Human moms prefer to hold their babies on their left sides. Although this does make it easier for right-handed parents to feed themselves and do other necessary tasks, scientists think the true explanation is deeper. Now, a study of walruses and bats has shown that mothers and babies in these species also cuddle on the left—even when the baby is the one choosing the side.

Repeated studies have shown a bias among human mothers, as well as chimps and gorillas, for holding their infants on t