Science Still Cannot Explain Why Women Sleep Around

Seed beetles are polyandrous – females mate with multiple males, and choose which sperm will fertilize their eggs afterward. Scientists long believed they did this to get the best sperm. But a new study shows the fittest males always lose.

A study published today in Science details a series of careful experiments Swedish researchers conducted on mating seed beetles (pictured). They want to find out what the benefits were to females who mated with multiple males, given that multiple matings could be dangerous to their health for a variety of reasons. The accepted wisdom is that females mate with many men because they can choose which sperm fertilize their eggs after mating. Basically, more men equals a bigger and better smorgasbord to choose from in the genetics department.

If this hypothesis were true, females would always choose sperm from the fittest males to fertilize their eggs. But they didn't. If you measure male fitness as the ability to produce a great number of offspring who themselves produce a great number of offspring, then the fittest males always lost. Inevitably, the females chose sperm from unfit males.

Why would these insects have sex with so many different men, only to choose the crappiest sperm? The researchers admit that they are baffled. Their experiment was only intended to determine whether females favored sperm from fit males – not to plumb the depths of the psychology behind female insect promiscuity.

However, what these Swedish researchers have done is eliminate one possible reason why female insects mate with multiple males. They're not doing it because of genetic benefits that come from the males. They are not picking sperm that have direct or indirect benefits on their offspring, as far as the researchers could tell. Though they do float the possibility that these females may be choosing sperm that are beneficial exclusively to female offspring. In other words, genes that make fathers fit may not make daughters who are fit.

According to researcher Göran Arnqvist:

The results support the suggestion that genes that are good for males may often be bad for their mates. Therefore, in beetles at least, multiple mating does not award females with genetic benefits.

So if multiple mating does not award females with genetic benefits, what exactly does it award them with? Is it possible that they're sleeping around just for fun?

via Science and Uppsala University