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Better Sex? (Optimal Gender Ratios) May 21, 2008

Posted by federalist in Open Questions.

A WSJ Op-Ed today offers a summary of the modern sociology of polygamy.  Indeed, monogamy may be the single greatest institution of human equal rights.

But this begs the question: Why are gender ratios evenly matched?

In large sexual animals, where gestation is a long and arduous process, it seems that there would be a tremendous benefit for a species to turn out females in much higher numbers than males. Consider elephants, for example: Females gestate babies for 15 months. Elephants don’t face significant predators, and males don’t appreciably contribute to the ability of a herd to forage or raise calves. If anything, they compete for scarce food. A single bull could conceivably keep a herd of 1000 females pregnant. A species of elephant that gave birth to many females for every male would seem to have a tremendous advantage in numbers and population resiliency over one that produced genders in equal numbers.

It doesn’t seem hard to imagine a diploid genetic process in which the sex trait is carried on a separate, extraneous chromosome, and whose propagation to gametes is highly skewed to exclusion.  In humans, for example, there would be additional advantages to carrying the male sex determiner on a 47th chromosome (whose absence results in a female): Males would not be as susceptible to recessive disorders on the X chromosome as they are now.

Are there any higher lifeforms in which sex is determined entirely by genetics and in which the proportion of male to female births is significantly less than one?  Why not?


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