This Valentine's Day, the battle of the sexes moves into competitive taste testing. While female bedbugs try to hide previous matings from males in hopes of receiving more sperm, the males take matters into their own … well, penises, where sensory hairs detect whether the female they're grasping has had a previous tryst. The sampling allows males to reduce their ejaculate if they find other bugs have been in their bedbug.
Love isn't pretty for bedbugs. Amorous males wield needle-like penises and mate by stabbing them in the midsection. A groove on the female abdomenal armor directs the penis and the ejaculate lands in a sack of cells just under her skin. This highly unusual "traumatic insemination" is a sign that males and females have conflicting goals--males appear to be mating with their mistresses against their will, says evolutionary physiologist Michael Siva-Jothy of the University of Sheffield in the United Kingdom. The female fights back with immune cells that eat much of the "contribution" of the first male to mate her in a given week. But Siva-Jothy and colleague Alistair Stutt have found that the males have other items in their bag of tricks.
The researchers allowed female bedbugs to mate with various numbers of males. On average, a female's first mating lasted 2 minutes, and subsequent matings lasted only about half that. The amount of ejaculate decreased accordingly. The researchers speculate that males may ejaculate less when females have already mated to save sperm, knowing their swimmers are safe from her immunological onslaught.
But how do the males tell if a female has mated? High magnification of the bedbug penis revealed little hairs that might be sensory. When the team dabbed sperm onto males' penises before the insects mated with virgin bedbugs, the mating time was reduced by about half, the team reports online this week in the Proceedings of the Royal Society B. Control penises dabbed with virgin bedbug juices led to first-time mating practices.
"This will be an important paper in sexual selection theory," says behavioral ecologist H. Kern Reeve at Cornell University in Ithaca, New York. He says that ejaculate detection is "the latest male innovation" in the evolutionary race of the sexes to assert dominance in mating. Since necessity is the mother of invention, time will tell what the females do next.