An anecdote from The Red Queen:
Learning that a cockerel could have sex dozens of times a day, Mrs. Coolidge said: “Please tell that to the president.” On being told, Mr. Coolidge asked, “Same hen every time?” “Oh, no, Mr. President. A different one each time.” The president continued: “Tell that to Mrs. Coolidge.”
We all know about these sort of truisms, and they are so banal that they can show up in Jay Leno’s monologue. In this vein, I was pointed by Abiola to this new paper, Genetic Evidence for Unequal Effective Population Sizes of Human Females and Males (be sure to check out Carl Zimmer’s excellent review). This isn’t unsuprising, last year another team published Reduced Y-Chromosome, but Not Mitochondrial DNA, Diversity in Human Populations from West New Guinea. About a month back I posted about Bobbie Low’s Why Sex Matters, which highlights affinal issues from the perspective of behavorial ecology and cross-cultural anthropology, that is, greater male variance in reproductive success seems almost universal. Genetic studies like the one above point toward the historical depth of this pattern, in other words it is not a reflection of current conditions. Human sexual dimorphism and comparative examination of human testicle sizes in the context of homonoids suggests a mild level of polygyny being normative for our species.
But one must also be cautious. When we think of polygyny we often conjure up images of harems ruled by potentates and guarded by enuchs. As Bobbie Low points out, many Western societies that are socially & legally monogamous tend to exhibit more male reproductive variance (skew) than female reproductive variance. Obviously, in a society where a small group of men monopolize sexual access to women through force of arms, the variance differential between the genders will be great and intuitively obvious, yet one may conjecture other scenarios that might result in the same gap.
Imagine a society characterized by endemic warfare where the mean age of death for males is 25. Many men will live past 30, but men will also die before the age of 20. The latter may never have an opportunity to father children. Additionally, high female mortality rates during childbirth might also mean the ancient men who make it past the age of 40 could be de facto serial monogamists (they have had several wives who have eventually died in child-birth). In contrast, imagine a society, such as some of the tribes of central Australia, which practices gerontocratic polygyny, where rather old males are “married” to the nubile females. In these circumstances, it has been attested that the official husbands often look the other way when their young wives have affairs with other men of the tribe. In this situation, you would see lower reproductive skew than one would expect from the official cultural practices.
As for the above study, I am curious as to possible long term differences between different populations because of reproductive skew and lower male effective population size. Low emphasizes the importance of a pathogenically constrained environment is favoring exogamous polygyny, while others have suggested that resource constraints might have imposed long-term monogamy upon certain populations. For example, would Northern Europeans and Inuit show less of a gap between male and female lineages? What about the Highland Ethiopians (who are generally monogamous) vs. the Sub-Saharan Africans in the lowlands around them?
Posted by razib at 01:13 PM