To prevent cannibalism, bring chocolate

Here’s another reason to show up with a box of chocolates: It doubles as a shield if she bites.

Edging slowly toward a female, male nursery web spiders clutch in front of their bodies their version of courtship candy: a big dead insect wrapped in white silk. “It’s pretty spectacular actually,” says Søren Toft of Denmark’s Aarhus University. It’s also prudent, he and colleague Maria Albo reported in the May Biology Letters. Sometimes female Pisaura mirabilis spiders just eat males that come calling. In a lab test, however, suitors bearing gifts were almost one-third as likely to be devoured as males that showed up empty-legged.

Males of diverse animal species go wooing with gifts of food or showy things. Biologists have long discussed the evolution of these nuptial gifts, including the possible benefits for male self-defense.

A different lab’s experiment with nursery web spiders had failed to find a defensive benefit because so few courtships in this species end in death. But Toft had been watching the spiders for years and had seen enough fatal flirtations to suspect the gifts, in part, function as shields. So he and Albo set up an experiment with enough mating opportunities to see lives lost — and saved. When a female pounced, “she actually hit the gift with her jaws,” he says. When that happened, the encounter turned from murder to mating.

Pouncing females are rare in this species. Typically, a female grabs the present and punches in her mouthparts to eat. The male then releases his grip and, while the female feeds, transfers sperm using organs near his mouth. The mating lasts, Toft says, “as long as it takes [her] to eat the fly.”

 In observations of nursery web spiders in the wild, “30 percent of males have a worthless gift,” reports Albo, now at Clemente Estable Institute of Biological Research in Montevideo, Uruguay. These sneaks have sucked the innards out of prey and packaged the inedible remains or some other debris into a silk-wrapped lump. Cheating doesn’t pay much: These matings last only as long as it takes the female to discover her pretty present is no gift.