Among the many similarities between our species and one of our very closest relatives, the common chimpanzee, is a fondness for red meat. (Okay, okay, so not all people have this fondness by any means, but we're generalising here.) 

Though much of their diet consists of plants and fruit, chimpanzees appear to relish meat – and they can be quite capable hunters of prey such as small antelope, bush pigs and especially monkeys. A freshly caught monkey or fawn can get an entire chimp group worked up to a frenzy, with a whole lot of beseeching hands outstretched to whatever ape has the delicacy in possession.

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An alpha male chimp tucks into a meaty meal, with hungry onlookers hoping for scraps. Image: Cat Hobaiter

Despite some broad similarities, however, chimps from different regions don't show all the same predatory preferences or traditions. And a new study from a long-term research project in Uganda's Budongo Forest – a large tract of tropical forest along the Albertine Rift – has revealed some fascinating differences in hunting and meat-sharing between neighbouring chimp groups.

Dr Catherine Hobaiter of the University of St Andrews and her colleagues compared the Sonso community of chimps, which has been closely studied since 1990, with their neighbours to the northeast, the Waibira chimps, which researchers began habituating in 2011.

"The differences in hunting between these communities are dramatic – so we wanted to try to understand why. They live in the same forest and have access to the same prey, but they hunt for different species and seem to share food differently," Hobaiter said in a press release about the new study.

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On the hunt for a Guereza colobus monkey, the favourite prey of the Sonso chimps. Image: Cat Hobaiter

Because the two populations inhabit adjoining ranges, behavioural disparities between them are less likely to be explained by fundamental ecological differences. And they probably wouldn't stem from significant genetic differences, either, because there's some dispersal between the two populations: when the team began following the Waibira clan, for example, they found several Sonso-born females as members. 

Habituating two chimp clans that share genetic flow and the same basic habitat gives biologists a rare opportunity to assess whether a behaviour unique to one group is "a socially acquired 'cultural' variant", as the researchers put it in a paper just published in PLOS ONE.

Both the Sonso and Waibira chimps hunt, but it turns out what they hunt and how they dine differs between the two communities. Sonso chimps seem to especially favour Guereza colobus monkeys, the prey in about 74 percent of observed hunts. Blue monkeys were the next most frequent target, with red-tailed monkeys, olive baboons, blue duikers and elephant shrews accounting for the rest. Once, and once only, Sonso chimps were seen to pursue a red duiker – and they came up empty. 

By comparison, Waibira chimps were most successful at hunting both blue and red duikers, though they also pursued Guereza colobus as well as blue and red-tailed monkeys. (That's a lot of blues and reds floating around.) The carnivorous component of their diet, in other words, was more evenly split between primates and non-primates.

That's not the only contrast the researchers uncovered. After Sonso chimps successfully killed an animal, high-ranking males tended to commandeer the carcass even if they hadn't done the capturing, often aggressively harassing subordinate chimps in the process. 

Adult male chimps claim the spoils, leaving younger group members to beg for scraps. Video: Liran Samuni

In the Waibira community, the actual hunter would much more frequently get to keep its prize – whether or not chimps higher up in the hierarchy were present during mealtime. Sonso-style harassment was less common, with more polite begging being the norm.

"One of the most remarkable differences we see is how the groups share meat," Hobaiter said. "In Sonso, it is rank-based – the most dominant chimp gets the meat, even if he wasn't the one that caught it. In Waibira, we sometimes see younger individuals, even young females, keep the whole carcass, and even if it's the alpha male who arrives, she'll refuse to share it."

Despite begging from a fellow group member, a young female (right) refuses to share her duiker dinner. Video: Liran Samuni

So why these differences between two chimp populations in the same forest? Let's start with prey preference. 

The researchers note that in a number of other regions, chimps seem to have a distinct appetite for the red colobus monkey as prey (including in places inhabited by both red and Guereza colobus).

"It appears that in the absence of red colobus, which are not present in the Budongo Forest, Sonso chimpanzees have instead specialised in hunting Guereza colobus monkeys," the team writes. "Feeding on the meat of other species appears less preferred, with individuals even discarding the carcass uneaten."

Waibira chimps, on the other hand, seem to get equally excited over a duiker dinner and a course of colobus. That's true even of the females in the community that were born into the Sonso group, which doesn't seem to have red duiker on the menu.

Interestingly, though, the Sonso chimps displayed less choosy hunting habits (similar to the Waibiras) in the early years of the Budongo Forest research. In other words, they hunted monkeys other than colobus as well as blue duikers (but not red) more often than they do now. (A similar gradual shift to a colobus-heavy diet has also been documented in the well-studied chimps of Tanzania's Mahale Mountains.)

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A dominant Sonso male with a meaty meal. His clan was once less choosy with the meat on the menu. Image: Jakob Villioth

Chimps are generally better at catching colobus when hunting in groups, whereas they can be quite successful solo hunters when chancing upon other prey species. Perhaps, the researchers speculate, habituating chimps to human presence in order to study them initially hampers the more elaborate, coordinated strategies the apes employ when targeting colobus. 

"Long-term research with wild chimpanzees brings real conservation benefits, 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," Hobaiter said.

But what about red duikers, which Sonso chimps did not hunt even during their less fussy meat-eating era? The Waibira group's preference for the antelope, the researchers propose, may be a case of distinct "socially learned traditions" – along the lines of a cultural difference.

Meanwhile, the differences in mealtime etiquette between the Sonso and Waibira chimps could possibly stem from habituation effects, but there may also be some other social process at play. The ins and outs of possessing and distributing meat vary between chimp populations (though, again, the Budongo Forest study is interesting because of the proximity of the two studied populations), and evidence suggests the process helps reinforce specific hierarchies and social bonds within a particular group.

The Budongo Forest raises some intriguing possibilities about how behaviours and customs arise within chimp communities – and how scientists themselves may influence the apes. 



Top header image: Catherine Hobaiter