Person:

Nelson, Adam

Abstract Sexual size dimorphism results when female and male body size is influenced differently by natural and sexual selection. Typically, in polygynous species larger male body size is thought to be favored in competition for mates and constraints on maximal body size are due to countervailing natural selection on either sex; however, it has been postulated that sexual selection itself may result in stabilizing selection at an optimal mass. Here we test this hypothesis by retrospectively assessing the influence of body mass, one metric of body size, on the fitness of 113 wild‐derived house mice (Mus musculus) residing within ten replicate semi‐natural enclosures from previous studies conducted by our laboratory. Enclosures possess similar levels of sexual selection, but relaxed natural selection, relative to natural systems. Heavier females produced more offspring, while males of intermediate mass had the highest fitness. Female results suggest that some aspect of natural selection, absent from enclosures, acts to decrease their body mass, while the upper and lower boundaries of male mass are constrained by sexual selection.

Background: Laboratory studies show that the sexual selection via mate choice and intrasexual competition can profoundly affect the development and fitness of offspring. Less is known, however, about the total effects of sexual selection on offspring in normal social conditions. For many animals, opportunity for mating success is determined by complex social interactions, such as dominance hierarchies. Social selection is an extended view of sexual selection that incorporates competition during sexual and nonsexual interactions, and predicts complex evolutionary dynamics. Whether social selection improves or constrains offspring fitness is controversial. Results: To characterize the consequences of social selection, we introduced wild-derived mice to seminatural competition for three consecutive generations (promiscuous lineage). In parallel, we bred a control lineage in cages using random mate assignment (monogamous lineage). A direct competition experiment using second-generation animals revealed that promiscuous line males had greater reproductive success than monogamous line males (particularly during extrapair matings), in spite of higher mortality and equivalent success in social dominance and sperm competition. There were no major female fitness effects, though promiscuous line females had fewer litters than monogamous line females. We confirmed a behavioral sexual attraction mechanism by showing that, when given a choice, females had both odor and mating preferences for promiscuous line over monogamous line males. Conclusions: Our study demonstrates novel evidence for the strength of sexual selection under normal social conditions, and shows rapid male adaptation to sociality driven largely by sexual trait expression, with tradeoffs in survivorship and female fecundity. The speed at which these phenotypes emerged suggests the possibility of transgenerational inheritance. Mouse population densities fluctuate dramatically in nature, and we propose that sexually selected phenotypes arise dynamically during periods of social competition.