Male nursery web spiders (Pisaura mirabilis) who offer up a fly wrapped in silk to females during courtship are more likely to survive mating encounters, a new study reports. His so-called "nuptial gift" shields him against the cannibalistic attacks of aggressive females, according to findings published in Biology Letters this week. 

Three non-mutually exclusive hypotheses have been proposed to explain the evolution of gift-giving behavior in male nursery web spiders. First, a nutritious gift may serve as parental investment, a contribution that might increase the number or quality of his offspring. Second, it showcases his mating effort and helps him obtain more matings. Or thirdly, the present protects him against the female’s cannibalistic acts. 

To investigate, Aarhus University in Denmark’s Søren Toft and Maria Albo collected immature spiders, and when they became adults, the females were divided into two groups: well-fed and poorly fed. After a week, females from each group were presented with either a gift-carrying male or a male bearing no gift. The females typically reacted in one of four ways: running away with the present without mating, cannibalism, mating, or male rejection. 

Most females had low levels of aggression, even if they had been starved for a week. But among aggressive females, gift-stealing is the most likely outcome if the male has a gift, and cannibalism if he has not. Female hunger had no major influence.

Females presented with no-gift males were more than six times as cannibalistic on average than females with gift-carrying males. Of the 79 females staged with no-gift males, 15 cannibalized their partner before copulation could take place. Only one of 28 females cannibalized her gift-carrying male, and this was committed after copulation. 

These findings provide the first experimental evidence for the anti-cannibalism hypothesis. Nuptial gift-giving, it seems, evolved partly as a counter-adaptation to female aggression. It helps to have strong forelegs: Sometimes a male would use his forelegs to keep the female’s mouthparts away from his body, redirecting it towards the wrapped-up prey item instead.