Operational sex ratio, sexual conflict and the intensity of sexual selection.

Modern sexual selection theory indicates that reproductive costs rather than the operational sex ratio predict the intensity of sexual selection. We investigated sexual selection in the polygynandrous common lizard Lacerta vivipara. This species shows male aggression, causing high mating costs for females when adult sex ratios (ASR) are male-biased. We manipulated ASR in 12 experimental populations and quantified the intensity of sexual selection based on the relationship between reproductive success and body size. In sharp contrast to classical sexual selection theory predictions, positive directional sexual selection on male size was stronger and positive directional selection on female size weaker in female-biased populations than in male-biased populations. Thus, consistent with modern theory, directional sexual selection on male size was weaker in populations with higher female mating costs. This suggests that the costs of breeding, but not the operational sex ratio, correctly predicted the strength of sexual selection

Data from: Male-male aggression is unlikely to stabilize a poison frog polymorphism

Phenotypic polymorphism is common in animals, and the maintenance of multiple phenotypes in a population requires forces that act against homogenizing drift and selection. Male-male competition can contribute to the stability of a polymorphism when males compete primarily with males of the same phenotype. In and around a contact zone between red and blue lineages of the poison frog Oophaga pumilio, we used simulated territorial intrusions to test the non-exclusive predictions that males would direct more aggression toward males of (i) their own phenotype and/or (ii) the phenotype that is most common in their population. Males in the monomorphic red and blue populations that flank the contact zone were more aggressive toward simulated intruders that matched the local coloration. However, males in the two polymorphic populations biased aggression toward neither their own color nor the color most common in their population. In sympatry, the rarer color morph gains no advantage via reduced male-male aggression from territorial males in these O. pumilio populations, and so male aggression seems unlikely to stabilize color polymorphism on its own. More broadly, these results suggest that the potential for divergent male aggression biases to maintain phenotypic diversity depends on the mechanism(s) that generate the biases and the degree to which these mechanisms persist in sympatry