Proximate Contributions to Adult Body Size in Two Species of Dusky Salamanders (Plethodontidae: Desmognathus)

Abstract

I used skeletochronological data to evaluate the contributions of propagule size, larval/juvenile growth, and age at first reproduction to differences in adult body size in two species of plethodontid salamanders of the genus Desmognathus. The traits in question were evaluated in populations of the larger D. quadramaculatus and smaller D. monticola in the southern Blue Ridge Mountains of North Carolina, USA. Gompertz and von Bertalanffy functions were fitted to the plots of standard length on skeletochronological age of each complete sample (larvae and/or juveniles, and adults) of earlier data sets; linear functions were fitted to data of immatures (larvae and/or juveniles). In order to examine the relationship of body mass and age, I conducted regressions of body mass on standard length in later samples of both species, calculated estimated body masses of the salamanders in the skeletochronological data sets, and then fitted a modified Gompertz function to each plot of body mass on age. The results showed that age at first reproduction is the principal factor contributing to differences between the species in size at first reproduction and adult size. Larger propagule sizes (i.e., hatching sizes) in D. quadramaculatus versus D. monticola may also be a factor; however, there is no evidence that a difference in larval/juvenile growth rate contributes to the adult size differential. Comparison of two populations of D. monticola suggested that small differences in adult body size result mainly from slight differences in age at first reproduction. Tradeoffs among propagule size, clutch size, growth, and age and size at first reproduction are discussed in the context of selective pressures that may have generated diversification in body size and life history in the genus.