Mammals prefer to cradle babies on the left, study demonstrates

Left-hand bias for human mothers is well-known, but new research shows it is more widespread among mammals than thought, suggesting an ancient origin. There is more to cradling an infant than meets the eye, according to new research revealing that, like humans, other mammal mothers and babies prefer to keep each other on the left when face-to-face.

A new study shows the phenomenon is also seen in Pacific walruses and Indian flying foxes, suggesting it is more widespread than previously thought.

“Our results suggest an ancient origin of cradling bias,’ said Dr Andrey Giljov, co-author of the new research from the Saint Petersburg State University.

“[The] right hemisphere controls many aspects of social behaviour,” said Giljov. “Therefore, the greater right-hemisphere involvement provides optimal processing of social information”.

That, he adds, might help mothers and their offspring to notice and respond to the behaviour of the other, with the right hemisphere known to be involved in tasks including recognition of faces and emotions.

Previous research has shown that female primates, including humans, preferentially hold their offspring on the left, while a number of animals mobile from birth, including reindeer, sheep and kangaroos, have been found to keep their mother on their left when approaching from behind.

However it was not clear whether primates are the only ones to show a preference for keeping each other in the left visual field when it comes to face-to-face cuddles between mother and infant.

Writing in the journal Biology Letters, Giljov and colleagues describe how they looked at the way in which female Pacific walruses and Indian flying foxes interact with their young – two species for which mothers and offspring are known to hang out in both side-by-side as well as in face-to-face positions.

In total, 73 walrus mother-offspring pairs and 266 flying fox pairs were observed.

The results reveal that when resting and facing each other, mother and calf walruses were more likely to keep each other in their left visual fields, and keep in the position for longer, than for the right visual field. Flying foxes were also found to prefer to keep each other in the left visual field for both resting and licking when face-to-face – although there was no preference for which nipple was chosen when suckling.

In addition, when side-by-side to their mothers, both the walrus and flying fox young preferred to keep their parent in their left visual field. Giljov said the left-biases are likely to be mutually beneficial to both mother and infant.

Dr Gillian Forrester, an expert in the field from Birkbeck, University of Londonsaid the preference for keeping others in the left visual field is probably about more than just better processing of social information, noting that previous studies in humans have found the left side of the face is more expressive than the right. “The mother is more efficient, quicker and more accurate in observing the baby’s welfare with left visual field than right. At the same time, she is providing the baby with the more expressive side of the face,” she said.

Forrester says the new findings tie in to the idea that the preference for keeping others in the left visual field originally arose as a way of keeping tabs on potential threats. “This looks like this is a really old evolutionary trait,” she said. “[By] understanding the evolutionary history of the way the brain is organised, we are going to be able to understand why we behave the way we do much better.”

The Guardian 

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