Sexual Prime Peaks When Males "Smell" Mates, Spider Study Shows

John Roach
for National Geographic News

June 16, 2006

For male Australian redback spiders, life is about sex and death. If
they are lucky, the former is part of the latter—female redbacks
eat their male mates during copulation.

But death comes first for 83 percent of the males, according to research.

To boost their chances at successfully fathering offspring, the male spiders can adjust how quickly they mature, depending on how many mating prospects are nearby, said Maydianne Andrade, a zoologist at the University of Toronto at Scarborough in Ontario, Canada.

"The male wants to be the first one to reach the female, but he also has to survive the arduous trip to find the female," she said.

The males mature more quickly when they smell that ample females are close by. But when pickings are slim, they bide their time to bulk up for an extended journey in search of sex, Andrade explained.

She and graduate student Michael Kasumovic reported their research in the April issue of the journal Current Biology.

Jutta Schneider is a biologist at the University of Hamburg in Germany who studies reproductive behavior in spiders. She says the finding is exciting and important.

"Surely most animals use external cues to adjust timing of breeding and migration and other major occasions in their lives," she said in an email.

"But the nice thing with the redback males is that they appear to flexibly adapt growth and maturation to expectations of fitness rewards."

Sexual Cannibalism

According to Andrade, Australian redback spiders—members of the black widow family—are ideal models for testing theories of sexual competition and selection.

(See a photo gallery about spider sex.)

The male encourages the female's cannibalistic tendencies, somersaulting over her mouth parts as he transfers sperm into her, she says.

"If he lets her eat him, it turns out she's less receptive to the next male, and he fathers almost all the offspring," Andrade said.

From a single mating, a female can produce eggs in batches of 100 to 300 about once every month for up to two years, she says.

Once a female mates and secures the sperm she needs, a male is of no value except nutrition, Andrade explained.

"Females are eating machines, and everything they eat contributes to reproduction," she said.

"Like a lot of [spider] species," she added, "male [redbacks] are tiny in relation to females. Males weigh 100 to 200 times less, so they are kind of like a snack, not a big meal."

For a male that stands less than a 20 percent chance of fathering offspring in no more than eight weeks of life, becoming snack food is a small price to pay for a shot at copulation.

"If you have a slim chance of surviving to find a female in the first place, you'll do anything you can to increase your success with that female," Andrade said.

Sex Experiments

In the laboratory experiment, Andrade and Kasumovic exposed males to pheromones, chemical signals that some animals give off as a form of communication, often of a sexual nature.

The spiders in the experiment could not see or touch other spiders around them. They instead were able only to sniff out potential mates and competition.

The researchers found that when a male smelled the presence of ample females, he matured more quickly in order to reach a female virgin before any others.

Once a female mates, she is unlikely to copulate again, Andrade explained.

And even if she does, her first mate will have plugged up her genitals to prevent passage of another spider's sperm. Virgins are therefore sought after by males.

But if a male smells only other males nearby and detects females in the distance, he'll bide his time to store up the energy reserves he'll need to survive the search for a mate.

The trip typically involves travel to distant webs and an extended courtship ritual.

The males don't eat or drink when on the search for a female and must compete with other males for the female's attention.

Without ample energy stored, he'll "soon collapse and die of starvation and desiccation [dehydration]," Andrade said.

Michael Maxwell is a biologist at National University in La Jolla, California, who studies sexual cannibalism, mate choice, and sperm competition in praying mantises, a type of insect.

He said Andrade's research shows that female presence has an effect on how and when the spiders develop, but he would like to see further study on how that decision plays out in the future.

For example, if a spider sniffs few females and thus takes longer to develop, "he's making the assumption that that ratio won't change over a few weeks. That assumption should be tested," Maxwell said.

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