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Sexual cannibalism makes mating dangerous for male spiders of many species.
The females may gobble up the male before, during or after mating, so males have evolved various ways to cheat death. In some species, they wait until the female is busy eating and rush in to copulate while her attention is elsewhere. In other species, they even bring an edible gift to distract their intended.
Male nursery web spiders have evolved a different kind of defense. They tie up the female before copulation, immobilizing her legs by wrapping them in strong spider silk so they can copulate and escape with their lives before the female gets free.
Researchers had noted that this behavior was probably defensive, given the generally high level of danger in spider mating. But Alissa G. Anderson, a graduate student in biology at the University of Nebraska, Lincoln, said that cannibalism had not been reported in nursery web spiders by other researchers.
That might have been because the males always tied up the females and so were never eaten. “I have never seen a male not wrap the female,” in nature, Ms. Anderson said. She and her adviser, Eileen A. Hebets decided to run some experiments.
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As they reported in Biology Letters last month, they used dental silicone to block some male spiders’ spinnerets — where the silk comes from. They left other males free to execute the usual plan. As expected, the males that couldn’t produce silk were eaten much more often than the other males.
Males with longer legs were also more successful at surviving, showing that size is important if you are in a species in which courtship makes mixed martial arts look like a Texas two-step.
Ms. Anderson, whose work on the spiders is part of her doctoral research, wants to determine if there is a benefit for the females as well as the males in this kind of mating.
The males who are able to wrap the females’ legs have more successful copulations, as measured by the number of insertions of the pedipalp, which delivers sperm to the female. And if the result is that the females who have been wrapped have more eggs, that could mean that both sexes benefit and the behavior is, at least in evolutionary terms, mutually beneficial.
For the males with blocked spinnerets, mating was usually terminal. Really terminal. The females don’t leave leftovers. “You may see a leg or two fall off,” as the females are eating, Ms. Anderson said, “but they usually eat the whole thing.”