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

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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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