Sexy to die for? Sexual selection and the risk of extinction

Abstract

Sexual selection is a field with a strong focus on the 'costs' of traits. However, whether such costs have an influence on the demography of the population is very rarely discussed. Here we present various processes through which sexual selection might have an impact on population viability and thus increase or decrease the risk of extinction. We argue that evolutionary 'suicide' - as sometimes suggested e.g. to have caused the extinction of the Irish elk - is unlikely in deterministic environments, except if costs are not paid by the same individual that bears the trait. Thus, intra- or inter-locus sexual conflict could in principle drive a population extinct, and we do not know why this does not frequently happen. Whether sexual selection increases or decreases extinction risks when populations face variable or unforeseen environmental conditions is likewise unknown, and we outline mechanisms that could account for either pattern. Inbreeding is another factor that could either increase or decrease population viability in sexually selected species. Inbreeding may be caused by a high mating skew, but it could also be reduced if females adaptively choose mates to avoid inbred offspring. Finally, when intraspecific competition for resources is taken into account, it is unclear how individual viabilities translate to extinction risks faced by the population. We show an example where greater mortality of males due to sexual dimorphism improves the carrying capacity of the environment, and thus presumably population viability.