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.
 |
 |
 |
| 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