A group of chimpanzees travels through the woodlands of Gombe National Park, Tanzania, where Jane Goodall first began studying their kind back in 1960. They come upon red colobus monkeys.

Chimps survey their prey. A hunt begins. Chaos ensues as monkeys fall from trees to the screams of chimps as they make their kill—all of it caught on video.

Ian Gilby, an anthropologist at Arizona State University and leader of a new study on the subject, originally filmed members of the habituated Kasekela chimp community in Gombe to learn more about how they share meat. (Read why chimps trade meat for sex.)

Reviewing the videos later, he noticed that chimps eat subadult prey—infants, juveniles, and adolescents—heads first. Chimps consuming adult prey show less of a pattern, he found.

This left him with a little-studied question that's relevant to how humans evolved: Why would the apes prefer to eat a particular body part first?

Brain Food

Gilby thinks it has to do with nutrition.

“We tend to just say meat is meat, but we know that the nutrient composition varies," says Gilby, whose study appeared recently in the International Journal of Primatology. "The whole carcass is valuable, but the brain is especially valuable."

Brains are high in fat and a source of long-chain fatty acids, which aid in neurological development.

View Images A chimpanzee captures a young red colobus monkey during a hunt. Photograph by Ian C. Gilby

And while a chimp might be able to crack a young monkey's skull with a bite, the brains of adult monkeys aren't as easy to access. Taking the time to try might cause the chimp to lose its kill to competitors.

Instead, when killing adult monkeys, chimps might find it more efficient to start with nutrient-rich organs like livers: Likely why the Gombe chimps sometimes targeted adult monkeys' torsos first.

"It might be one of the first quantitative studies about how exactly a prey item is eaten by chimps," says Jill Pruetz, a biological anthropologist specializing in primatology at Texas State University and a National Geographic explorer.

Nutrition vs. Culture

There's also debate about why the omnivorous apes hunt at all, considering meat isn't a diet staple.

Pruetz supports the hypothesis that chimps are seeking nutrition, but she says it can’t explain all differences across study sites.

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For example, some chimps eat eggs while others don't. And at Fongoli, when preying on baboons, chimps mysteriously sometimes give the heads away, and guts are often discarded.

She suspects that cultural traditions and learned traditions among populations may be more at play.

Meat Drove Evolution?

The species that led to early hominids started eating more meat, Gilby says, and his study suggests that a need for fat may have been the motivation.

"One of the best ways we have of understanding early hominids is using chimps as a model," Pruetz says.