Feature

The bizarre sex lives of spiders

Some even give Valentines...

What girl could resist a tasty treat hand-wrapped by a gentleman caller on Valentine's Day? Female Paratrechalea ornata spiders certainly can't: When males from this South American species come calling, they bring a silk-wrapped piece of insect prey as a courtship gift to up their chance at mating.

Plenty of animals, including some birds, insects, and spiders, know that a sure way to win female's affection is with a gift, and will give potential mates some small token before attempting to reproduce with them. For P. ornata, the gift itself should be good, but new research says that presentation counts, too.

Researchers from Uruguay collected these spiders and painted the mouthparts of some of the males white, to make it look like they were carrying a silk-wrapped present. When the females were introduced to the painted and unpainted males, they showed a clear preference for the ones with white mouths. They spent more time interacting with them, touched them more and mated with them sooner and more frequently than with the plain-faced males. The researchers also looked at how different males wrapped their gifts and found that hungry spiders do a pretty poor job. They wrap fewer layers around the prey and let it show through the silk, which makes it appear darker. Well-fed males do a much more thorough wrap job and add more silk to give the gift a uniformly white appearance.

These results led the scientists to believe that, for females, a big part of the appeal of these gifts is the bright white wrapping. The white silk helps the nocturnal spiders spot their suitors in the dark, and the quality of wrap can also tell females about what kind of condition a male is in and how good a mate he might be.

But even if his gift looks a little dull, just showing up with one gives a male spider a leg or eight up on competitors who arrive empty handed. Across the spider species that use gifts as part of their courtship, females show a clear preference for guys who bring gifts over those who don't, and accept them for more, and longer, mating sessions.

It's an oddity of spider sex that makes the gifts such an important part of their mating game. When spiders get it on, the male's sperm doesn't head right for the egg. Instead, the female stores it in an organ called a spermatheca until she's ready to use it. Her eggs don't get fertilized until she wants them to, and by controlling the amount of sperm she accepts and stores from different males, she also gets some say in who does the fertilizing.

By bringing a gift, a male spider not only buys himself more time to transfer sperm to a female, but also ensures that she'll also hold on to more of it. When researchers gave female nursery web spiders (Pisaura mirabilis) equally long mating sessions with males who brought gifts and males who didn't and then dissected them, they found that the females stored almost twice as much sperm from the males who brought a present. With that one little bit of food, a male spider gains two different advantages that raise his chances he'll be the father of his mate's next batch of spiderlings.

Males who come bearing gifts seem to get preferential treatment because they've proven that they're able to hunt for food. The traits that contribute to that are ones that a spider mom-to-be would want to pass on to her offspring, making a gift-giving male a good mate and father. This assessment doesn't always work out for the females, though, because males will sometimes try to trick her with a fake present.

Instead of wrapping up a tasty insect, a male looking to mate without the hassle of hunting down some prey will give a female a plant seed or some other bit of inedible junk. He'll start mating with her while she unwraps it, and by the time she realizes she's been duped, he's already transferred some of his sperm to her. When she realizes the deceit, the female will shove the male away, costing him precious mating time and putting him at a disadvantage in the competition between his sperm and other spiders'. The blow isn't too bad, though: Studies have shown that males who use fake gifts still get to mate longer than ones who offered nothing at all, and females will lay almost the same amount of eggs fertilized from males bearing fake gifts as ones with the real deal.

And if all that's not weird enough for you, consider the spider Nephilengys malabarensis, which often leave its lover a much more personal souvenir. During sex, the male spider snaps off his paired sperm-delivering genitals, called palps, inside the female, leaving them behind to continue pumping sperm while he fights off other males who want to mate with her. Since the females also wind up killing and eating the males around three quarters of the time, this remote detonation tactic also lets the male keep transferring sperm even if he's been turned into a snack or decided to run for his life.

Suddenly, your last one night stand seems downright tame.

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