Why mums and babies prefer to keep to one side of each other

Stefanie Neumann/plainpicture

By Alice Klein

Mothers hold their children more on the left and wild mammals seem to keep their young more on that so too, at least when fleeing predators.

Now it seems many mammal babies prefer to approach their mother from one side too – and the explanation may lie in the contrasting talents of each half of the brain.

In mammals, the brain’s right hemisphere is responsible for processing social cues and building relationships. It is also the half of the brain that receives signals from the left eye.

Some researchers think this explains why human and ape mothers tend to cradle their babies on the left: it is so they can better monitor their facial expressions with their left eye.

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Now, Janeane Ingram at the University of Tasmania, Australia, and her colleagues have looked at whether animal infants also prefer to observe their mum from one side.

The team studied 11 wild mammals from around the world: horses, reindeer, antelopes, oxen, sheep, walruses, three species of whale and two species of kangaroo.

Whenever an infant approached its mother from behind, the researchers noted whether it positioned itself on its mum’s left or right side. They recorded almost 11,000 position choices for 175 infant-mother pairs.

Infants of all species were more likely to position themselves so that their mother was on their left. This happened about three-quarters of the time.

The observations tally with a recent human study, which found that when children approached adults, they tended to do so in a way that kept the adults on their left.

Better bonding

Ingram and her colleagues found that mammal infants who keep their mother on their left are better able to keep track of her and hence increase their chance of survival.

When baby whales and horses move around with their mother on their left, for example, they are more likely to bond with her by rubbing up against her body, and less likely to be accidentally left behind.

However, if a threat emerges, the roles often reverse, Ingram says. “Infants keep their mother on their left in normal situations such as moving forward or suckling,” she says. “But when faced with stressful situations such as when fleeing, mothers prefer their infant on their left side so they can better monitor them.”

Human mothers cradle their babies on their left side while they are young and vulnerable, but this may switch as the children age and become more independent, Ingram says.

The consistent use of the right hemisphere in mother-infant interactions across all studied mammals hints that it has an evolutionary advantage, she says.

Lesley Rogers at the University of New England in Armidale, Australia, agrees. “If you’ve got different functions to perform, you can do that more effectively if you allocate different kinds of processing to each brain hemisphere,” she says. “So it makes sense for the right hemisphere to be dedicated to social behaviour.”

Journal reference: Nature Ecology & Evolution, DOI: 10.1038/s41559-016-0030