September 17, 2007

Strange BiologyThe Male Praying Mantis — Dying to Mate?

A female mantis chews off the head of a male.

Females of the praying mantis and Chinese mantis species are infamous for their habit of devouring male mantises before, during, or after mating with them. For years scientists have been arguing over how and why this gruesome relationship could have evolved. The female certainly benefits from having a large meal that essentially delivers itself to her spiny embrace. What can account for this suicidal form of insect chivalry on the part of the male?

In the evolutionary scheme of things, any organism that reproduces and passes on its genes is considered successful, even if it involves personal sacrifice. Have male mantises not evolved a way to avoid being eaten by females because their main objective — passing on genes — is achieved? Have females evolved in a way that leads to selecting males who cooperate in their own death? Do males who fight for their lives never get a chance to mate and pass on their genes? If you're a female mantis, especially a hungry one, why choose a mate who's going to put up a fuss when you try to bite his head off?

Some point out that the nutrition offered to females in the form of a male probably improves the odds that the sperm deposited in a female will end up resulting in a new generation of offspring. If a male simply mated with a female and walked away, as is the case in many animal species, then the female would have to fend for herself and the few hundred eggs she carries. A well-fed female should fare much better than a hungry female.

Despite these compelling, logical arguments, some scientists insist that a male who could mate with a female but also live to mate another day would be more successful in reproductive and evolutionary terms. So why hasn't evolution lead to male mantises adapted to the female's aggression? Is there no way to mate without losing your life?

William Brown of the State University of New York, Fredonia, decided to study Chinese mantises to see if males demonstrated any ability to avoid death at the hands of their mates. Brown and one of his undergraduate students, Jonathan Lelito, experimented with introducing male and female mantises to each other under different conditions.

"We predicted that if the male is complicit, he's just going to march into her jaws," Brown said. "But if the male does not want to become her meal, then he should avoid the risk of being eaten." They placed some males in containers with females who had not eaten recently, and placed others with females who had already feasted on crickets. They also experimented with how the males approached the females. Did males fare better if they approached from behind?

The results of Brown's experiments showed that males are quicker to approach females that are full. When presented with hungry females, males will jump onto them from a greater distance, possibly to avoid being grabbed and killed before getting a chance to mate. Males also tend to stay on hungry females for longer, perhaps to wait for the female's hunger to pass. Males who approach females from behind are also less likely to be eaten.

"It's clearly a case of sexual conflict," Dr. Brown said. "Males are not willing partners." While the work of Brown and Lelito shows that male mantises are not gung-ho about being cannibalized during sex, the fact that as much as 63% of a female mantis's diet is composed of male mantises suggests that the urge to mate is stronger than the urge to live.

More to Explore

• This Can't Be Love: The Curious Case of Sexual Cannibalism
• ScienceDaily: Male Praying Mantids Prefer Not To Be Victims Of Sexual Cannibalism
• Praying Mantids Wise to Sexual Cannibalism Risk
• Evolution: Library: Mantis Mating

Last updated September 17, 2007 | Comments (5) | Permalink | Strange Biology