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Echoes and Mirrors» Blog Archive » Evolution & Economics

Evolution & Economics

Male pond skaters forced to ‘sing’ for sex

“This is exactly the kind of thing you might expect to evolve in a system where females are vulnerable to forced matings,” comments Tim Birkhead of the University of Sheffield, UK, who in 2007 published evidence that female mallard ducks have intricate vaginas allowing them to send unwanted sperm into a cul-de-sac.

“The authors are aware that there are no winners in such arms races, but at any point in time, one sex may be ahead of the other – and here, it seems that the female water striders are ahead,” says Birkhead.

To win their prize, courting red-backed strider males must tap out three consecutive songs. Firstly, the males grip the female from behind and manoeuvre her into the mating position with their forelegs. During this ritual, they extend their long mid-legs forwards and begin tapping rhythms – dubbed “grasping signals” – on the water.

These defense mechanisms develop when there are more males than females – and the gains are seen most by the stronger of the males. I wonder what the population ratios over time have looked like, pre-barrier and post-barrier. If there are fewer females, therefore demand goes up, thus raising the ‘price’ (development of this tapping ritual via evolution of the vaginal barrier). Markets in everything. Same goes for more males (more demand, less supply, ‘price’ increase).

It’s a very simple model, but I would be willing to wager that it could be applied to women too. The human equivalent is the bar scene, which is proportionally more complex and hazards a much higher number of defense mechanisms, but is still very similar.

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