Crickets increase sexual signalling and sperm protection but live shorter in the presence of rivals.

The sociosexual environment animals experience through their life can shape the evolution of key life history traits, including longevity. Male-male competition, for instance, may influence the resources allocated to traits involved in male reproductive success. Here, I test whether lifelong exposure to a competitor male influences male investment in pre- and post-copulatory sexual traits (calling effort and sperm quality) and how this affected the oxidative status and longevity of male field crickets (Gryllus bimaculatus). As expected, the visual exposure to a mating competitor promoted a higher calling effort in the male crickets but resulted in a decline in the haemolymph antioxidant content. The haemolymph antioxidant content negatively covaried with the antioxidant content of the sperm but interestingly, only when the males were exposed to a competitor. This suggests that when mating competition is high, males may prioritize sperm antioxidant protection against somatic maintenance. Finally, I provide evidence that male-male competition imposes additional costs to reproduction, as males exposed to a mating competitor lost a significant proportion of body antioxidant defences and body mass over time and in long term, had a reduced lifespan. Overall, this study strongly suggests that intrasexual competition may impose additional oxidative costs during reproduction for males, with negative consequences on lifespan. Moreover, the results highlight the role of oxidative stress as a physiological mechanism underlying the trade-off between reproduction-longevity and through which sexual selection may contribute to the evolution of senescence patterns.