Red colobus monkeys in Uganda's Kibale National Park are being hunted to extinction—by chimpanzees. According to a study published May 9 in the American Journal of Primatology, this is the first documented case of a nonhuman primate significantly overhunting another primate species.

(The taxonomy of Ugandan red colobus monkeys is in dispute. Some scientists consider them a species, Procolobus tephrosceles, whereas others identify them as a subspecies, P. rufomitratus tephrosceles.)

The study examined nearly 33 years of primate census data (covering 1975 to 2007) and found that the endangered Ugandan red colobus monkey population dropped 89 percent during that time, mainly as a result of being hunted by common chimpanzees (Pan troglodytes).

There were other factors affecting the colobus monkeys' survival, including disease and competition from other herbivorous monkeys, which are being crowded into smaller areas as their forest habitats shrink, but those were found "to be relatively insignificant compared with predation by chimpanzees," according to the paper. The decline was furthered by the fact that chimpanzees preyed more on juvenile monkeys which had not grown old enough to reproduce, hurting the population's chance to bounce back.

Meanwhile, the authors found that the number of chimpanzees in the area has risen by 53 percent.

The decline in red colobus monkeys had previously been observed in a 2008 paper published in the journal Primates, but that study only analyzed population data spanning three years. Also, in 2008 the International Union for Conservation of Nature and Natural Resources listed the Ugandan red colobus as endangered on its Red List of Threatened Species, because of chimpanzee predation and severely fragmented subpopulations.

So why are the chimpanzees thriving and overhunting the red colobus monkeys, yet leaving other monkey species alone? That's unclear—it's uncertain whether any other source of chimpanzee food has declined or if drought or any other factors may have influenced the change in hunting patterns.

What is clear is that in the last few years chimpanzees have actually reduced their hunting activity for some reason. As a result, younger chimpanzees are not as skilled at it, which means they red colobus may yet have another chance in the area.

Photo: Red colobus monkey in Uganda by Rob Gipman. Via Flickr under Creative Commons license