Sexual cannibalism is the act of one partner eating the other after sex. In the orb-web spider Argiope bruennichi, the female tries to grab and wrap up the male at the onset of mating so she can snack on him during sex.
In the lab, only about 30 percent of the males survive their first mating, but by letting the female gnaw on them, the males prolong the sex act, making it more likely they will inseminate their partner.
Of these survivors, half go on to find a second mate, while the others try again for the same female. Due to the male's anatomy, two copulations is the limit.
"Two main hypotheses explain the evolution of sexual cannibalism," study researcher Klaas Welke, of the University of Hamburg in Germany, said.
The males might be offering themselves up "to gain access to mating opportunities and to prolong their mating duration."
Or, Welke said that it could be a "paternal investment into their own offspring, and they provide females with nutrients."
In the case of the orb-web spider, males tend to be much smaller than their mates; in this species, they are only one-tenth as heavy, and researchers were not sure how much nutritional benefit the females can gain from such pipsqueak partners.
The researchers thought that perhaps the nutritional bonus from multiple mates and meals would have an impact on females. But it did not.
"We were surprised, because we had expected that any effect of male consumption would increase with every additional male consumed," Welke said.
"Our findings suggest a paternal investment of males into their offspring.
"Sexual cannibalism may increase male reproductive success and may be very beneficial in a species with a high paternity insurance and a low rate of polyandry as found in Argiope bruennichi," Welke added.
The study has been published in journal Animal Behaviour.