Tag Archives: Zoology

Study suggests famine can affect the ratio of boys and girls born to a population

A sociological study of the famine that happened during China’s Great Leap Forward suggests that, like many other species, humans modify their birth rates to reflect times of feast or famine. Specifically, more girls are born in starving communities than boys.

This and other types of environmentally based birth modifications have been observed elsewhere in the animal kingdom. Alligator offspring’s sex are determined by the temperature of the environment just prior to birth. Studies also show that “handedness” – whether you predominantly use your left or right hand – can be affected by stress on the mother prior to birth. But this is the first indication that such modifications happen on a large-scale based on poverty of resources. That a community-wide change can be observed in time of poverty has huge implications for social science.

The report does not have a definite conclusion as to why the disparity in gender production. But one possibility suggested is that, since malnourished males produce less offspring than comparably malnourished females, this may be nature’s way of maintaining the continuation of the species.

What do lefties and male crocodiles have in common?

Life. Its all interconnected and we humans – high as we may be on the food chain – are very much a part of the living systems around us. We are governed by the same set of biological imperatives. These are axioms of our modern world, argued over perhaps, but rarely contemplated for their own sake.

Occasionally, research lays bare just how mechanistic that relationship really is. And I suspect many will find it slightly upsetting. For example, we generally assume that the combined genetics of our father’s and mother’s lineage determine what we will be like when we come out. Our hair color, eye color; our preference for reggae music. All of it predetermined by a process that is a vast, nearly-unknowable operation involving all kinds of complex… stuff.

We don’t know how it works. But its got something to do with beans. And some German guy, according to our high school science education.

More and more, though, we are finding that not to be the case. Today is just such an occasion. In the Wall Street Journal today is an article entitled “The Health Risks of Being Left-Handed.” What stuck out in my reading of that article was this passage:

…More important, researchers say, are environmental factors—especially stress—in the womb. Babies born to older mothers or at a lower birth weight are more likely to be lefties, for example. And mothers who were exposed to unusually high levels of stress during pregnancy are more likely to give birth to a left-handed child…

Got that? In other words, while genetics probably do play a role in determining left or right handedness, its the specific conditions just prior to birth that appear to have the most direct effect on your final preference for signing your name.

So, how does all this relate to male crocodiles? Well, crocodile eggs are unusual in that they have no determined sex until just before hatching. The temperature around the nest at the time of hatching will determine the sex of the hatched crocodile-lets (wait. What is the name of a baby crocodile? Meh. I don’t feel like looking it up).

Environmental variations affect at least the sex of crocodiles and the handedness of humans. But the question is: why? And what else is determined in this purely environmental way?

In the case of temperature-dependent sex determination, the jury’s still out as to why. But one theory goes that colder temps mean longer seasons, and a longer season ensures that there will be more females at full maturity to lay the next generation of eggs. Is there some evolutionary reason that humans determine their hand preference so late in development? Is there some benefit?

The above-linked article doesn’t delve much deeper on the subject than the quoted paragraph, unfortunately. However, it does go on to list some of the traits which seem to accompany left-handedness. Are these all connected, or simply a collection of traits all changed by some of the same environmental conditions?

One thing is for sure: events which are consistently reproduced in nature rarely happen accidentally or without purpose. What that purpose is for lefties is, I guess, something for the next set of researchers to work out.