. Future studies need to focus on prey availability, measurements of insect food intake (if possible through direct observations), nutritional analyses of prey, plant and fruit species, and sex-age class differences in insectivory at different sites.

Isra Deblauwe

I would like to thank MINFoF, MINRESI, Service de la Conservation de la Réserve du Dja for supporting PGS; RZSA (CRC), UA and other Belgian foundations for funding my research at the Dja Reserve periphery; and my faithful trackers Parfait & Roger.

Isra Deblauwe is finalizing her PhD thesis on insectivory by chimpanzees and gorillas in southeast Cameroon. For her PhD she worked during a total of two years in the field with Projet Grands Singes.

References
Cipolletta, C. et al. (in press) Termite feeding by western lowland gorillas (Gorilla gorilla gorilla) at Bai Hokou, Central African Republic. Am. J. Primatol.
Deblauwe, I. et al. (2003) Insectivory by Gorilla gorilla gorilla in southeast Cameroon. Int. J. Primatol. 24, 493-502
Deblauwe, I. & Janssens, G. P. J. (submitted) New insights in insect prey choice by chimpanzees and gorillas in Cameroon: the role of nutritional value.
Doran, D. M. et al. (2002) Western lowland gorilla diet and resource availability: new evidence, cross-site comparisons and reflections on indirect sampling methods. Am. J. Primatol. 58, 91-116
Fossey, D. & Harcourt, A. H. (1977) Feeding ecology of free-ranging mountain gorillas (Gorilla gorilla beringei). Pp. 415-447 in: Clutton Brock, T. H. (ed.) Primate ecology: Studies of feeding and ranging behaviour in lemurs, monkeys and apes. London: Academic Press
Ganas, J. & Robbins, M. M. (2004) Intrapopulation differences in ant eating in the mountain gorillas of Bwindi Impenetrable National Park, Uganda. Primates 45, 275-279
Harcourt, A. H. & Harcourt, S. A. (1984) Insectivory by gorillas. Folia Primatol. 43, 229-233
Nishihara, T. (1992) A preliminary report on the feeding habits of western lowland gorillas (Gorilla gorilla gorilla) in the Ndoki Forest, Northern Congo. Pp. 225-240 in: Itoigawa, N. et al. (eds.) Topics in Primatology, Vol. 2, Behaviour, Ecology and Conservation. Tokyo: University of Tokyo Press
Remis, M. J. (1997) Western lowland gorillas (Gorilla gorilla gorilla) as seasonal frugivores: use of variable resources. Am. J. Primatol. 43, 87-109
Tutin, C. E. G. & Fernandez, M. (1983) Gorillas feeding on termites in Gabon, West Africa. J. Mammal. 64, 511-513
Tutin, C. E. G. & Fernandez, M. (1985) Foods consumed by sympatric populations of Gorilla gorilla gorilla and Pan troglodytes troglodytes in Gabon: some preliminary data. Int. J. Primatol. 6, 27-43
Tutin, C. E. G. & Fernandez, M. (1992) Insect-eating by sympatric lowland gorillas (Gorilla gorilla gorilla) and chimpanzees (Pan troglodytes troglodytes) in the Lopé Reserve, Gabon. Am. J. Primatol. 28, 29-40
White, L. J. T. et al. (1995) Herbaceous vegetation in different forest types in the Lopé Reserve, Gabon: implications for keystone food availability. Afr. J. Ecol. 33, 124-141

Western gorilla overview

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