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 the left. Handedness doesn't seem to explain it; left-handed mothers have the same tendency, for one thing. Rather, researchers think mothers cradle their babies this way because the left eye is linked to the brain's right hemisphere, which processes social and emotional information. The bias might be useful for the baby too, since a baby cradled in his mom's left arm looks back at her with his own left eye. Biologists at St. Petersburg State University in Russia wanted to look for the left-side phenomenon in mammals that aren't as closely related to humans. They chose Pacific walruses and Indian flying foxes, a kind of large bat. Mothers of both species snuggle with their babies face-to-face, similar to a human mom cradling her infant. From a cliff on a Russian island, the researchers observed several dozen pairs of walrus mothers and calves resting in shallow water. When they hung out face-to-face, as in the photo above, these moms and babies were more likely to keep each other on their left sides. And when calves floated horizontally next to their mothers before diving underwater to nurse, as in the photo below, they usually kept their moms to their left.
The researchers also observed more than 250 mom-and-pup flying fox pairs hanging from tree branches in Sri Lanka. When pups were resting against their mothers or licking their mothers' faces (a and b, below), they preferred the left.
About half the flying fox pairs were nursing when researchers observed them. In this position, pups were equally likely to be on either side. But when pups were simply hanging by their mothers' sides, they usually kept Mom in their left field of vision. The scientists think this is evidence that many mammal moms prefer to keep their left eye on their babies, and vice versa. The left eye means the right brain, "which plays a crucial role in a variety of social cognition tasks ranging from face and emotion recognition to spatial coordination and social learning," the authors write. Unlike baby bats and walruses, human infants don't get to position themselves. But when their mothers hold them on their left sides, it might be what the babies prefer too. Images: Giljov et al.