Here's a site that provides citations on both sides--or more properly, all sides--of this argument. Citing a great deal of the-peer-reviewed literature. I'd suggest reading it, and then reading the various sources it cites.
http://www.beyondveg.com/index.shtml
More specifically, this page addresses the question of how the diets of the great apes have been and should be categorized.
http://www.beyondveg.com/billings-t/comp-anat/comp-anat-2a.shtml
Here's a way-too-long excerpt:
"Vegetarian" apes: a misconception of the past
First, as mentioned above, the idea that apes (or primates in general) are strict vegetarians in the normal human sense of the word is a misconception of the past. This point is clarified in Sussman [1987, pp. 166, 168]:
In fact, most species of primate are omnivorous (see Harding [1981]) and omnivory should be considered an evolutionarily conservative and generalized trait among primates. Primates evolved from insectivores....
Thus, omnivorous primates are mainly frugivorous and, depending upon body size, obtain most of their protein from insects and leaves. In all large, omnivorous, nonhuman primates, animal protein is a very small but presumably necessary component of the diet.
In the above, the term omnivore has the usual definition; e.g., from Milton [1987, "By definition, an omnivore is any animal that takes food from more than one trophic level. Most mammals are in fact omnivorous...". ["Trophic" refers to the different levels of the food chain.] Note that some experts use a different, more precise definition for the term omnivore, and disagree that mammals are omnivores--instead they suggest using the term faunivore for animals that regularly include fauna (other animals) in
Insect food
Regarding consumption of animal foods by primates, Hamilton and Busse [1978, note:
Many primate species once considered herbivorous are now known to expand the animal-matter portion of their diet to high levels when it is possible to do so...
Insect food is the predominant animal matter resource for primates. Insects are eaten by all extant apes, i.e., chimpanzees (e.g., Lawick-Goodall 1968), orang-utans (Gladikas-Brindamour1), gorillas (Fossey2), gibbons (Chivers 1972, R.L. Tilson3), and the siamang (Chivers 1972). The amount of insect matter in most primate diets is small, but may expand to more than 90% of the diet when insects are abundant and easily captured...
Preference for animal matter seems confirmed.
Note that the footnote numbers in the quote above refer only to Hamilton and Busse [1978].
Rationalizations about Dietary Deviations among Primates
Fit Food for Humanity does include notes on "Dietary Deviations Among the Primates" (pp. 11-12). It is interesting to note that most (but not all) of the references cited therein are encyclopedia entries--which usually do not reflect the latest research. The response in
Fit Food for Humanity to the information that anthropoid apes are not strict vegetarians could be characterized as reliance on outdated information, rationalizations, and hand-waving.
Let's review some of the claims. ("FFH" is used as an abbreviation for
Fit Food for Humanity in the material below.)
FFH: Gorillas are total vegetarians.
REPLY/COMMENTS: Both lowland and mountain gorillas consume insects, deliberately and indirectly, that is, on the vegetation they consume. The above quote from Hamilton and Busse [1978] cites Fossey (personal communication) regarding insect consumption by mountain gorillas. Tutin and Fernandez [1992] report consumption of insects by lowland gorillas in the Lope Reserve, Gabon: termites (whose remains were contained in 27.4% of gorilla feces) and weaver ants. Note that both insects mentioned are social insects; the consumption of social insects is efficient, as their concentration in nests allows easy harvesting of significant quantities. Of further interest here is the information that termites are known to contain significant quantities of vitamin B-12; see Wakayama et al. [1984] for details. Insectivory by mountain gorillas is discussed further later in this section.
FFH: Orangutans consume 2% insects; from p. 11: "the 2% digression may be seen as incidental and insignificant."
REPLY/COMMENTS: The quote from FFH does not specify whether the 2% is by weight or feeding time. Due to the difficulties in estimating weights of foods consumed, the 2% figure is probably by feeding time. Galdikas and Teleki [1981] report that orangutans at Tanjung Puting Reserve in Indonesia consumed 4% fauna (insects, eggs, meat) by feeding time. Kortlandt [1984] reports that (p. 133), "orang-utans eat honey, insects and, occasionally, bird's eggs, but no vertebrates."
