Maternal competency and natural selection?

So, as a woman, am I wired to be a mother? Is there something on my X-chromosomes, or in my lack of a Y-chromosome, or in my female hormones that programs me to know how to care for an extremely helpless newborn baby? In those early days, I wanted that to be true, but I wished I felt more certain about it all.

When I seek evolution as a comfort in all this, my thoughts fall into a habit of thinking through natural selection. I try to check off the boxes, which are basically, that the trait is 1) variable in a population, 2) affects the evolutionary fitness of the individual (how many offspring survive and, in turn, reproduce), and 3) is heritable. If all these things are true, then 4) the trait should increase in the population if it is good or decrease in the population if it’s bad.

As far as maternal behavior being variable in a population, I think that is self evident. Isn’t it? Isn’t that why there are so many different books on the subject, so many philosophies, so much judging about it? In a paper called “Evolutionary roots of intuitive parenting: maternal competence in chimpanzees” (Bard, 1994) the author evaluates maternal competence in chimps. It’s done in captivity at the Yerkes Primate center, though if we’re interested in hard-wired, genetic behavior that shouldn’t matter. Key points were that mothers spent a lot of visible time cradling their infants (~70%). Competent mothers cradles and gazed at their infants more, and stimulated them, encouraging them to develop their motor skills. “Marginal” mothers showed a real difference in these skills, failing to respond as much to infant crying and not keeping them as close by.

Infants of the marginal mothers didn’t fare as well, some were injured. This brings us to #2, does maternal behavior affect evolutionary fitness of the individual? Does it matter if a mother is competent or marginal? This is a longer term question, and I don’t feel like I have sufficient research or knowledge to answer this one yet. On the one hand, there are plenty of studies out there to prove infants need a mother not just for nutrition, but for nurturing also. Here’s a brief look at the saddest set of experiments ever, by Harry Harlow:

I haven’t gone back to the source papers on these, but what I learned in Psych 101 in college was that monkeys reared without mothers are not properly socialized, which I assume affects their ability to find mates and mother offspring of their own. It’s hard to apply this to a more naturalized setting, but it seems fair to assume better mothers have better adjusted offspring. Does that translate to increased reproductive success? Because we should only expect to see behaviors selected for if they increase reproductive success. I could imagine a contrary scenario; what if cold, marginal mothers ignored their offspring and just continued to have lots of them? Maybe it doesn’t matter if the mother is marginal, in which case you shouldn’t see any selection on competent mothering.

But what you see in monkeys, apes, humans, is extremely helpless infants, that are carried by their mothers and form an important and long-lasting relationship, perhaps the most important, long-lasting relationship (depending on species). Competent mothering seems like a set of traits that must be selected for, since helpless infants (with their large, slowly developing brains) have clearly been selected for.

Then there’s that whole heritability part. Is “mothering” as a set of traits coded for by genes that we can locate? Is there evidence for this? Are there mothering genes, and do the forms you get predict whether you are a competent or marginal mother?

Because the alternate hypothesis would be that mothering is largely learned, culturally transmitted as we learn from our own mothers and mothers around us.

Next time: cultural transmission of mothering practices.


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