Wild Neighbors—Redback Blues: The Spider’s Dilemma

By Joe Eaton
Thursday October 30, 2008 - 10:55:00 AM

Last week, inspired by some recent mantid encounters, I wrote about the dilemma of the male mantid. Female mantids are hungry little beasts, and a male may become a meal as well as a mate. All that is undisputed. What’s controversial is whether the male is complicit in his own demise—whether he behaves so as to increase the likelihood of being eaten—and what the evolutionary roots of such behavior might be. 

Biologists who believe most if not all traits exist for a purpose have come up with plausible reasons for the male mantid to offer himself up: if being consumed helps his mate produce more eggs, if his sacrifice limits the access of other males to the female, if his chances of future mating encounters are limited. Others, like the late Stephen Jay Gould, tend to see evolution as a quirkier process, with organisms, like Vonnegut’s Malachi Constant, the victims (or beneficiaries) of a series of accidents. 

Gould’s skepticism about the sacrificial behavior of male mantids is supported by recent laboratory research, in which the subjects did their best to avoid becoming a post-nuptial snack. Some female mantids use pheromones to lure males, although it’s not clear whether that happens in the test species. If so, you have to give these guys points for staying clear-headed enough to judge how hungry the female was, and approach her accordingly. 

But what’s true of mantids may not follow for another well-known group of sexual cannibals, the spiders. The behavior is widespread among these arachnids, from black widows to orb weavers. Male jumping spiders perform elaborate dances for their potential mates. If a female is unimpressed, she eats him. She doesn’t even let him finish his routine. 

Arachnologists have found some support for the complicit-male model. Maydianne Andrade of the University of Toronto worked with the notorious Australian redback spider, a relative of the black widow. (Those who listened to Dr. Demento’s radio program in the early 80s may recall an Aussie song entitled “Redback on the Toilet.”) She observed male redbacks executing suicidal somersaults into their partners’ jaws while mating. Paternity analysis showed that if the female redback consumes her mate, his sperm will fertilize more of her eggs than if he somehow makes his getaway. Andrade also reported that females who ate their mates took longer to become receptive to other suitors, which would allow the sperm of the deceased a head start. 

I’m not sure if this applies to all spiders, but the mechanics of reproduction would doom male redbacks in any case. Male spiders use a pair of leglike structures called palps to transfer their sperm to the female. A redback’s palps break off inside the female, and he’s pretty much a goner. (Male mantids, on the other hand, are capable of mating again if they survive the encounter.) 

But against that case, there are numerous examples of male spiders who don’t behave suicidally. Male crab spiders try to restrain females with silk before mating. (Yes-spider bondage.) Some orbweaver males prefer females who are shedding their skins, a process that slows them down considerably. Others approach a female only if she’s occupied with a prey item. Male long-jawed spiders use those jaws to hold the female’s jaws open during mating.  

So, despite the fate of the male redback, it seems unlikely that male spiders are hardwired by evolution to volunteer for certain death. It may just be that a female spider’s default mode is to eat whatever comes near her, mate or otherwise. Since female spiders are so much larger than males in many species, it’s even hard to buy the dietary-bonus model of female advantage. There’s just not that much meat on a male spider. 

I’m sure the debate will go on, though. Which is fine. Having just read a brace of books on the Kitzmiller v. Dover trial and watched Randy Olson’s film Flock of Dodos, I’m aware that opponents of evolution-old-line creationists or Intelligent Design advocates-like to exploit disagreements among evolutionary biologists as evidence that the whole concept of evolution is fatally flawed. If these scientists can’t agree among themselves, why should we take them seriously? 

But that’s the way science works. The debates continue until one side or another amasses conclusive evidence. Someday, one way or another, the sexual cannibalism question will be resolved, just like the ones about phlogiston, or mesmerism, or whether swallows hibernate underwater. 

In the meantime, the fact that individuals like Gould and the arch-adaptationist Richard Dawkins can disagree about how the evolutionary process works doesn’t negate the scientific consensus that the process exists—and that there’s no place in it for a designer’s hand. There are, and will continue to be, legitimate controversies among scientists. Evolution versus Intelligent Design is not one of them.