Why Do Mammal Moms Prefer Cradling on the Left Side?

[Frabjous]

Maybe the right-hand rule isn’t arbitrary and is due to the cognitive properties of the left side of the brain.

Many Mammal Mamas Carry Kids on the Left​

Regardless of whether they are left- or right-handed, human moms tend to cradle their babies on the left side of their bodies, especially in the early months. This left-handed bias likely has to do with the human brain’s lopsided layout: sensory information on the left side of the body is processed on the right side of the brain. The brain’s right hemisphere is also where emotions are processed, so holding and observing the baby on the left may help transmit social information to the right side more efficiently. Babies seem to prefer to keep their mother in the left visual field, too. Fascinatingly, researchers recently documented left-side bias in non-primate mammal mothers. Observed off the coast of a Russian island, walrus moms tend to keep their babies on the left while bobbing along the waves, and their calves swam over to their mother’s left side before diving to suckle. Ditto for flying fox moms dangling from tree branches in Sri Lanka who seemed to favor keeping their babies on the left.

from https://www.smithsonianmag.com/science-nature/14-fascinating-facts-about-moms-180977677/

 

[Borek]

Sounds plausible, still it just puts the question back in time: why the left/right split of information processing in hemispheres? Was it simply a random thing, or was there some deeper reason?

(Somehow makes me think about theories why our biochemistry is based on L amino acids instead of D.)

 

[Klystron]

Probably not a coincidence that the (human) heart and related major blood vessels occur on the left side of the body for most mothers. Not sure of the heart's position in other mammals that hold their offspring in front of their bodies but holding a baby close to the mother's beating heart likely has benefits for mother and child.

The relations among asymmetric brain structures, visual fields, length of time to rear independent offspring and 'right-handedness' could be connected to internal asymmetries in the position of major organs in the adult body. This discussion begs the question of how a single heart located on the left corresponds to left-right symmetric lungs and, as the OP proposes, how left and right hands with apparent identical functions depend on specialized functions in the brain.

IOW do we recognize a left-right preference in other paired bilaterally symmetric organs? I suspect that voluntary controlled bilateral body parts such hands and eyes exhibit more propensity for operational asymmetry than involuntary functions such as breathing using two lungs through two nostrils or excreting urine via two kidneys into a single bladder.

 

[hutchphd]

But of course there is an unexpected "handednes" to physical law as explained wonderfully by prof Feynman.

 

[BillTre]

IOW do we recognize a left-right preference in other paired bilaterally symmetric organs? I suspect that voluntary controlled bilateral body parts such hands and eyes exhibit more propensity for operational asymmetry than involuntary functions such as breathing using two lungs through two nostrils or excreting urine via two kidneys into a single bladder.

There are unobserved and sometimes subtle asymmetries throughout the body. The convoluted path of the GI tract (in particular the stomach and intestines) through the abdomen are markedly asymmetric. This seemingly results in asymmetries of most abdominal organs (liver, spleen, pancreas).
In addition, any tube or string shaped components crossing the midline of a bilaterally symmetrical organism will have to do so in an asymmetric manner (those from one side will have to go over or under their contralateral hemi-pair when they meet at the midline (same cell on the other side)).
Besides being a geometric requirement, this has been observed in real animals.

Asymmetries are also known in other animals. Several crustaceans, for example, have a large and smaller pincer (first walking leg, like lobster claws). They are used for different purposes (often characterized as cutting or crushing). The ganglia (CNS in crustaceans) innervating these structures are also asymmetric (invertebrates are often like that).
Once developed, large legs can be transplanted to small legs and visa versa. The appropriate behavior goes with the innervation the legs receive from their ganglia (one per leg), not the large or small leg structure.

There are mutations that do not do away with body asymmetries, but will randomize them which way they go.
For example, instead of the heart being on the left side all the time, it is 50-50 in those carrying two doses of the mutation.
This mutation breaks a developmental process that normally sets up which way the asymmetry goes. Further developmental processes results in the organ asymmetries. Mutations like this have been found in fish, mice, and humans.

Looking at these kinds of human mutations would be an obvious approach to determining some of the basis for handedness of the baby holding.