Underage Spiders: Males show unexpected interest in young mates

To the surprise of biologists, a male Australian redback spider will mate with a juvenile female before her reproductive tract has an external opening. The male bites through the immature female’s outer covering and by doing so, protects his own life.

DANGEROUS LIAISON. Among adult Australian redback spiders, the male (in upper position) is much smaller than the female. A. Mason

This discovery adds a new twist to a textbook example of extreme mating practices. Until now, biologists had focused on these Latrodectus hasselti males’ apparently suicidal sexual behavior with adult females.

While delivering sperm to a mature female, a male typically flips his body and ends up dangling above her fangs, explains Daniela Biaggio of the University of Toronto at Scarborough. Many of these males become lunch, but studies have shown that their sacrifices boost their share of their mates’ offspring (SN: 11/13/99, p. 312).

The studies had overlooked an alternative male tactic, Biaggio reported Aug. 14 in Snowbird, Utah, at the annual meeting of the Animal Behavior Society. She’s found that given a chance in the lab, males mate non-suicidally with females that haven’t yet molted the final time.

Biaggio says that she discovered this alternative mating strategy while observing a practice that’s widespread among spider species: adult males hanging around immature females. As Biaggio was arranging her subjects after an experiment, she found one pair entangled. “I freaked,” she says.

Her coauthor Maydianne Andrade, also in Scarborough, says that she was “incredulous” when Biaggio first told her the story. However, they took a new look at female anatomy. About 2 days before a female’s final molt, her internal reproductive organs look fully formed.

Biaggio then paired adult males with females at various phases of their final premolt periods. The only mating occurred among males paired with females just a few days from molting. Of 18 males in this group, 72 percent bit through the female’s cuticle and delivered sperm. They rarely somersaulted toward the female’s mouth, says Biaggio.

Afterward, the females molted normally and laid fertile egg clutches. Biaggio says that males won’t mate again if they’ve used both of their non-refillable sperm-delivery organs.

Andrade says, “After we had substantiated [Biaggio’s] observations, I realized I had likely seen this behavior in the field but wrote it off as misguided males mounting juvenile females.”

Arthropod behaviorist Linda Rayor of Cornell University says that she hasn’t heard another report of male spiders breaking through the outer covering, or exoskeleton, of juveniles to mate. “This is really weird,” she says.

Yet she adds that it fits with the extreme pressures on males among many insects and spiders, including the redbacks, to be a female’s first mate. That early suitor of adult females often fathers the largest percentage of her offspring.

Eileen Hebets, who studies arthropods at the University of Nebraska in Lincoln, notes that insects don’t always respect the integrity of each other’s exoskeletons. Male bedbugs, for example, routinely ignore female reproductive-tract openings and poke their own openings for mating.

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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