Mate-guarding strategies and male competitive ability in an orb-weaving spider: results from a field study

Competition between males to reproduce has led to various forms of resource-guarding behaviour. A recent theoretical investigation suggested that guarding strategies should be closely linked to the risk of take-over and the competitive ability of males, but experimental evidence is still scarce. We investigated precopulatory mate guarding of premoult females in a solitary orb-weaving spider Zygiella x-notata (Araneae: Araneidae). We tested the hypotheses that larger males should (1) be more successful at guarding, (2) acquire their mates by usurping them from other males (take-over), and (3) start guarding females as close as possible to their final moult. In a field study, we recorded the mating activity of marked males throughout a breeding season and conducted a male removal experiment to suppress male–male competition near randomly selected guarded females. We found strong male–male competition for guarding premoult females. Male take-over occurred frequently in the population, even though guarding was efficient at locally reducing male turnover near females. Large males were overall more successful in their guarding attempts because of their higher competitive ability, but guarding success did not increase linearly with body size. Large males mainly acquired females by evicting previous guardians. They spontaneously started guarding females closer to their moult than small males; however, this was not significant when no direct competition occurred between them. We discuss possible mechanisms to explain variation in guarding duration and success and the evolution of male body size in this spider.