Cannibalism: A Perfectly Natural History(13)

One hypothesis reasoned that the male mantis’s brain actually inhibited sexual behavior. With their heads removed, however, males became “disinhibited,” found the rhythm, and eventually pumped out a full load of sperm. Other mantid mavens suggested that getting oneself cannibalized made sense for praying mantis males that might have limited opportunities to mate over their lifetime. It made evolutionary sense, therefore, to fatten up the only female they might ever run into—especially one now carrying their sperm. Furthermore, it would be a plus for both sexes since headless males reportedly pumped out more sperm than those equipped with heads, leading to more fertilized eggs and more offspring. These accounts contributed to an overall impression that the decapitation of male mantises was a normal and perhaps necessary copulatory stage and, soon after, the concept became entrenched in textbooks and the popular literature. Unfortunately, what never quite made it into print was the fact that most observations of mantis cannibalism were made in laboratory settings and only after females had been deprived of food.

In reality, cannibalism varies across this large and diverse group. The behavior has gone unobserved in most species, not necessarily because it doesn’t happen, but because it hasn’t been studied. Researchers now believe that rather than being a required component of mating behavior, the consumption of males is more likely to be a foraging strategy employed by hungry females unable to wrap their raptorial forelegs around an alternate form of nutrition.

Support for this hypothesis comes from studies on a wide variety of mantis species, including those in which worse-for-wear females that cannibalized their mates later exhibited improved body condition, produced larger egg cases (ootheca) and more offspring. Significantly, well-fed female mantises showed no cannibalistic tendencies during mating encounters.

Before we blame mantid cannibalism on captive conditions or starvation, though, the fact remains that both wild and captive males exhibit extreme caution as a normal preamble to copulation. Depending on the species, the males’ initial approach can vary from simple (slow and deliberate movement toward the female, followed by a flying leap onto her back) to complex (the male fixes its stare on the female, goes through a series of stereotypical movements like antennal oscillations and abdominal flexing, then takes a flying leap onto her back). Researchers believe that these movements serve to either circumvent or inhibit the females’ aggressive, predatory response. It is, therefore, extremely unlikely that these forms of cautionary behavior by males would have ever evolved if there weren’t at least some risk of being attacked by females.

And what about the male’s famous ability to “keep the beat” even after losing its head? Biologists Eckehard Liske and W. Jackson Davis have an explanation for that phenomenon as well. They believe that, rather than acting as a stimulus for copulation (by releasing sexual movements), decapitation artificially induces the behavior. This would be similar to the way in which lopping off a chicken’s head artificially induces locomotor movements that can temporarily propel a headless bird around a barnyard. According to these researchers, from an evolutionary perspective, reflexive abdominal contractions and the subsequent release of sperm may insure that fertilization takes place, even if the males are left feeling a bit lightheaded after sex. As such, it serves as a prime example of how cannibalism can benefit the individual being cannibalized.

As in the praying mantises, in certain well-known spiders, truth has been masked by myth. After several papers in the 1930s and 1940s reported that female Latrodectus mactans spiders devoured their mates after copulation, L. mactans and two additional North American species became widely known as black widows. Although most of the initial observations turned out to be anecdotal, cannibalism and black widows became forever linked, appearing in an array of literature that ranged from storybooks to college textbooks on evolution, ecology, and animal behavior. The cannibal association continued through the 1970s and 1980s, even though researchers working with these spiders were beginning to discover that the behavior in black widows was actually a rare occurrence. They determined that not only did most male spiders depart unharmed after copulation, but some of them lived in the female’s web for several weeks, even sharing her prey.

“The supposed aggressiveness of the female spider toward the male is largely a myth,” said spider expert Rainer Foelix. “When a female is ready for mating, there is little danger for the male.” Foelix did add that all bets were off if a male mistakenly showed up in the web of a hungry female.

Readers who might be disappointed to learn that the black widow’s reputation is apparently worse than its bite may be consoled by the fact that sexual cannibalism has been reported in 16 out of 109 spider families (although the list is described as not “exhaustive nor definitive” regarding frequency). One of the most interesting examples takes place in the black widow’s Aussie cousin, the redback spider (L. hasselti). In this species, males go to extreme lengths, not only to guarantee their own demise, but their consumption as well.

The Australian redback spider is common throughout Australia, and in a country renowned for its notorious creatures, the redback ranks among the most dangerous. The reasons behind this spider’s bad reputation start with a neurotoxic bite that can cause severe pain and swelling, and in rare instances, seizures, coma, and even death. Like the North American black widows, Australian redbacks are often found in close proximity to human residences, especially sheds and garages offering the spiders undisturbed areas full of clutter. Presumably because of the abundance of flies, both black widows and redback spiders were once common in outhouses, where their fondness for living under privy seats was never quite as unpopular as their habit of biting anything that blocked their escape routes. Slim Newton’s song “Redback on the Toilet Seat” details one such encounter. “There was a redback on the toilet seat when I was there last night—I didn’t see him in the dark but boy I felt his bite.” The song reached #3 on the Australian pop record charts in August 1972, well ahead of artists like The Bee Gees and Elton John.