Video footage shows male capuchin monkeys carrying at least 11 howler monkey babies between 2022 and 2023. The monkeys' motivations remain under investigation.

The scientists nicknamed one capuchin kidnapper Joker because the scar at the side of its mouth reminded them of the "Batman" villain.

Capuchins are also known for developing odd social rituals across their range. Scientists studying mainland populations have noted monkeys poking fingers up each other’s noses and in each other’s eyes, biting tufts of fur off of each other, drumming on a partner’s belly or back, and sticking hands in other capuchins’ mouths, says Perry.