Funnelweb spiders: a knockout date.

Some male spiders knock out their mates before sex. Females, meanwhile, can view dinner and date as one and the same. Whereas funnelweb males overpower their partner with pheromones, male redbacks, who get a single stab at sex, go to equally elaborate lengths to be eaten, the meeting of the Animal Behavior Society in Corvallis, Oregon, heard today.

Male funnelweb spiders (Agelenopsis aperta) leave their webs in search of mates and are attracted by a female's pheromomes. Using an elaborate dance of abdomen-waving and web-vibrating, they manoeuvre themselves face-to-face with their mate.

"Suddenly the male will walk forward, and at the same time the female will collapse," says Fred Singer of Radford University in Virginia. Out cold, males take advantage of the senseless female for between several hours and a day. Although she may wake up occasionally, the female is simply re-sedated.

The sedative pheromone has probably evolved to defuse females' tendency to eat everything they encounter. Females benefit from this too, says Singer, "if she doesn't [pass] out, she isn't going to mate." Females may also judge whether males make good mates from the strength of their pheromones, he speculates.

Although males are susceptible to the knockout pheromone, they seem to resist it in the wild - perhaps because they fire it in one direction. But this can backfire in the act: "Occasionally she'll wake up and eat him."

Athletic act

The male redback spider (Latrodectus hasselti), meanwhile, embraces his post-copulative fate. While mating, he somersaults to place his abdomen over the female's fangs - with predictable results.

"The male has two copulatory organs," explains Maydianne Andrade, of the University of Toronto at Scarborough, Canada. "He inserts one, somersaults, goes back - when he's been partly eaten - inserts the second one, and somersaults again. It's pretty fabulous."

Redback spiders: a tasty dinner-date.

Males offer their bodies as food because finding another mate is risky, Andrade told the meeting. More than 80% of males die en route from their web to that of a female, Andrade found, by studying redbacks in Western Australia. Besides falling prey to their own species, males succumb to ants and lack of food and water.

Given such a tiny chance of mating twice, "the male may as well put everything he has - literally - into the one mating opportunity," says Phil Taylor, who studies spider behaviour at the University of Cincinnati, Ohio.

Female fidelity is the reward for sacrifice - females are less likely to mate again if they eat their first male. Even if they do, they fertilize a higher proportion of eggs with the sperm of males they consume.

Males that are eaten father about 40% more offspring than those that live to mate again, Andrade found, by comparing the benefits of sacrifice against the low probability of finding a new mate.

Why female redbacks play by the rules is a mystery - they never eat males before mating. "Females are not being forced in any way," says Andrade, who thinks they may be showing some taste in partners.