When humans get themselves killed in creative ways, they are usually considered unfit individuals. But for some species, self-sacrifice of the ultimate kind is a common, adaptive part of mating. The process is called sexual cannibalism, and it's every bit as gory as it sounds.
You probably know that black widow females sometimes consume their mates (along with 95 percent of their young) and many species, from crickets to scorpions, indulge similarly cannibalistic appetites after sex. One fly, Serromyia femorata, administers a death kiss, sucking the body of her mate through his mouth as a post-copulatory snack. But the masochism prize in the mating game goes to the suitor of another poisonous widow, the Australian redback spider Lactodectus hasselti. He actually tries to feed himself to his partner.
Before mating can begin, the male redback spider must go through a few preparations. First he spins a special web and deposits sperm on it. Then he sucks the sperm packets into his pedipalps, two appendages on the front of his head that he might otherwise use to hold food. Once he's primed and ready, this inconspicuous brown male searches for the rare web containing the irresistible black and red female, who is fifty times his weight.
The drama begins in the standard spider mating position: He stands on her large abdomen and inserts a palp into one of her genital openings. But then the redback male does something different. He uses his palp as a pivot to somersault 180 degrees and land on the female's jaws. The female may then pierce his abdomen and inject enzymes, beginning the digestion process. The male is ready for this outcome-he hopes for it-and his abdomen is compartmentalized to slow his demise. Often he will manage to insert his other palp in her second genital opening, then leap into her jaws again. Eventually he is devoured, mating all the while. If he survives his daring leaps, it is only because the female isn't hungry. (She is typically ravenous.)
Other male widow spiders don't feed themselves to their mates. So what drives the male redback to such lengths? In the lingo of evolutionary biologists, "What's in it for him?" Scientist Maydianne Andrade of the University of Toronto performed an experiment to find out: She mated redback spiders in the lab and determined how many eggs males fertilized when they were eaten-or spared. After a first pairing the females were allowed to mate again. Dr. Andrade found that when the first male was eaten, the female mated with him for twice as long (while eating him), and the "male meal" fertilized roughly twice as many eggs as an uneaten male. She was also much less likely to mate again with a second male. The male fertilizes more eggs, which is the goal.
But is it worth it for a male to lose the chance to mate again? For the male redback spider, the answer is yes. He only lives eight weeks, compared to the female redback's two-year lifespan. Only 20 percent of males ever find one female, let alone two. Even if a redback male did get lucky twice, he would be functionally sterile, because his palps are damaged during mating and emptied of sperm. Cannibalized, the male doubles his "paternity benefits"-offspring. The female he inseminated can store his sperm for the rest of her life, potentially producing thousands of offspring. Generation after generation, this paternity advantage has cemented risky behavior in males. Each suitor performs the same suicidal somersault, trading his life for more offspring.
Widow spiders aren't the only animals with a penchant for cannibalism. Several species of praying mantis also eat their mates. Mantises are aggressive hunters, occasionally catching much larger prey, including hummingbirds. (One biologist often says the only thing mantises pray for is a good meal.) Most species only cannibalize mates regularly in captivity, but one species in particular deserves its bloodthirsty reputation. The European Mantis (Mantis religiosa) is a common, two- to three-inch green mantis with an uncommon appetite. In the wild, the female eats a third of her partners. She eats even more in the lab when the males can't escape. She is so voracious that the male European mantis forgoes mating displays in favor of surprise. No foreplay for her! He sneaks up on the female from behind, so she won't get any ideas about his juicy head. If he can get close enough, he'll leap onto her back and begin to mate.
If the female spots her mate's head during the process, she'll snatch it and begin to dine. The decapitated male only has one goal at this unfortunate juncture. He thrashes wildly, and though headless, manages to mate with her. Mantis females eat all parts of the male they can reach. At least the male redback spider's death is voluntary. Cannibalism in the European mantis shows no sign of male buy-in. He makes a tremendous effort to avoid being eaten, his wariness a good indication that sexual cannibalism has shaped the evolution of his behavior.
Sexual cannibalism may be rare and extreme, but it has evolved several times in the animal kingdom. For species like the redback spider and the European mantis, reproduction trumps even death. It always does-natural selection ensures that the fittest genes are perpetuated, even when the process is painful for the participants. So when the male redback spider gives his life as a snack, he can honestly claim: "my genes made me do it."
Andrade, M.C.B. 2003. "Risky mate search and male self-sacrifice in redback spiders." Behavioral Ecology, 14: 531-538.
Lawrence, S.E. 1992. "Sexual cannibalism in the praying mantid, Mantis religiosa: a field study." Animal Behavior, 43: 569-583.
Johns, P.M., and Maxwell, M.R. 1997. "Sexual cannibalism: who benefits?" Trends in Ecology and Evolution, 12(4): 127-128.
Andrade, M.C.B., and Banta, E.M. 2002. "Value of remating and functional sterility in redback spiders." Animal Behavior, 63: 857-870.
Andrade, M.C.B. 1996. "Sexual selection for male sacrifice in the Australian redback spider." Science, 271: 70-72.
Mukerjee, M. October 1995. "Giving your all." Scientific American.
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