Plus, researchers could hear adult howler monkeys calling back and forth with the infants, who would sometimes try to crawl away. Camera footage showed one occasion when an adult howler came and tried to retrieve a baby, and capuchins scared it off.

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

A new trend is catching on among bored young male capuchins: kidnapping baby howler monkeys, in what scientists say is the first time animals have been recorded stealing another species' infants