The adaptive significance of paedogenesis in North American species of Ambystoma (Amphibia: Caudata): an hypothesis

Abstract

Paedogenesis in varying degrees is exhibited by many species of North American salamanders, particularly of the genus Ambystoma. On the basis of a consideration of the ecology of these species, it is proposed that paedogenesis is evolutionarily adaptive and the following hypothesis is presented. In most habitats where ambystomatid salamanders occur, they exhibit the typical amphibian life history, consisting of aquatic larvae and terrestrial adults. However, where unusually harsh conditions, such as severe temperature fluctuations, lack of suitable cover or food, and low humidity, exist on land then the adjacent aquatic habitat is more suitable for salamanders provided it is permanent and provided fish are rare or absent. Under these circumstances paedogenesis will evolve because salamanders which spend their whole lives as larvae in the water have an adaptive advantage over those which metamorphose and become primarily terrestrial. Supporting evidence for the hypothesis is based primarily on patterns of similarity in the ecology of paedogenetic species of Ambystoma. It is suggested that paedogenesis is most commonly reported from the western mountains because this area contains more permanent fish-free ponds than any other area of North America. Exceptions to the hypothesis are discussed and some speculation on the genetic basis of paedogenesis is offered.