Chimps are known to be methodical hunters, surrounding an unfortunate monkey victim before striking it dead, tearing the prey apart, and sharing the spoils. But it turns out that this behavior may be influenced by human observers, who have spent decades habituating the apes to allow them to watch.

Researchers observing these chimps in the forests of Uganda have noticed that two separate communities display vastly different hunting behaviors, despite living in the same environment and having access to the same prey. While one group, known as the Sonso community, focuses almost exclusively on black and white colobus monkeys, another called the Waibira chimpanzees seem to show an equal preference for small antelopes, while also catching a wider variety of primates.

The primatologists also noted that once a hunt was finished, they divvied up the meat differently. In the Waibira community, whoever caught the prey (regardless of status) got to keep hold of the lion's share. However, in the Sonso group, the alpha male would take control of the meal even if he failed to take part, while the rest of the community begged for scraps.

What is fascinating is that the researchers think that these differences may have resulted from humans trying to habituate the apes to their presence. In order to study animals in the wild, researchers often have to spend years gaining their trust, until the animals in question no longer see humans as a threat and, in theory at least, act naturally. While people have been studying the Sonso group for 27 years, the Waibira have only been habituated to humans for the last six.

After looking at the data collected from when the Sonso community had just been habituated, they found some striking similarities between how they hunted back then and how the Waibira group hunt now. At first, it seems they shift to a more "opportunistic" hunting style in the presence of humans. 

When researchers first started watching the Sonso group, the chimps hunted a much wider variety of prey and focused a lot of attention on small antelope. After about 14 years, they began focusing almost exclusively on one species of primate. The researchers suggest it has taken this long for the apes to finally return to their natural behavior.

"For Sonso – most of the current generation of adults were born with us being there, so they're really incredibly relaxed about our presence," said Dr Catherine Hobaiter, who led the research in PLOS One, to BBC News. "But [for] Waibira, some of the young ones have started to grow up and become very comfortable with us, but some of the adults would be 30-40 years old when we started, and five years of us following them round is a fraction of their lifetime."

“Long-term research with wild chimpanzees brings real conservation benefits,” explains Hobaiter in a statement, “but we have to remember that our presence can affect their behaviour; in this case the group hunting used to catch colobus monkeys may take years to re-establish.”

This raises interesting questions regarding wild ape research, particularly at the start of the habituation process, and how careful researchers need to be when interpreting behavior.