Lunglessness in Plethodontid Salamanders is Consistent with the Hypothesis of a Mountain Stream Origin: A Response to Ruben and Boucot

The salamander family Plethodontidae is characterized by the absence of lungs. In 1920, I. W. Wilder and E. R. Dunn proposed that lunglessness evolved as an adaptation for life in flowing streams. However, J. A. Ruben and A. J. Boucot recently suggested that protoplethodontids had no access to the mountainous terrain associated with fast-flowing stream habitats. They further suggested that plethodontids lost lungs for reasons other than ballast. We cite evidence contradicting Ruben and Boucot's geological interpretation. We contend that the Wilder-Dunn hypothesis remains a robust one and argue that the life-history pattern exhibited by the primitive members of the family (e.g., Gyrinophilus, Pseudotriton) suggests that lunglessness evolved as a rheotropic adaptation that promoted primarily larval, not adult, survival in streams. We review evidence on the life history, ecology, morphology, and physiology of larval salamanders that supports the Wilder-Dunn hypothesis.