April 10, 2006 — Male redback spiders can speed up their sexual development in order to beat rivals to nearby females, says a new study published in the April 4 issue of Current Biology.

"It's the first time that it's been shown that males are actually changing their development in response to both sexual and natural selection," said Michael Kasumovic, a PhD candidate at the University of Toronto at Scarborough, in an April 5 press release. Kasumovic is a coauthor of the study with Professor Maydianne Andrade.

The Australian redback spider (Latrodectus hasselti), a cousin of the black widow spider, exhibits extreme sexual dimorphism; the marble-sized female weighs 100 to 200 times more than the male, which is about the size of a grain of rice. Though the female can live up to two years, the tiny male only lives for four to eight weeks.

The male remains on the web where he was born until wind conditions are right, then disperses and builds his own web. After molting he begins building up fat reserves to help with the strenuous task of locating and courting a female. Upon reaching sexual maturity the male stops eating and focuses on finding a mate.

Competition is fierce, and as many as eight males will compete for the attentions of a single female. During mating the male deposits a small piece of his copulatory organ inside the female. Called a sperm plug, it prevents the other males from fertilizing the female's eggs. So in order to pass on his genes, the male redback spider has to be the first to mate with the female.

"On the one hand, they want to provision themselves to survive this trip," said Andrade in the release. "On the other hand, they have to develop quickly so that they become sexually mature in time to mate with these virgin females before the females mate with someone else."

Andrade and Kasumovic found that male redback spiders can determine at an early age whether it would be more beneficial to speed up sexual development or to steadily build ample fat reserves.

"We call it 'scent of a woman,'" said Andrade in the release. "They do this without contact — they smell how many females are around them."

Kasumovic reared male redback spiders in the presence or absence of females in a laboratory at the University of Toronto at Scarborough, said the press release. The spiders were not allowed to see or touch one another, so the only cue to a female's presence was her pheromones.

"What we were mimicking there was a situation where a male is developing as a juvenile in a habitat with lots of females close by — in other words, he doesn't need to be well-provisioned — versus a situation where female webs are very far away and he would need to be well-provisioned to survive the trip to find her," said Kasumovic in the release.

When males could smell females they developed rapidly, trading body size for the ability to get to the females before their rivals, said Kasumovic. In the absence of females, males developed at a normal pace.

"This provides a really good explanation for why there is so much size variation (among males) in the wild," said Kasumovic in the release. "Everybody always states that the small males are a by-product of them not being able to get enough resources, but we're showing that they're intentionally developing that way."

The study was funded by the Natural Sciences and Engineering Research Council of Canada, the Canada Foundation for Innovation and the Ontario Innovation Trust.