During courtship, males of the spider Pisaura mirabilis offer a prey wrapped in silk to the female as a nuptial gift.
It sounds awfully chivalrous, but researchers say the behavior is not as selfless as it sounds.
Scientists have hypothesized that nuptial gift-giving proves a male's fitness as a mate. It might also ensure their future offspring are well nourished. However, a new study suggests there may be an additional reason for the gift giving ritual: Self-preservation.
In a paper in the journal Biology Letters, researchers found that male spiders who approached potential mates with no gift were more likely to be cannibalized by aggressive female spiders before copulation than those who came with a gift.
"The gift-giving is very serious business for the males," said Søren¿ Toft, an entomologist at Aarhus University in Denmark, who led the study. "Even if just a few of the females are aggressive, male fitness will be reduced to zero if they are unlucky."
The Pisaura mirabilis is one of just a few known spider species that engage in nuptial gift behavior. However, other examples of gift giving can be found throughout the animal kingdom.
For example, some hermaphroditic snails and slugs stab each other with a sharp spear called a love dart just before mating takes place. The dart is covered with mucus that delivers a hormone-like substance to the snail's partner that allows sperm to survive for a longer amount of time.
Katydid males, also known as bush crickets, provide their mate a more tasty treat.
"Some katydid males transfer a spermatophore [a capsule of sperm], most of which consists of a nutritious secretion from the male's genital system," he explained. "It is eaten by the female after being transferred to her, and while the sperm is running to her spermathecae," a reproductive organ.
Toft noted that the nuptial gifts are always produced by males and therefore always benefit the male. They usually benefit the female as well, but not always.
For example, substances in the semen of some fruit flies influence female behavior in the male's, but not the female's, best interest, he said.
P. mirabilis can be found throughout Europe. The males have a relatively short adult lifespan -- about one month. Females, who guard the eggs and take care of juveniles, live a bit longer -- about two to three months.
In a series of more than 280 spider couplings in the lab, the researchers found that females paired with no-gift carrying males cannibalized their partner 15% of the time. When males were carrying a gift, cannibalization happened just 3.6% of the time.
The authors note that most of the time females were not aggressive to non-gift-giving males, even when the females had been deliberately starved for two weeks by the research team. The authors propose that the male habit of gift giving might in part be a precaution against especially aggressive females, even if they are few and far between. Kind of a better-safe-than-sorry scenario.
"Precautions against a lethal attack must be important for males as soon as the risk is non-zero," they write.
However, there are other reasons for the gift giving procedure as well, Toft said.
The bigger the gift, the longer a male gets to spend copulating with his spider-lady love.
"The gift increases the chance that [he] gets to mate in the first place, and increases the duration of the mating, and the amount of sperm he transfers to the female," Toft said.
Since the females mate with several males (collecting yummy silk-covered treats each time), the spider who earns the most mating time is also more likely to fertilize the eggs and pass his awesome gift-giving genes along.