Females know best: dispersal polymorphism maintained by sex-specific foraging

Polymorphisms are maintained over time by trade-offs that alternatively favor morphs over space or time. In a polyphenic population of amphibians (Arizona tiger salamander; Ambystoma mavortium nebulosum), two morphs—paedomorphs and metamorphs—exhibit a trade-off in dispersal capacity. Larvae select between two discrete ontogenetic pathways based on environmental cues—adult paedomorphs remain in their permanent, natal pond, while adult metamorphs disperse between ponds of varying hydroperiods and overwinter in the terrestrial environment. This polyphenism is maintained in part by sex-specific reproductive advantages within each morph, with paedomorphic males achieving more reproductive opportunities and metamorphic females achieving higher egg production. In this study, we examined the role of a high-quality, primary prey taxon (fairy shrimp)—which is accessible to metamorphs only—in balancing the sex-specific component of this dispersal trade-off. Among the 95 metamorphs for which we evaluated diet and body condition, we found that most of the high-condition individuals were females that contained predominantly fairy shrimp in their stomachs. In addition, females specialized on fairy shrimp at much lower fairy shrimp densities than males, indicating that the consumption of this prey taxon may have differential fitness benefits across metamorph sexes. Our findings align with the expectations of parental investment theory, in which female reproductive success is most limited by the energetic resources necessary for egg production, while male reproductive success is limited predominantly by access to females.