Disruptive selection for alternative life histories in salmon

Abstract

Many salmon species include males which mature as much as 50% younger and as small as 30% of the adult body size of other males in the population^1–8. In the semelparous Pacific salmon Oncorhynchus spp., these small males are known as ‘jacks’, and they compete with larger late-maturing ‘hooknose’ males for opportunity to spawn on the breeding grounds. The existence of jacks is problematical as it is believed that salmon populations should have a single optimal age or size at maturity^9–11. I now show, however, that these two alternative life histories are evolutionarily favoured by frequency-dependent disruptive¹² selection on the breeding grounds. In coho salmon (Oncorhynchus kisutch), small and large males gain access to females by sneaking and fighting respectively. By contrast, intermediate-sized males are at a competitive disadvantage. Jacks, which are specialized at sneaking, and hooknose males, which are specialized at fighting, have negatively frequency-dependent fitnesses from these alternative breeding tactics. Calculations suggest that the lifetime fitness of jacks is similar to that of hooknose males. Thus, age of maturity in salmon has probably not evolved as a single optimum, but rather as a ‘mixed evolutionarily stable strategy^13,14 in which precocious maturity is an evolutionarily viable alternative life history strategy.