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Thursday, February 17, 2011

Why do females sleep around? The geometry of promiscuity.


In the United States, we are all familiar with the human sexual double standard from our developmental years in High School. Girls who "sleep around" are sluts but guys who "hook up" with lots of girls are studs. Sexist? Sure. Unfair? You bet. Biologically meaningful? Maybe just maybe.

The very first thing I wrote about in this blog was the fundamental asymmetry of sexual reproduction. One of the two sexes works really hard to acquire the resources to make and/or care for offspring and the other sex works really hard to mate with as many of the opposite sex as possible. Or at the very least tries to prevent other members of the same sex from joining in the fun.

Typically (but not in every species) females are the sex investing most heavily in the production of offspring because they bear the burden of making eggs. Think of eggs as a lunchbox the mother packs for the developing embryo to build its body from. Males can potentially sire large numbers of offspring by mating with multiple females (that double standard studliness). Females on the other hand, receive more than enough sperm by mating with one male. Why would they need to mate with two males? To do so would seem rather .. well, slutty.

If you think about it, the double standard in humans mostly benefits men because it minimizes the chance males will be "cuckolded" (a charming word that derives from cuckoo birds known to sneak their eggs into the nests of unsuspecting neighbors). Does the double standard benefit women? Perhaps, if through sexual fidelity a female gains higher reproductive success than if she does some cuckolding.

How interesting, then, that females of many animal species mate with multiple males during a single fertile period. I will leave stories about human postmen ringing twice and turn instead to Masked Julies.

Julidochromis transcriptus, the Masked Julie

Female Masked Julies (Julidochromis transcriptus, a cichlid fish found in Lake Tanganyika in eastern Africa) routinely mate with multiple males. But I would hardly call them sluts. I would call them shrewd.

Female and male Julies cooperate in providing parental care to eggs. In theory the eggs are more likely to hatch with two parents caring for them. Many female Julies do indeed form an exclusive pair with a single male. But wouldn't the eggs do even better if three parents looked after them? Probably so, but the problem is that male Julies are not so different from male humans: they chase off other suitors.

But females have found a very clever way around this problem.

Masanori Kohda and colleagues at Osaka City University in Japan recently found that female Masked Julies prefer to deposit their eggs in wedge-shaped crevices. Big males can enter the crevice, but cannot squeeze their large bodies all the way into the narrow end of the wedge.

Which is precisely where the females lay some of their eggs.

Now cue the little guy. The 98 ounce weakling who would normally be chased away by the big male. The small male finds safe haven (and unfertilized eggs!) in the narrow end of the crevice. He fertilizes and cares for those eggs, the big male cares for eggs in the wide end of the wedge, and the female gets two babysitters for the price of one. What a bargain!

If anybody ever tells you girls are no good at math, remind them that female Julidochromis transcriptus won the battle of the sexes with geometry.

Kohda et al. (2011) recently published their study in the Proceedings of the Royal Academy B.






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