Having a lot of wives boosted the evolutionary fitness of Mormon men in 19th century Utah, but decreased that of the sister-wives, says a recent study. The study suggests that – for every sister-wife a woman had – each wife dropped her reproductivity by about one child.
And, as it turns out, human polygamists share this phenomenon with fruit flies, according to Michael Wade of the University of Indiana, Bloomington, and colleagues Jacob Moorad (Duke University), Daniel Promislow (University of Georgia), and Ken Smith (University of Utah), the team that authored the study.
In the calculus of evolution, your ability to represent, gene-wise, in future generations is your measure of evolutionary fitness. All you have to do to grasp the differential here is watch an episode or two of Big Love. You didn’t think a piece about Mormon polygamy would go without mentioning that, did you? The father in Big Love — who has three wives — represents in the next generation big time with 8 or 9 children (I stopped watching in Season 2), while the sister-wives each have only 2 or 3. Male benefit, relative female loss.
For anyone familiar with the relative ability of a man and a woman to conceive during the human gestational period, the findings of this recent study about Big Love likely isn’t Big News. After all, a pregnant sister-wife is always going to be on at least a nine-month delay, while the polygamist husband can continue visiting — and conceiving with — all of the other sister-wives.
Plain old math also had to have something to do with the enhanced evolutionary fitness of 19th century polygamist men. While the husband could mate constantly, given the will and the wherewithal, the sister-wives could only mate in turn.
As for fruit flies, the authors note in their paper, published in the March 2011 issue of Evolution and Human Behavior, that these 19th century sister wives and fruit flies both are fine examples of the Bateman gradient in which polygamy benefits the male polygamist but not the female. A.J. Bateman, he of the gradient, uncovered the same relationship in fruit flies, although it’s unlikely that the flies understood that they were polygamists.
This advantage of polygamy to the polygamist male is not confined to 19th-century Utahans and fruit flies. Male mice also appear to benefit from mousing around with lots of mouse wives in the form of amped up sperm production, more offspring, and possibly even the formation of giganto-sperm.
By the way, polygamy benefits only the men who practice it. By examining data on birth, marriage, and death during a critical period in the Mormon church involving a drive to phase out polygamy, the researchers showed that ending polygamy lessened the sexual selection pressure on all males and spread around some of that fitness among the men. Under polygamy, fewer males had access to wives, and the presumably sexually fit and studly males who did have wives had the lion’s share of mating and reproductive success. Indeed, Wade notes that polygamy in general isn’t that great for most males of a species. In a press release, Wade noted:
When the ratio of sexes is about equal, for every male that has three mates, there must be two males that have none. If a male has even more mates, then the disparity among male ‘reproductive’ haves and have-nots can become quite great.
That’s exactly what Wade, Moorad, Promislow, and Smith saw with the 19th-century Mormon data. But with the phasing out of polygamy and Big Love and the growth of a one-man/one-woman mating trend, those men who had been left cold and lonely had a better chance of finding a mate, reproducing, and enjoying some improved evolutionary fitness.
Dr. Emily Willingham came to EarthSky from The Biology Files. Her background includes a PhD in biological sciences, a bachelor's degree in English, and a published book: The Complete Idiot's Guide to College Biology. She is a scientist, writer, editor, teacher, autism & ADHD parent, and "all around opinionator." Says Emily: "Got an English BA & biology PhD, & I'm not afraid to use them, often together."