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?

Canadian Wildlife - - CONTENTS - By Jay In­gram Il­lus­tra­tion by Went­ing Li

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

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