For photos of a wild orangutan eating insects, see Knott [1998], p. 42; and for a photo of a wild orangutan eating a vertebrate--a rare event--see Knott [1998], p. 54.
The claim that insect consumption by orangutans is "insignificant" is clearly an unproven assumption. Insects and other animal foods are nutrient-dense foods: they supply far more calories and nutrients per gram of edible portion than the same weight of most of the plant foods commonly consumed (i.e., fruits other than oily fruits, and leaves).
FFH: The principal rationalizations given for termite and meat-eating by the chimps of Gombe Preserve are:
- The Gombe Preserve is small, and surrounded by human-populated areas.
- Chimps are "natural" imitators.
FFH then implies (assumes) that the behavior of the chimps of Gombe is in imitation of human behavior. Other writers (elsewhere, not in FFH) suggest that chimps eat meat in imitation of baboons.
REPLY/COMMENTS: The reality is that predation on vertebrates by chimpanzees is widespread throughout tropical Africa. Regarding chimpanzee predation, Teleki [1981, p. 305] reports that:
Moreover, predatory behavior involving vertebrate prey has now been recorded at all major study sites in equatorial Africa, from Uganda and Tanzania to Sierra Leone and Senegal. I expect that the known geographical distribution of predatory behavior will continue to expand as new chimpanzee projects are launched, though it is probable that some populations practice this behavior little or not at all, while others do so regularly and systematically (Teleki, 1975).
Note in the above remark that predation by chimps has been found at all major study sites, although it is possible that some groups of chimps hunt rarely, or not
Over and above the reality that predation and meat-eating by chimps is widespread, the claim that chimps do so in imitation of humans or baboons is both unproven and dubious. Chimps have lived in proximity to both humans and baboons for approximately 2-2.5 million years. This alone suggests that sufficient time has elapsed, in evolutionary terms, for chimps to adapt to such allegedly "imitation" behavior. Once evolutionary adaptation occurs, the "imitation" behavior would no longer be an imitation (supposing it were that, in some hazily conceived past)--it is natural. This reasoning suggests that the "imitation" argument is dubious at best. Another problem with the "imitation" argument is that imitative learning in captive chimps is common, but in wild chimpanzees it is rare; see Boesch and Tomasello [1998] for discussion on this point.
Chimp/Baboon Interaction. The interaction between baboons and chimps is quite interesting and serves to illuminate the shallow nature of the "imitation" argument. Teleki [1981, pp. 330-331] comments on:
...the anomalous nature of an interspecific relationship that includes play with baboons, consumption of baboons, and competition with baboons for at least one kind of prey...
[T]he Gombe chimpanzees removed 8% of the local baboon population in 1968-1969 (Teleki, 1973a) and 8-13% of the local colobus population in 1973-1974 (Busse, 1977). How is it possible, then, that the primates serving as prey to chimpanzees in Gombe National Park, and possibly also at other sites, have not developed more successful defensive tactics? Any answer other than the proposition that chimpanzees have only recently acquired predatory inclinations, for which there is no supportive evidence at all (Teleki, 1973a), would be welcome."
Note: The above quote is included to specifically inform readers that there is no evidence that predation by chimps is a "new" behavior, and that there is extensive, complex, baboon/chimp interaction.
Insect Consumption by Chimps is Universal. Kortlandt [1984] also discusses insect consumption by chimps (p. 133):
Chimpanzees, on the other hand, spend a remarkable amount of time, mental effort and tool use on searching out insects and feeding on them in every place where they have been intensively studied. Hladik and Viroben (1974) have shown that this insect food is nutritionally important in order to compensate for a deficiency of certain amino acids in the plant foods, even in the rich environment of the Gabon rain-forest.
GO TO NEXT PART OF ARTICLE
(Ape Diets: Myths, Realities, and Rationalizations, cont.)p. 93]:their diet.p. 761]at all.
I'll just point out this is 1) not peer reviewed literature, and 2) this is an organization with an agenda, which might just affect how they will describe the diets of those species they wish to "defend.".
Beyond Veg is also not a peer-reviewed site, but it at least provides citations for its discussion.