Who's Dying for Sex?

Science News, Nov 13, 1999 by Susan Milius

Scientists refine the calculation of when it's really worth it

Maydianne Andrade still remembers the first time she saw a true femme fatale--and her willing partner--in action.

Andrade, then a graduate student at the University of Toronto, had placed a male and a female Australian redback spider together and settled back to watch. As in many spider species, the two made an odd couple, she acknowledges: "The female's about the size of a marble, and the male's a rice grain."

Just a year before, in 1992, New Zealand researcher Lynne Forster had published the first description of mating redbacks. Her observations suggested that this desert species presents a real-life example of what had been only a tentative prediction at the extremes of mating-behavior theory.

Males of some other species, such as the redback's North American relative the black widow, occasionally lose their lives in close encounters. Before this report, however, there wasn't a clear example of males that encouraged females to chew them up. Forster's description of redbacks suggests that males don't just risk death to mate; they beg for it.

The fatal courtship can take quite, a while, Andrade found. "The male moves around the web, making what seem to be vibrational signals for several hours," she remembers. "Then, he's climbing onto the female's abdomen and climbing off--basically there's a bunch of false starts. I was watching, and it's 6 hours later, and I was thinking, `This is never going to happen.'"

After 7 1/2 hours of fidgeting, Andrade saw the spectacle that Forster had described. "When it finally happened, it was incredible," she recalls.

Like many other male spiders, a redback can fertilize females with either of a pair of structures that uncoil from his head. Called palps, they look like boxing gloves, according to Andrade.

The male inserts a palp into one of the openings of the female's reproductive system--he has a choice of two abdominal targets. Then, using the inserted organ as a pivot point, he flips around to dangle close to her mouth parts.

"They end up in this posture where his abdomen is above her fangs. She's piercing his abdomen and releasing digestive enzymes, and you can see his legs pulsing in time with sperm transfer," Andrade says. "In the building where I was working, I remember late at night running around looking for someone to tell."

Since then, she's found many people to tell, as she and other researchers have worked to refine the understanding of the pros and cons of sexual cannibalism. Now at Cornell University, she's documenting the male's side of the story, starting with what benefits might make it worthwhile to sacrifice his life for 10 to 25 minutes of sex. Recently, she's been studying whether he has any romantic future to lose when he flips onto those fangs.

Other researchers have looked at similar questions of mating theory in another legendary system, that of praying mantises and their relatives, as well as in spiders of lower celebrity status. As Andrade puts it, "Some of the most powerful tests of a theory are at the extremes."

"At one time I tried to get the emotion out of `cannibalism,' but it's a word that just won't go away," sighs Gary Polis of the University of California, Davis. His 1981 review of scorpion cannibalism, or as he called it, "intraspecific predation," raised the question of whether males are just victims or whether evolutionary pressures push them toward collusion in their own demise. Mates eat mates all over the animal kingdom, but sorting out which selection pressures act on whom has been difficult, Polis says.

When Andrade first saw the redback mating, she already knew that theorists like Ruth Buskirk of the University of Texas at Austin and her colleagues had predicted that suicide during sex could be adaptive.

"The theory predicted it in the extreme, but to many scientists it seemed really unlikely that it would ever be that extreme," Andrade says. In 1984, Stephen J. Gould of Harvard University, for example, wrote an essay dismissing the possibility that evolution could ever make sexual suicide the best course.

To see if the spiders really embody this extreme phenomenon, Andrade studied benefits that a male might reap by offering himself as a snack. In nature, 65 percent of redback matings end with a dead male. In theory, the unlucky suitor might serve as a nutritional boost to give his offspring a head start in life, but at 1 to 2 percent of the female's body size, he hardly even counts as a square meal.

Comparing cannibalistic matings with less traumatic ones, Andrade found no significant difference in egg weight or number of eggs per sac.

She did notice brisk competition among males in the wild. The several hundred eggs that a female lays in one egg sac don't necessarily have the same father. Providing dinner might give a male some kind of edge in the contest for egg share, Andrade speculated. So, she offered laboratory females two consecutive partners and compared their success.

The trick to fathering more eggs seemed to be spending more time fathering. And the trick to prolonging that transfer of sperm seemed to be getting eaten. "A female allows the mating as long as she's preoccupied with consumption," Andrade says. The mate-eating females chowed down just seconds after the male dangled his body over her fangs. Her mouthparts kept moving throughout the mating.