Male spiders do something out of the ordinary during lovemaking - but it's not merely kinky behavior, but instead a method of self-preservation.
They appear to practice bondage during sexual intercourse, wrapping their female partner's legs with silk to reduce their risk of being cannibalized, according to a new study.
Publishing their findings in the journal Biology Letters, behavioral ecologists Alissa Anderson and Eileen Hebets of the University of Nebraska have paired male and female nursery-web spiders to find out how this restraining practice protects males. Some of the males ably spun protective strands, while the rest were barred from spinning.
Those who couldn't restrain their partners mated almost as much as males that could bind theirs. However, unprotected male spiders appeared to be much more likely to be treated by the females as post-coital grub.
Previous studies have explored on this unusual mating behavior among male spiders, leaving researchers to wonder if having longer legs would enable them to restrain the hungry females and survive cannibalism.
"[Cannibalizing is] quite beneficial for the female," Anderson told Live Science, explaining that the male sex partner likely serves as a useful resource for the developing offspring of a newly fertilized female spider. In one species, a female spider consuming the male post-lovemaking leads to more children and increases the offspring's survival rate and weight.
These competing interests - the female's need for nourishment and the male's need to stay alive - lead to the sexual quirk.
Silk has multiple purposes in this part of the animal kingdom. They can bring the spiders back to safety during falls or other threats, or help them catch prey. In sex-survival spinning, males twist strands of silk around female legs, while others drape their lover with the material.
Males with longer legs and bigger bodies, added the authors, which successfully wrapped up their partners, have most probably walked away after sex. Tying their partners up also emerged as a way to create more insertions and have better chances at successful fertilization.
But regardless of their size, male spiders wrapping up females in silk were about seven times less likely to be eaten during or after mating, and also about seven times more likely to penetrate females twice.
The researchers will continue to investigate the benefits and costs of silk wrapping during and after sex whether the critters are male or female.