Breeding Chronology, Sexual Dimorphism, and Genetic Diversity of Congeneric Ambystomatid Salamanders

Many amphibians are explosive breeders, but the key factors that influence migrations to (and from) breeding sites are not well understood for all species. We used a comparative approach to compare and contrast the complex relationships among breeding chronology, environmental conditions, sexual dimorphism/body size, sex ratio, and genetic variation in Small-mouthed (Ambystoma texanum) and Eastern Tiger Salamanders (Ambystoma tigrinum tigrinum). We intercepted 171 A. texanum and 330 adult A. t. tigrinum on their annual migration to breeding ponds over two consecutive breeding seasons (2003-04). Both species immigrated over a short period of time (4-17 days) and displayed no clear pattern regarding whether males or females typically arrived first at breeding ponds. Sex ratios were skewed toward males in both species but varied between years. Consistent with their expected sexual selection regimes, intraspecific sexual dimorphism was subtle in A. texanum but pronounced in A. t. tigrinum. There was no size-ordered immigration or emigration for either species examined; migration events were triggered by temperature and precipitation. We genotyped four hypervariable microsatellite loci and found no gross difference in the overall level of genetic variation between species but document that our study populations are genetically diverse (mean of >18 alleles per locus and heterozygosities >0.75 in each species), presumably as a result of historically large effective population sizes.