Wild Things

Why do mothers usually hold their infants on the left side? Why is there a similar tendency in other species, from walruses to flying foxes?

Why do human mothers usually hold their infants on the left side? And other species too, from walruses to flying foxes?

HUMAN MOTHERS WITH NEW babies hold them on the left side 90 per cent of the time. This is also true of left-handed mothers, so the first explanation that comes to mind, that is, holding the baby on the left frees the dominant right hand, seems not to be true. The most popular explanation now builds on the differences between the left and right hemispheres of the brain, and new research suggests the same is true for many animals.

The hemispheric explanation is both more exotic and more subtle than another previously popular and now largely discarded idea that holding babies on the left positions them directly over the comforting rhythm of the mother’s heart. Instead, the brain explanation relies on an understanding of the perception and expression of emotion. While it is not apparent to us as we experience the emotional ups and downs of life, it is the brain’s right hemisphere that plays the dominant role.

The right hemisphere is more perceptive of emotion displayed on the face and, in turn, expresses emotions more strongly on the left side of the face, the side it controls. With the baby cradled in the left arm, the expressive left side of the baby’s face, turned upward, is seen in the mother’s left visual field — and so is communicating directly with the right hemisphere. Mother and baby are emotionally locked in.

But that’s humans. We know a lot about the different roles of our brain hemispheres, but for decades such differences were overlooked in animals. No longer.

The young of several species, including horses, muskox and orcas, tend to position themselves on their mother’s side when travelling or resting, and chimps and gorillas go further, holding their infants on the left side, as we do. But now the infant-holding behaviour has been shown to be more widespread. In February 2018, scientists at St. Petersburg State University in Russia published a report of their observations of two very different species: flying foxes and walruses.

Flying foxes are large fruit-eating tropical bats, an ancient lineage, which, unlike North American bats, do not echolocate but rely instead on very good vision. The Russian scientists observed mother-baby pairs in Sri Lanka and noted that when engaged in face-to-face resting, face-to-face licking and simply hanging upside-down resting, there was a strong preference for the baby to be on the left side.

They then turned to observing walruses and their offspring from a perch overlooking the Chukchi Sea, attending particularly to pairs resting face-toface vertically in the water or with the baby floating horizontally just before nursing. Again, baby on the left was the preferred arrangement.

These observations add to a growing body of evidence for left-side preference in the mother-baby bond, but also to the idea that a wide variety of animals, from mammals through birds to amphibians, have specialized left and right brain hemispheres. (There is a healthy debate over what exactly the advantages might be of having the two hemispheres specialized, but for whatever reason it is so common that it must be of some evolutionary value.)

An equally compelling conclusion is that if we humans have a left-holding preference for emotional reasons, and many other animals have a left-holding preference too, then they are probably doing it for emotional reasons too.

There are still some puzzling aspects to all this. First, given that the two hemispheres, at least in humans, communicate extremely rapidly, it seems odd that such a strong preference would be given to accepting and expressing emotional messages with the right hemisphere. It can only be a split second before the other hemisphere is aware. Also, the fact that human males seem to position their babies randomly might be good material for comedians, but does it really mean that fathers aren’t as emotionally engaged with their offspring? Hormonally they’re not too different. I’m not aware of any animal observations that would shed light on this.

One final mystery: while data supporting the idea of a preferred brain hemisphere leading to left-side holding in humans has continued to accumulate, there is one puzzle that has not yet been solved. Several centuries worth of art depicting mothers and their infants presents an unexplained inconsistency: in Europe for a period of time from the mid-1400s to the mid-1700s, the predominance of left-side holding dropped precipitously, only to climb back up in the centuries after. Even more curious, in Mexico from about 300-600 CE, left-holding made up about 80 per cent of those artworks examined, while at the same time in South America, the number was only 50 per cent. Was it artistic fashion? So far there is no explanation.a