These monkeys turn into meat-eaters when plants are scarce. There’s something very human about it, says scientists

When researchers in Brazil were studying how capuchin monkeys use tools, they soon noticed that the primates were actively hunting and consuming vertebrates – so they quickly started recording more observations.

Between 2006 and 2010, they studied two groups of bearded capuchins in Fazenda Boa Vista, northeastern Brazil. They observed more than 5,000 hours of the capuchins’ behaviour and documented 446 instances of them preying upon small animals – mostly lizards and rodents.

• “It looked so wrong”: Researchers find never-seen-before footage of capuchins engaging in unprecedented behavior

• “They make the hardships seem worthwhile.” Researchers spent 33 years trekking through the Costa Rican forest. Here’s why

The study also captured the first recorded instances of bearded capuchins eating opossums, bird nestlings and an iguana.

One of the most interesting findings, published in the journal PLOS One, was that the monkeys demonstrated an increase in hunting when their preferred plant foods (such as fruits) were less accessible.

And the researchers think there is an interesting comparison between the behaviour seen in capuchins, and early humans.

• Do any primates use weapons to kill?
• What's the biggest monkey in the world? 

• Early humans may have learnt something from capuchin monkeys

“Hunting small prey could have been an important source of nutrition for human ancestors, during periods of food scarcity, providing a stepping stone to the exploitation of larger prey,” says Susana Carvalho, director of science at Mozambique’s Gorongosa National Park and one of the lead authors of the study.

“The fossil record now needs re-analysing to look at evidence of small vertebrate predation.”

However, they warn that studying these primates cannot tell us directly how the human diet might have evolved.

• What's the difference between humans and apes?

Instead, “these behaviours remind us that the incorporation of animal foods and the behavioural flexibility required to obtain them are not unique to humans or to the great apes,” explains Noemi Spagnoletti, from the Sapienza University of Rome and one of the study’s lead authors.

“Looking at how similar feeding strategies have evolved independently in other primates can therefore help us better understand the evolutionary processes that may have shaped different components of the human diet.”

The researchers also discovered that the monkeys often ate the brains and guts of prey first, seemingly prioritising these fat-dense tissues. Compared with female monkeys, male monkeys went after more dangerous prey such as snakes and hunted prey more frequently.

Top image: adult male capuchin feeding on bird prey. Credit: Luciano Candisani, CC-BY 4.0