This photo shows a female with a white gift, and a male about to transfer sperm to the sperm storage organs under the rear part of the female’s body. The male has silky threads going to the gift from the silk glands at the rear of its abdomen. This enables the male to remain attached to the gift. (Photo : Allan Lau)

Male spiders have been documented wrapping flies and other "gifts" in their silk and presenting the package to a female with the hopes of mating. New research suggests that these gifts give the males a better chance at fathering offspring.

After studying the behavior of the nursery web spider (Pisaura mirabilis), researchers from the Spiderlab at Aarhus University in Denmark suggest that when a male spider wraps a delicious fly or other edible insect in silk and presents it to a female, the act buys the male more time to mate and increases the odds of passing on his genes.

Upon presenting the gift to the female, the male spider will begin to mate with her while she unwraps and eats the gift. Doing so provides the male an opportunity to transfer more spider sperm into the female while she eats and extends the overall copulation time, which is in the male's interest because it raises the chances of fatherhood.

Spider copulation is fickle. After a male releases his sperm into a female, she stores it in an internal organ until she has eggs ready to deliver it to.

The Spiderlab researchers found that the female stores more sperm in this organ if the male remembers to bring a gift. The greater sperm storage leads to an increased probability of the male being the father of her offspring.

The female spider's ability to regulate how much sperm she stores enables her to show preference towards certain males, the researchers suggest, adding that a male who comes bearing gifts is allowed to give more sperm.

"The female presumably prefers sperm from the gift bearer because it shows that he is resourceful and good at hunting and catching food," the researchers wrote in a statement. "If these are good hereditary traits, the female can transfer the qualities to her offspring by favoring the male. The female spider thus selects sperm from the males she prefers, and has the benefit of passing on their good characteristics to her male offspring."

Interestingly, only 2 percent of male nursery web spiders were observed wrapping and presenting gifts to their mates. This might be because the male is typically killed by the female after copulation either way.

The research is published in the journal Proceedings of the Royal Society B.