Like to Hold Your Baby on Your Left? So Do Walruses

By Elizabeth Preston | January 12, 2018 1:19 pm

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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.

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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.

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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.

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  • OWilson

    Maybe the left side is more comforting because a baby can better feel Momma’s heart beating?

  • JC Hamner

    Is this “social cognition” theory of sidedness a new one? I’ve never heard of it.

    Also, it seems to me that on the left hip, the the left eye would have the baby out of focus and in the periphery more than the right.

    I’m right handed and have a preference for carrying (mammals at least) on my right side. Am I just distrusting of my dexterity, or do I view carrying in a different capacity (or 3–this is nonsense.)

    • Elizabeth Preston

      The idea that holding babies on the left is linked to social processing in the brain has come up in many papers in the past. It’s not universal to carry babies on the left, though–in one study, it was about 3/4 of moms who had a consistent left preference, and it was strongest before the baby was 12 weeks old.

  • Clebla

    @inkfish/ Ms. Preston: I was reading a few of your blog posts, and then ran across a tweet by you regarding your apparent relief that docs no longer suggest that breast feeding women should wash their nipples with acid. I would have let that lie, but then you provided a citation wherein boric acid was suggested as the acid, and that it was used as an antimicrobial/antifungal to ease the symptoms of a colicky child. The ignorance in your tweet is profound, and is suggestive of an uninformed person with little science in their background, or a person capable of ignoring the wealth of information the internet provides your fingertips.

    Let’s start with things this biologist remembers from first exam, first semester freshman inorganic chemistry taken 20 odd years ago. If you had bother to do an Internet search of boric acid and pKa/Ka, you would see that this acid is a rather weak one. It’s slightly more acidic than the vitamin c tablet that wouldn’t dissolve your flesh if you had one this morning. It’s less acidic than the vinegar you may have made a dressing with last night, or used as a probiotic if you are a cider therapy fan. It’s also a weaker acid than that found in lemon juice, kimchi, and perhaps even the lactic acid in some cheeses.

    Next let’s consider that boric acid has had a VERY LONG HISTORY of usage as an antimicrobial/antifungal agent. In fact, on my stove right now a boric acid/saline solution is cooling down. It’s destined to be used to wash the eyes and ears of my old pet, who is prone to infections owing to his age and pedigree. What are the odds I’d be washing close to my pets brain (the sensory epithelium in his eyes, and ears) with a dangerous acid? What are the odds my vet would suggest doing so if it were some bizarre and dangerous practice. Would NIH provide documents that back it’s use in both humans and animals of it were dangerous. Would docs and vets have promoted its use, or the stronger acid called acetic acid found in vinegar, as a topical antifungal if it were dangerous. Both of these, by the way, have also been used in oral and vaginal cavities to influence microbial and fungal communities in ways that promote less pathogenic populations.

    The wording of your acid nipple post is Trumpesque in its omission and twisting of science and facts. I dare you to compare the toxicity and safety of a boric acid solution that would be used as a topical antifungal/antimicrobial by a nursing mom with the commonly prescribed antifungal nystatin that breast feeding moms use to eliminate yeast from their nipples, and docs use to rid a baby’s oral cavity of thrush. With the latter, the drug is being swallowed by the child. I can assure you that a drug whose mechanism is to create nonspecific channels across any cell membrane it contacts, which includes both pathogenic yeast causing the thrush and any epithelial cell the drug happens to bump into, will more toxic and damaging to mom and baby’s membranes than a dilute solution of a weak acid like boric acid would be. As a science reporter and blogger at a site associated with Discover, you should be ashamed by such carelessness tweets and commentary. As a reader, I’m left wondering what else posted by Inkfish and you has similarly disregarded science you could have used for creating an accurate, informed opinion or comment.

    Shame on you. Your implication that boric acids use by nursing moms was somehow bad, and less safe than what is commonly used by clinicians today in a similar setting is inexcusable.

  • Clebla

    Eighth or ninth attempt to post an innocuous comment about this author’s willingness to make imply misleading facts.

    [This comment was not relevant to the blog post and has been deleted.]

    • Elizabeth Preston

      Hi–your comments all went to the spam folder, which is why they weren’t appearing here. My tweet about doctors of the early 20th century recommending that mothers rinse their nipples with boric acid before every nursing session was meant to be lighthearted, and not a condemnation of boric acid, a perfectly nice chemical toward which I bear no ill will. I was bothered by the implication that mothers’ nipples somehow get so dirty in the 90 minutes or so between newborn feedings that they need to be washed with soap and water, followed by boric acid. (This was advice for treating crying or colic, not some sort of skin or mouth infection.)

  • wright gregson

    I have read that the beat the heart is more pronounced on the left side. making left side more comforting too baby

  • Rich

    “The left eye is linked to the brain’s right hemisphere” — actually, no, the left-of-center field of view in each eye is linked to the right hemisphere. Pretty much the same significance in this context, but let’s get the facts as straight as we can.

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Inkfish

Like the wily and many-armed cephalopod, Inkfish reaches into the far corners of science news and brings you back surprises (and the occasional sea creature). The ink is virtual but the research is real.

About Elizabeth Preston

Elizabeth Preston is a science writer whose articles have appeared in publications including Slate, Nautilus, and National Geographic. She's also the former editor of the children's science magazine Muse, where she still writes in the voice of a know-it-all bovine. She lives in Massachusetts. Read more and see her other writing here.

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