Can’t get enough of those yummy colobus (Tim Laman/National Geographic/Getty)

Humans have a bad reputation for overhunting some animals, but a new study suggests we’re not the only ones. Chimpanzees in Uganda have overhunted red colobus monkeys, causing their local population to fall to one-tenth of what it was just 33 years ago, a new study has found. This is the first time a non-human primate has been shown to overhunt another, leading to a population decline.

The chimps in question live in the forests of Ngogo, in Uganda’s Kibale National Park. Like the chimps in Tanzania and Ivory Coast, they are skilled hunters and work together to catch their monkey prey – typically the red colobus (Procolobus rufomitratus tephrosceles).

Thomas Struhsaker of Duke University in Durham, North Carolina, and colleagues have been regularly surveying the chimp and red colobus populations in Kibale, along with the populations of another six monkey species. They have now compiled all their census data between 1975 and 2007 to see how the different species have fared

Ups and downs

All the populations changed significantly, with some growing and others shrinking. The biggest change was seen in the red colobus population, which decreased by about 89 per cent. A previous study suggested a similar decline, but did not test its statistical significance.


Estimating changes in chimpanzee populations is more difficult due to the tendency of chimps to spend time on their own, says Struhsaker, but his team noticed that the number of chimps they have sighted over the years has risen by 53 per cent, which he says suggests that they were prospering at the increasing expense of their prey.

The number of red colobus killed by the chimpanzees in Kibale National Park had increased from an estimated 167 per year in the late 1990s to 322 in 2002. During that time, the chimpanzees killed between 15 and 53 per cent of the red colobus population each year, taking a particularly heavy toll on young animals that had not yet reproduced.

Bounce back

Struhsaker examined other factors that might have caused the red colobus to decline, such as disease, competition with other monkey species, changes to their habitat, and predation by crowned eagles. All had a much smaller effect than predation by chimpanzees, suggesting the chimps were mostly to blame.

David Watts of Yale University in New Haven, Connecticut, says the findings are in line with his own observations of the Ngogo monkeys. “The rate at which I encounter red colobus has gone way down,” he says.

Watts says the chimps are now hunting less than they did, so young males are not getting as many opportunities to acquire the skill. That means the red colobus may not face as great a threat from them in the near future, and so could yet bounce back.

Journal reference: American Journal of Primatology, DOI: 10.1002/ajp.20965