Drivers of life history variation in a paedomorphic mole salamander

Phenotypic variation is ubiquitous among organisms and is thought to provide an evolutionary advantage for species living in variable environments. Paedomorphosis, or the retention of juvenile traits in sexually mature adults, is a model system for understanding the ecological causes of phenotypic variation. In the facultatively paedomorphic mole salamander (Ambystoma talpoideum), the species' complex life cycle can follow three pathways that ultimately lead to the expression of two discrete adult phenotypes. An individual may undergo traditional development from egg to aquatic larva but will (1) remain in the water as a reproductive, gilled adult (paedomorph), (2) metamorphose into a terrestrial adult, or (3) delay metamorphosis and overwinter in an immature state in ponds, eventually developing into either the terrestrial or aquatic phenotype. We assessed environmental factors influencing variation in the occupancy and abundance of paedomorphic and overwintering larval A. talpoideum across natural populations in western Kentucky. We sampled 20 ponds in western Kentucky in 2013-2015 and 2022, and measured abiotic (e.g., pond depth, canopy cover, and dissolved oxygen) and biotic (e.g., larval and predator densities) factors to compare several hypotheses concerning phenotypic outcomes in A. talpoideum. We created and compared eight models based on ecologically plausible explanations for phenotypic variation. Paedomorphs occurred more often and in greater numbers following larval periods with favorable aquatic conditions including low larval densities and greater body sizes. The opposite was true for overwintering larvae, which were found more often after unfavorable aquatic conditions, high summer larval densities, dense canopy cover, and low dissolved oxygen.