Chimpanzees have almost hunted one of their prey – the red colobus monkey – to local extinction. The population size of the red colobus monkey has dropped dramatically, by 89% between 1975 and 2007. Since decimating the red colobus monkey population, chimpanzees are turning to new prey, and reducing their population size too.

Some non-human primates (NHPs) hunt other NHPs as prey. The chimpanzees and red colobus monkeys are a classic example. In the forest in Kibale National Park, Uganda, chimpanzees prey heavily on red colobus monkeys. Researchers observed 556 hunts between 1995 and 2014. 356 of these attacks were directed at red colobus.

In the study, researchers note that “the number of red colobus killed was about 15 times the number of guerezas, the second most common prey, and 15 times the combined number of kills of redtails and gray-cheeked mangabeys.”

Researcher David Watts told BBC Earth’s Michael Marshall that the chimpanzees “have been so effective as predators that they really have knocked down the local population.”

The success rate for red colobus hunting has been “unusually high”. As Marshall explains, red colobus are being killed at an incredible rate because these monkeys are choosing to “stand their ground” instead of just running away. While this could work in an even one-to-one fight, chimpanzees hunt in large groups. In the end, the red colobus doesn’t really stand a chance.  

Researchers set out to investigate whether the rapid decline of the red colobus monkey population is forcing chimpanzees to mix it up and hunt for new prey. They note that chimpanzees are not “obligate carnivores” – meaning they don't only eat meat – and most of their diet is comprised of fruit and figs.

Chimpanzees could be continuing to hunt prey for meat – even when there’s a low abundance – for its nutritional value. Meat could also be an important tool for mating and other social relationships within chimpanzee communities.

^{Red colobus monkeys in Uganda. Sam DCruz/Shutterstock.}

The study, published in the International Journal of Primatology, found that the rates at which chimpanzees hunt mangabeys and redtails have gradually increased. Researchers found that after 2002, when there was a peak in red colobus predation, there was an increase in hunting of other groups. The overall hunting has not declined for chimpanzees, which could indicate that the hunting of alternative prey has increased enough to offset the decline in red colobus.

The switch in prey is bad news for rarer monkeys like the black-and-white colobus, who are also being hunted. Watts told BBC Earth “I've been here a little over a month. The chimpanzees have hunted quite often in that time. They've hunted black-and-white colobus eight times. I've never seen anything like that before.”

It remains unclear whether the reduced red colobus hunting frequency and increase in prey switching have allowed the population to recover. Renewed long-term sampling of the area from 2006 through 2009 will help to provide an answer, but the data of that work is not yet available. Researchers are also unable to say whether young chimpanzees who have matured and started to hunt regularly since the predation peak in red colobus in 2002 are less effective than chimpanzees who matured earlier as they’ve had less experience hunting them.

[H/T BBC Earth]