Lately I’ve been running into mantises, or, more properly, mantids. A friend in Vacaville has a sort of colony in the shrubbery outside her condo. On a recent visit she pointed out a mantid egg case, or ootheca, from which legions of miniature predators will emerge. On a hike near Mare Island last week there was a large brown mantid perched atop an animal dropping in the middle of the trail, not at all camouflaged. Waiting for flies? Preparing to lay eggs? Not a clue.
These insects are classic ambush predators. Many are green and leafy in appearance, and some tropical representatives assume elaborate floral disguises. The placement of their huge compound eyes gives them superb binocular vision. Like those of dragonflies, mantid eyes have a fovea-like region for greater visual acuity. The ommatidia—the individual eye structures—automatically adapt to night vision, causing the whole compound eye to darken.
At about this point you’re probably saying, “Fine, but when do we get to the sexual cannibalism?” All right then.
It is true that female mantids have been known to eat their mates. It is also true that having his head bitten off does nothing to dampen a male mantid’s enthusiasm. There’s a large ganglion—a bundle of nerves—in the mantid’s neck that has an inhibitory function. Remove it and what’s left of the male becomes a veritable sex machine.
Evolutionary theorists have had a field day with the mantids. Sexual cannibalism is so prevalent among them—and a few other groups of arthropods, notably spiders and scorpions—that it’s considered an adaptive behavior. It’s easy to see what’s in it for the female mantid. Since males, although not as large as females, can be good-sized insects, she gets a nice nutritional package that helps her produce all those eggs.
But for the male? As with most male animals, his best strategy for leaving lots of his genes in the next generation would seem to be mating as often and with as many females as he can. Becoming his mate’s meal on the first go-round would pretty much blow that option. Yet some biologists claim the male goes willingly to his death, provisioning his future offspring in the ultimate paternal sacrifice.
The late Stephen Jay Gould was skeptical about this. Gould was fiercely critical of what he called adaptationist just-so stories: the notion that every existing structure and behavior has been favored by natural selection because it served a purpose. Instead, he was strong on contingency—the idea that not just the origin of a trait but its persistence could be a matter of chance—and the constraining role of developmental pathways.
Back in the 1980s, in an essay called “Only His Wings Remained,” Gould questioned the reality of sexual cannibalism in mantids and spiders: “I can find no quantitative data on the percentage of eating after mating either in nature or even in the more unsatisfactory and artificial conditions of a laboratory.” Further, for mantids at least he found “no evidence for the male’s complicity in his demise … Male mantises become headless wonders; male black widows remain on the female’s web. Both behaviors may be useful; but we have no evidence that either arose by active selection for male sacrifice.”
This ticked off some biologists. John Alcock, best known for his natural history essays about the Sonoran Desert, weighed in somewhat belatedly (in 2000) with a scathing critique. Alcock accused Gould of using his long-running column in Natural History to attack human sociobiology and the adaptationist ideas that supported it, in the process dismissing “adaptive sexual suicide” in mantids as “utterly implausible, not worth even speculating about.” (He didn’t indicate what the mantids had to say about the biological roots of human behavior; probably just as well.)
Despite all this, it was only a couple of years ago that Jonathan Lelito and William Brown of the State University of New York, Fredonia, brought male and female mantids together in their lab for the first time. They reported in 2006 that male mantids were not passive victims: “To the contrary, our results show that males assess risk of cannibalism and that, given this risk, they behave in a manner to reduce the likelihood of cannibalism.”
Lelito and Brown used lab-reared Chinese mantids (Tenodera aridifolia sinensis) which had been fed on crickets. Then, just before introducing the males, they cut the cricket rations of some of the females. The males were somehow able to tell whether their potential mates were ravenously hungry or not. Confronted with a cricket-deprived female, males approach-ed more slowly, displayed more intensely (by flicking their wings and rhythmically bending their abdomens), and made the final leap onto her back from a greater distance.
Score one, posthumously, for Gould. Spiders may be a different story. More on that later.