Who wins when a man has multiple wives?
Polygamy, as practiced by 19th century Mormons, had a curious effect on women: they tended to have fewer children than their monogamous peers. Lightening the burden of housework and childcare, it would seem, came at an evolutionary cost.
Scrupulously kept birth, marriage and death records from the Utah Population Database, examined by a team of US researchers, highlight shifting birth rate patterns. In total, they tallied up around 800,000 people who lived between 1830 and 1894. This was an important period in the history of the community, as it was around this time that polygamy was being phased out.
They found that the more wives a Mormon woman lived with, the less she reproduced. For every additional woman in the household, each wife had approximately one less child. The implication would be that polygamy is bad for women, as well as working to the disadvantage of most men.
A polygamous mating system, as opposed to a broadly monogamous system, creates stark divisions between the men who succeed and those who are condemned to failure. If one man has five wives, for example, that is great news for him and his genes, but assuming an equal balance of the sexes, it leaves another four competitors whose genes are unlikely to be passed on.
This study reveals that polygamy is detrimental to the majority, with none but high-status males living up to their reproductive potential. In the case of the one man with the five wives and the four disgruntled rivals, that is nine people out of ten,having fewer children than they otherwise might.
The scientists working on this study used a measure known as Bateman gradients to calculate their subjects’ reproductive success. The Bateman gradient, named after Angus Bateman who performed a seminal study in 1948 on fruit flies, plots a change in reproductive success against a change in mating success. In other words, it looks at how the number of sexual partners correlates with the number of offspring.
The women’s reduced fecundity in this situation is in line with what the scientists were expecting. Under a polygamous system, a woman must compete for resources in a way she would not if she were monogamous. Bateman himself had noted a similar trend in his fruit flies, but this is the first time the theory has been tested in a human population.
Why does polygamy continue at all, if it’s so bad for a woman’s reproductive success? Scientists still don’t have a full answer to the conundrum, but it seems to have a lot to do with the pressures of sexual selection on males.
“One thing we know now,” said Michael Wade, whose work informed the study,
“is that selection can be so strong on males that it can drag the entire species off of a naturally selected viability optimum. Take the peacock. Its tail is magnificent for attracting females and bad for attracting predators. Males trade high risks to their lives in order to gain large numbers of mates and thereby offspring.”
Polygamy, then, may have evolved as a way for men to demonstrate their genetic fitness. While risky for a species as a whole, successful males win a harem of wives – an evolutionary jackpot for those with the best genes, despite being broadly unfair on everyone else.
Image: courtesy of Andee Duncan under Creative Commons License 2.0
Wade, M., & Shuster, S. (2010). Bateman (1948): pioneer in the measurement of sexual selection Heredity, 105 (6), 507-508 DOI: 10.1038/hdy.2010.8
Bateman, A. (1948). Intra-sexual selection in Drosophila Heredity, 2 (3), 349-368 DOI: 10.1038/hdy.1948.21
Jacob A. Moorada, Daniel E.L. Promislow, Ken R. Smith, Michael J. Wade (2011). Mating system change reduces the strength of sexual selection in an American frontier population of the 19th century Evolution and Human Behavior, 32 (2), 147-155
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One has to keep in mind that some STD-s like Chlamydia can cause infertility. Polygamous households would have a greater chance of having an STD infection, which would cause infertility and would also correlate with the number of wives. I suggest this as one of the causes.
I guess there’s a few trade-offs taking place. If the peacock’s tail becomes to cumbersome the individual will be eaten or even the species be wiped out. In general, sexual selection won’t select the same characteristics as natural selection (because they are already selected) so it must, on average, be selecting less adaptive characteristics (natural selection wise). OTOH sexual selection may actually increase the species fitness by wiping out the less fit individuals faster - the weak peacock still grows a tail which may remove him from the gene pool. We might also note that peacock tails don’t grow infinitely large - there are both upward and downward pressures.
In the case of humans, the tendency for polygamous women to do less well and gradually die out is at odds with the advantage of polygamy to the successful male. Perhaps women should avoid polygamists but perversely they should be attracted to successful males.