Praying Mantis Mating and Cannibalism

The female praying mantis is known for cannibalistic mating behavior: biting off the head or legs of her mate and eating them. This behavior, which occurs in less than 30 percent of all mating sessions in the wild, may have evolutionary advantages for the praying mantis species.

Background

Rumors of the praying mantis' cannibalistic tendencies began when scientists observed their mating behavior in a laboratory environment. Entomologists would offer a potential mate to a captive female; after mating, the female would bite the head or legs off the smaller male. For a long time, these laboratory observations were considered proof of mating habits in the mantid world. 

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However, after scientists started observing praying mantis mating in a natural setting, the behavior changed. By most estimates, sexual cannibalism by praying mantis females occurs less than 30 percent of the time outside the lab.

How the Praying Mantis Chooses a Mate

Given a choice between females, male praying mantises will move toward females seen as less aggressive (i.e., ones they hadn't just seen eating another male) more often than the more aggressive females.

The males also tend to prefer to mate with females that appear fatter and more well-fed than others, as the skinnier and hungrier mantises are more likely to eat their mates during or after mating. This could also point to the male praying mantises being more attracted to females that are healthier, for the betterment of their offspring. 

Evolutionary Explanations

There are interesting evolutionary advantages to this behavior. The male praying mantis brain, located in the head, controls inhibition, and a ganglion in the abdomen controls the motions of copulation. Without his head, the male praying mantis will lose his inhibitions and continue mating, which means he can fertilize more of the female's eggs.

Paradoxically, then, the female praying mantis's sexual cannibalism may have an evolutionary advantage for both the female and the male. The male will have more of his genes passed on to the next generation if he fertilizes more eggs, and more eggs are laid by females who eat their mates—88 vs. 37.5, in one study. (However, if a male can mate more than once, that also increases his odds of having his genetics passed on.)

In addition, a slow-moving and deliberate predator like the praying mantis is not going to pass up an easy meal. If a male chooses a hungry female for a mate, there's a good chance he won't survive the mating session.