Some Male Spiders Give Worthless Gifts to Their Prospective Mates

The finest silk wrapping can keep females occupied for just long enough.
A mating pair of Paratrechalea ornata spiders with a nuptial gift.

Male Paratrechalea ornata spiders offer silk-wrapped gifts to their prospective mates. In this image, a female (left) and male (right) are both holding a gift. 

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Maria Albo

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Joshua Learn, Contributor

(Inside Science) -- Male spiders that give females worthless gifts seem to bank on the idea that it's the thought that counts -- even gifts that have no value give them more time to mate. 

Paratrechalea ornata is a roughly bottle cap-sized semi-aquatic spider found in Uruguay and surrounding South American countries. Like other male spiders across the world, the males of this species give females nuptial gifts -- silk-wrapped packages presented as a form of courtship. But P. ornata males also often use a form of deception to increase their chances of mating -- they give away worthless gifts.

"You all the time find males with worthless gifts," said Maria Albo, an evolutionary biologist at the University of the Republic in Uruguay. Some males will wrap up their dried-up prey leftovers, while others will even stoop so low as to wrap up plant bits. 

In a study published recently in the journal Ethology, Albo's team wanted to see how successful this tactic was for the males, and whether the females counteracted the deception by cutting the mating short. The researchers gave males the option of wrapping a dead fly -- a nutritious morsel that would make a worthwhile gift for the discerning female -- or the worthless, empty casing left by a molting beetle larva. 

In another recent study, the researchers found that the males will often wrap up the larva molt for the female even when they have the option of giving a better gift. Albo and her colleagues found that the male invests more silk in wrapping the worthless gift, but it takes the female longer to open it up. Albo speculated that while the female is distracted, the male can mate with her for longer. Based on these new findings, the researchers believe that the males may give worthless gifts to extend their mating times longer than would a nutritious gift.

The worst thing is, the male doesn't even fully give the gift up, worthless or not. Instead, he holds onto the silk with his third pair of legs during mating. "The silk can help him retain the gift if the female wants to run away," Albo said. 

In general, the idea of nuptial gifts is that they provide females with more food, helping to sustain them through pregnancy. Clearly, that doesn't happen if the gift is worthless. Albo and her colleagues are now studying whether the worthless gift tactic evolved as a response to low food availability. 

Author Bio & Story Archive
Joshua Rapp Learn (@JoshuaLearn1) is an expat Albertan based in Washington, D.C. He reports on science for publications like National Geographic, New Scientist, Smithsonian, Scientific American, Washington Post, The Atlantic, Science and Hakai.