Exaggerated sexually-selected traits, occurring more commonly in males, help individuals to increase reproductive success, but are costly to produce and maintain. These costs on the one hand may improve population fitness by intensifying selection against maladapted males, but on the other hand may increase the risk of extinction under environmental challenge. However, the impact of sexually selected traits on extinction risk have not been investigated experimentally. We used replicate populations of a male-dimorphic mite, Rhizoglyphus robini, to test if prevalence of an elaborate, sexually-selected weapon affected the risk of extinction under gradual temperature increase (20C per generation). As temperature increased, individual survival decreased, but this effect was much more dramatic in populations with high weapon prevalence, compared to populations in which weapon expression was low. Consequently, the former was significantly more prone to extinction than the latter, with 75% vs 8% populations going extinct, respectively. Extinctions occurred despite partial suppression of the weapon expression at increased temperature, and were not explained by increased male mortality. Our results provide the first, to our knowledge, experimental evidence demonstrating dramatic effect of elaborated sexual traits on the risk of extinction under environmental challenge.

Natural host populations differ in their susceptibility to infection by parasites, and these intra-population differences are still an incompletely understood component of host-parasite dynamics. In this study, we used controlled infection experiments with wild-caught guppies (Poecilia reticulata) and their ectoparasite Gyrodactylus turnbulli to investigate the roles of local adaptation and host genetic composition (immunogenetic and neutral) in explaining differences in susceptibility to infection. We found differences between our four study host populations that were consistent between two parasite source populations, with no indication of local adaptation by either host or parasite at two tested spatial scales. Greater host population genetic variability metrics broadly aligned with lower population mean infection intensity, with the best alignments associated with Major Histocompatibility Complex (MHC) ‘supertypes’. Controlling for intra-population differences and potential inbreeding variance, we found a significant negative relationship between individual-level functional MHC variability and infection: fish carrying more MHC supertypes experienced infections of lower severity, with limited evidence for supertype-specific effects. We conclude that population-level differences in host infection susceptibility likely reflect variation in parasite selective pressure and/or host evolutionary potential, underpinned by functional immunogenetic variation.