Gorillas May Not be Vegetarians

Reputations in the mist

Mar 26th 2009
From The Economist print edition

GORILLAS are gentle giants, chimpanzees aggressive killers and bonobos sex-crazed vegetarians. That, at least, is the PR which has been created around the three species of ape most closely related to man. They are simple stories—and simple stories sell books, TV programmes and even, whisper it softly, newspaper articles.

In nature, of course, things are a little more complicated. Chimpanzees are undoubtedly successful hunters, but the “wars” seen between neighbouring bands seem to have been brought on by human encroachment on their habitat rather than original sin. Bonobos, too, have been shown in the past year to hunt animals for food, and are losing their promiscuous aura as more data come in. Only gorillas have retained their reputations intact. Until now.

Part of the reason for the stories was that apes are hard to watch in the wild, and few people are prepared to put in the time needed to do so. Early observations were assumed to be representative, but were based on small samples. One way to extend those observations is to study animals in zoos.

And that is what Steve Ross, a primatologist at Lincoln Park Zoo in Chicago, has done. He has looked, in particular, at the interactions between zoo-held apes and local birds and small mammals. Some of his findings cast further doubt on the stereotypes.

Mr Ross and his colleagues, who have just published their results in the American Journal of Primatology, asked 71 zoos around the United States about interactions between the local wildlife and any bonobos, chimpanzees, gorillas or orangutans in their collections. The chimps did live up to their reputations. Four-fifths of the zoos that had them reported that they had killed at least one local animal during the past five years. Two-fifths of zoos with orangutans had likewise seen those animals hunt successfully over that period—which might surprise the layman, but such hunting has been seen in the wild. The most aggressive apes of all were the bonobos. In every zoo that kept them, these allegedly vegetarian creatures had killed visiting wildlife in the past five years.

But it was the gorillas that surprised the researchers most. In the wild, they seem to eat only plants and insects. Yet 30% of the zoos in the survey reported that their gorillas had killed some local wildlife, and in at least one case had eaten a bird.

This seemingly uncharacteristic aggression may be the result of captivity. Aberrant behaviour, often brought on by boredom, is not unusual in zoo animals. In fact, bonobo promiscuity seems to be an example of this. Wild bonobos (those few that have been watched carefully, at any rate) do not seem overly promiscuous. It is also possible, though, that gorillas hunt in the wild, but that no zoologist has yet observed them do so. If that is the case, then another carefully constructed zoological reputation will bite the dust.

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