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Our Killing Cousins

By Elizabeth Preston
May 24, 2011 9:54 PMNov 5, 2019 12:22 AM


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If you're feeling any guilt about belonging to the one species that encroaches on other animals' habitats, hunts them for sport, and drives them to extinction, consider this: we're not alone. There's another species capable of hunting its neighbor to near annihilation. Of course, it's our closest relative.

Chimpanzees eat mostly fruit and other plant matter. They were believed to be complete vegetarians until Jane Goodall first witnessed a chimpanzee hunt in the 1960s. An organized hunting party may chase a young monkey through the treetops, some chimpanzees blocking its escape routes while others wait in ambush. Meat is not only a nutritional resource for a chimpanzee community, but a social and political tool.

One of chimps' favorite animals to prey on is the red colobus monkey. And a new paper says that in Uganda's Kibale National Park, chimps are taking more than their fair share of monkeys. The red colobus population has declined sharply over the past few decades, and if they don't rebound soon, they could be wiped out entirely.

Previous studies had suggested that the red colobus monkey population in the park was declining, and that chimps might be to blame. So an international group of researchers set out to settle the question. They gathered data in the park for nine years, then added it to data from several previous studies to produce almost 33 years' worth of observations.

Rather than trying to find and count every primate in the forest, researchers took a kind of cross-section by walking along a predetermined route and recording what monkeys and apes they crossed paths with.

What they found was quite pronounced: between 1975 and 2007, the population of red colobus monkeys decreased by 89%. Their decline was tied to a significant, though less dramatic, increase in the chimpanzee population. The researchers ruled out various other explanations for the apparent red colobus loss, including disease, lack of food, shyness around strolling scientists, and being eaten by eagles. Habitat loss and poaching, the usual human-based causes of species loss, were ruled out by the national park's protections. The chimpanzees, with their suspiciously expanding population, were the only remaining culprits.

This is the first time scientists have observed one species eating another species into a serious population slump. Usually, non-human species are forced to maintain a balance with one another. But the chimps at Kibale are managing an eerily human overuse of resources.

Of further concern is the fact that both species are already endangered, thanks to humans. The red colobus population might not be so fragile if we hadn't driven its numbers down to begin with. Or perhaps overhunting is a new chimpanzee behavior, an adaptation that would have appeared without our interference. Either way, now that one endangered species is threatening the survival of another, can conservationists afford to stay out of the animals' business?

The authors of the study think there's hope for the red colobus in Uganda, because their population's plummeting seemed to slow or stop near the end of the study period. Observations by other scientists suggest that the monkeys in the park have dramatically expanded their ranges since the 1970s; that is, individual monkeys seem to roam over a much wider area of land than they used to. This may be their attempt to avoid hunting chimpanzees. If it works, the monkeys will have saved their species from not one, but two primate threats.

Images: Thomas Lersch/Wikimedia Commons, Olivier Lejade/Wikimedia Commons

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