It's long been observed that mothers tend to cradle their infants on their left side, and this has long been at least informally attributed to handedness (so that right-handed mothers have the right hand free).
Now researchers report in the journal Nature Ecology & Evolution that "positional bias" is in fact observed in multiple mammal species, and they say the reason for it is likely neurological, as the BBC reports.
Turns out that this positioning on the left activates the brain's right hemisphere, responsible for processing social activities like communication and bonding. "We suggest that this bias is even more widespread and may be a characteristic of all mammals, with few exceptions," one researcher says.
Studying wild animals—including horses, walruses, reindeer, sheep, and kangaroos—the team found that young animals tend to keep their mothers on their left and watch her with mainly their left eye.
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In dangerous situations, however, the mothers would swap spots to keep their young on their left for better monitoring, reports New Scientist. The scientists recorded nearly 11,000 position choices across 175 pairs of infants and mothers.
That the positional bias is so widespread among mammals suggests, as one researcher put it, that the mechanism is "ancient and really basic." Meanwhile, the Orange County Register reports on a thrilling drone sighting of a mother and calf whale migrating south—with predictable positioning.
(Here's why viruses go easier on females.)
This article originally appeared on Newser: 'Ancient' Signal Dictates Where Mom Holds Baby