A capuchin named Joker started the apparent trend, leading researchers to suspect "there was something a little quirky about him, or that he was kind of lonely in a way.”

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.