Abstract
Large male body size is typically favored by directional sexual selection through competition for mates. However, alternative male life-history phenotypes, such as "sneakers," should decrease the strength of sexual selection acting on body size of large "fighter" males. We tested this prediction with salmon species; in southern populations, where sneakers are common, fighter males should be smaller than in northern populations, where sneakers are rare, leading to geographical clines in sexual size dimorphism (SSD). Consistent with our prediction, fighter male body size and SSD (fighter male∶female size) increase with latitude in species with sneaker males (Atlantic salmon Salmo salar and masu salmon Oncorhynchus masou) but not in species without sneakers (chum salmon Oncorhynchus keta and pink salmon Oncorhynchus gorbuscha). This is the first evidence that sneaker males affect SSD across populations and species, and it suggests that alternative male mating strategies may shape the evolution of body size.
Highlights
Both natural and sexual selection can influence the magnitude and direction of sexual size dimorphism (SSD) between males and females
Large body size in females is favored by fecundity selection, whereas large size in males is often favored by sexual selection through malemale competition or female preference for larger males (Andersson 1994; Fairbairn 1997)
The frequency of male parr maturation decreased with latitude across populations of Atlantic and masu salmon
Summary
Both natural and sexual selection can influence the magnitude and direction of sexual size dimorphism (SSD) between males and females. If mating skew results from size-dependent male competition, small sneaker males should reduce the relative fitness of all large “fighter” males or decrease the strength of directional selection on fighter male body size if larger fighter males disproportionately lose copulations to sneakers. These nonexclusive mechanisms lead to the common prediction that fighter males will be smaller in populations with sneaker males than in populations without sneaker males, such that SSD will be less pronounced or even female biased (Neff 2001)
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