Paedomorphosis in Ambystoma Talpoideum: Effects of Density, Food, and Pond Drying

The expression of metamorphosis and paedomorphosis in the salamander Ambystoma talpoideum was examined in a three—factor experiment using artificial ponds. This experiment was designed to measure simultaneously the effects of initial larval density, food level, pond drying regime, and their interactions on the percentage of individuals metamorphosing or becoming paedomorphic. A higher percentage of individuals metamorphosed from drying low—density, and high—food ponds than from constant water level, high—density, and low—food ponds. A higher percentage of individuals became paedomorphic in constant water—level and low—density ponds than in drying and high—density ponds. Food level had no effect on the expression of paedomorphosis. Initial density, food level, and drying regime all had significant effects on the mean snout—vent length of larvae in ponds. The percentage of paedomorphic individuals in a pond was positively correlated with mean snout—vent length in the population. Analysis of covariance using snout—vent length as the covariate removed density and food level effects on the percentage of metamorphic individuals but did not remove the effects of drying regime. Use of snout—vent length as covariate in an analysis of covariance for the percentage of paedomorphic individuals failed to remove significant effects of density or drying regime. Only initial density affected the survival of larvae. The factors manipulated in this experiment affected the expression of paedomorphosis both directly through the physical environmental conditions that they created and indirectly through the growth and body size of larvae. These results suggest that differences in environmental factors among populations can affect the life—history pathways of individuals and lead to a predominance of paedomorphosis in permanent aquatic habitats with favorable growth environments.