Coitus Interruptus

by Katie Pratt

You know what would be really upsetting? Getting eaten while copulating. And, were you to copulate in the vicinity of a mighty predator, out in the open, your risk of getting fatally jiggy might be pretty high. Us humans generally (though not exclusively) avoid this by having sex in a house or hotel room, and our natural predators are few and far between. But what about all the other creatures on the planet?

This question led evolutionary biologists to the hypothesis that animals would minimize copulation time to avoid predation (something that dolphins, with their approximately 40-seconds-to-ejaculation phenotype, have all figured out. At least, that’s what they tell the ladies). This idea was based on the observation that most species are less than vigilant when engaged in sexual activities, and the accompanying noises and movement might alert the attention of a hungry predator.

However, there is little support for this hypothesis, with numerous examples of lengthy copulation documented (such as the 50-plus hour-long sessions of the lovebug, Plecia nearctica). But in this month’s edition of Current Biology there is a report that neatly demonstrates this theory in action.

A team of German biologists set up a video camera in a cowshed in Marburg and observed the activity of a colony of Natterer’s bats. These bats eat insects, and are particularly fond of diurnal flies. Within the cowshed, Musca domestica flies tend to spend the night on the ceiling rather than flying around. The reason they stay still is to prevent an attack by the resident bats, who cannot distinguish between a fly and the surface on which it is sitting using echolocation. What the scientists noticed, however, was that if the flies started mating, they had 26% chance of getting attacked by a bat.

To check that the bats were actually attracted to the sounds of the flies mating (they make buzzing sounds with their wings when courting and copulating) the researchers played recordings in the cowshed. Sure enough, this attracted the odd hungry bat, which then used its tail membrane to try and locate the pair of flies. The sounds the flies made were critical, as without them the bats were unable to target a dummy fly couple.

These data also prompt another interesting question: If flies are at such a high risk of death when they mate, why do they continue to copulate in this way? The authors suggest that perhaps since the male is the most at risk in this situation (being positioned on top), the behavior is under selection pressure as it is a true means of assessing reproductive fitness. A male that will do it in the face of death is a keeper.

Or…maybe not.

Katie Pratt is a postdoc in Molecular Biology at Brown University. She has a passion for science communication, and in an attempt to bring hardcore biology and medicine to everyone, she blogs jargon-free at www.katiephd.com. Follow her escapades in the lab and online on Twitter.

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