Males sacrifice their legs to pacify aggressive females in a sexually cannibalistic spider

Abstract

Monogynous male mating strategies have repeatedly evolved in spiders along with female-biased sexual size dimorphism (SSD) and extreme male mating investment. As a manifestation of sexual conflict, male African golden-silk spiders, Trichonephila fenestrata, are regularly attacked by females during copulation, and sexual cannibalism is common. Curiously, attacked males actively cast off (autotomize) their front legs and copulation continues while the female is feeding on these legs. Since the loss of legs is costly in reducing males’ ability to mate guard, it should yield significant benefits. In a series of experiments, we investigated the behavioural mechanism of male leg ejection and tested three hypotheses. First, we performed feeding experiments to test whether conspecific male legs are particularly attractive for females and act as a sensory trap. Second, we conducted mating experiments with sibling and nonsibling pairings to test whether males preferentially invest in high-quality mates. Third, by offering male legs during copulations, we tested whether male leg ejection serves to distract and pacify females. In support of the female pacifier hypothesis, our results confirm a significantly reduced probability of attacks in females that had been offered a male leg, but we found no relationship between simulated leg ejection and male survival. While there was no evidence for special properties of male legs, females accepted male legs significantly more often than insect food. The degree of male leg sacrifice did not depend on male–female relatedness, but large males lost more legs than small males, and small males achieved more copulations. However, total copulation duration was unrelated to male size. Male leg sacrifice in T. fenestrata may represent a rare example of an evolutionary transition in which the antipredation behaviour of autotomizing body parts has changed its function into a sexual context, here to pacify females and to facilitate undisturbed copulations.