While chimps’ diet is mainly vegetarian, they will sometimes supplement it with other types of nutrients. Turns out, crabs are also on the menu.

“Our study is the first evidence showing that non-human apes regularly catch and eat aquatic fauna,” says Kathelijne Koops, one of the study authors.

Chimpanzee fishing for crabs. Image credits: Kathelijne Koops.

Chimps eat a lot of plants, including fruits, leaves, nuts and seeds. They’ll also occasionally eat meat or even honey, which they have advanced methods of accessing. However, they’re still largely vegetarian, with animal products accounting for around 6% of their nutrient intake. A new study, however, found a new entry on the chimps’ menu: crabs.

Koops and colleagues found that chimps in the rainforests of the Nimba Mountains in Guinea searched for streams in the forest. After finding a stream, they scratch its surface, churning the riverbed and looking for crabs to eat.

Chimps ate crabs all ear round, regardless of whether there were ripe fruits available as an alternative. Their preference for crab was also equal in the dry and rainy season, as even during the dry season, there’s still enough water to host some crabs. The one thing that was counterbalanced by crabs was ant consumption: the more insects they ate, the fewer crabs, and vice-versa, suggesting that crabs and ants have similar nutritional value for chimps.

This wasn’t the only surprise, however.

“Female chimpanzees and their offspring fished for crabs more often and for longer than adult males, which we had not expected,” says anthropologist Koops. A possible explanation for this is that the nutrients in the crabs (especially fatty acids) are better for maternal and infant health, but it’s unclear if this is the case. Mothers were also found to teach their children the crab fishing technique.

These findings don’t just shed new light about our closest living relatives, but they could also teach us a thing or two about our own evolution. Anthropological research has shown that for hominins, aquatic fauna has become a more and more important source of nutrition over time, with one theory stating that consuming aquatic fauna helped evolve bigger, more potent brains, as they contain high amounts of unsaturated fatty acids.

“The aquatic fauna our ancestors consumed likely provided essential long-chain polyunsaturated fatty acids, required for optimal brain growth and function,” said first author Dr Kathelijne Koops.

For now, however, this is still a theory. What is clear is that chimps sure love their crab.

Journal Reference: Koops et al. Crab-fishing by chimpanzees in the Nimba Mountains, Guinea. Journal of Human Evolution. Doi: 10.1016/j.jhevol.2019.05.002.