Male Mate Choice Dependent On Male Size in Salmon

Abstract

[Non-anadromous sockeye salmon males (kokanee), Oncorhynchus nerka, were tested to determine if they select mates according to their absolute size (fork length) or if male mate choice was governed by the size of the male. In a series of four experiments, involving three size groups of mâles and females, the size of females presented and males tested were systematically varied. The results demonstrated male choice is dependent on male size and not solely on the absolute size of females as previously suggested in salmon and other fishes. Males of all sizes discriminated against females smaller than themselves, but did not discriminate between females of their own size and those larger. Thus, male selectivity increases with male size in salmon. The smallest males are the least selective of mates whereas the largest males arc the most selective, preferring the largest females. This varying selectivity corresponds to the availability of females on the spawning grounds. Small males have the most limited range of potential mates (because of the effects of size in intrasexual competition and female choice) whereas large males have the widest range in potential mates. The experimental results presented are consistent with two observed patterns of association of male and female salmonids on the spawning grounds: a) the number of males aggregated around females increases with increasing female size; and b) pairing between the sexes is assortative by size. Assortative mating by size was demonstrated to be further increased by male intrasexual competition. These results suggest that intrasexual competition and mate choice by the opposite sex can lead to the evolution of varying strategies of mate choice within sexes, just as it can lead to the evolution of varying strategies of seeking mates., Non-anadromous sockeye salmon males (kokanee), Oncorhynchus nerka, were tested to determine if they select mates according to their absolute size (fork length) or if male mate choice was governed by the size of the male. In a series of four experiments, involving three size groups of mâles and females, the size of females presented and males tested were systematically varied. The results demonstrated male choice is dependent on male size and not solely on the absolute size of females as previously suggested in salmon and other fishes. Males of all sizes discriminated against females smaller than themselves, but did not discriminate between females of their own size and those larger. Thus, male selectivity increases with male size in salmon. The smallest males are the least selective of mates whereas the largest males arc the most selective, preferring the largest females. This varying selectivity corresponds to the availability of females on the spawning grounds. Small males have the most limited range of potential mates (because of the effects of size in intrasexual competition and female choice) whereas large males have the widest range in potential mates. The experimental results presented are consistent with two observed patterns of association of male and female salmonids on the spawning grounds: a) the number of males aggregated around females increases with increasing female size; and b) pairing between the sexes is assortative by size. Assortative mating by size was demonstrated to be further increased by male intrasexual competition. These results suggest that intrasexual competition and mate choice by the opposite sex can lead to the evolution of varying strategies of mate choice within sexes, just as it can lead to the evolution of varying strategies of seeking mates.]