Saturday, June 24, 2006

Trivers-Willard in the hiz houze   posted by Razib @ 6/24/2006 11:07:00 AM

The Trivers-Willard Hypothesis was proposed a generation ago, and there has been some empirical support for it. Now, Anna at Sepia Mutiny pointed me to this story who goes toward suggesting that different nutritional regimes can effect sex ratios. How would this work?

Well, some assumptions....

1) Among mammals the conception rate of male zygotes is going to be higher than female zygotes because the Y chromosome is smaller than the X chromosome.

2) Because of lethal unmasked sex-linked traits, and immune suppressive impact of testosterone, the rate of spontaneous abortion of male fetuses should be higher than that of female fetuses

3) Ergo, the 104:100 ratio of males and females at birth in the United States is a reduction from a higher ratio at conception.

Obviously data from other mammalian groups should be relevant for human beings. If we assume that more male fetuses are marginal than female fetuses (their fitness being reduced by non-lethal recessives unmasked because of operational haploidy) then stress during pregnancy might be assumed to disproportionately effect male fetuses. So you have a mechanism here to equalize sex ratios in a facultative fashion in stressed environments, or, bias toward males in an environment of abundance.

Does this work for humans? Here is a list of sex ratios by country. I'll leave it to others to generate the appropriate regressions and correlations (obviously some data points, like China, should just be discarded)....