Males are expected to mate with as many females as possible, but can maximize their reproductive success through strategic mating decisions. For instance, males can increase their own fitness by mating with high quality females that produce more offspring. Additionally, males can adjust mating effort based on the relative distribution of females and male competitors. To test factors that influence male mate choice, we assessed male mating decisions in the golden-web spider, Trichonephila clavipes (Nephilidae), a species in which females are polyandrous and males guard females before and after copulation occurs. We tested the hypothesis that males spend more time guarding high quality females that are spatially isolated, and when the risk of sperm competition is higher. We assessed solitary and aggregated female webs in the field and quantified female quality (i.e. female body condition), the risk of sperm competition (i.e. number of males in each female web), and mate-guarding duration (i.e. number of days each male spent in each web). We found that mate-guarding behaviour is influenced by the presence of male competitors. In addition, the type of web seems to moderately influence male mating decisions, with males spending more time guarding a female when on solitary webs. Finally, female body condition seems to play a small role in mate-guarding behaviour. As mate-guarding duration considerably increased per each additional male competitor in the web, and guarding behaviour prevents males from seeking additional mates, it seems that males do benefit from guarding females. We conclude that failing to guard a sexual partner promotes high costs derived from sperm competition, and a male cannot recover his relative loss in fertilization success by seeking and fertilizing more females. In addition, the search for more sexual partners can be constrained by possible high costs imposed by predators and fights against other males, which may explain why the type of web only moderately influenced male mate choice. Following the same rationale, if high-quality females are not easy to find and/or mating with a high-quality female demands much effort, males may search females and guard them regardless of female quality. In conclusion, the factor that most influences male mate-guarding behaviour among T. clavipes in the field is the risk of sperm competition.
Audience effects in the Atlantic molly (Poecilia mexicana)–prudent male mate choice in response to perceived sperm competition risk?