Territorial Behavior and Ecological Divergence in a Polymorphic Salamander

Polymorphic species provide an opportunity to examine the process of sympatric divergence as it occurs. The Eastern Red-backed Salamander, Plethodon cinereus, is a polymorphic species that has served as a model organism in behavioral and ecological studies. Recent work suggests that the two most common color phenotypes (striped and unstriped) exhibit weak assortative mating and are diverging along a number of niche dimensions including temperature optima, diet, and response to predators. Males and females of P. cinereus are territorial and this behavior is thought to function in the context of prey and mate acquisition. Striped males have been shown to gain access to larger, and presumably more fecund, females. We posited that this pattern emerges through differential territorial behavior between the two phenotypes. We predicted that striped and unstriped salamanders would differ in their use of cover objects in the field, and in their aggressive responses to intruders in the laboratory. We examined salamander cover use and movement by placing artificial cover objects (ACOs) on the forest floor and monitoring them for 3.5 years. We compared residency time, number of recaptures, and number of ACOs occupied between the two phenotypes. The proportion of striped salamanders that were territorial residents was significantly greater than the proportion of unstriped salamanders that were territorial residents. Striped salamanders also exhibited significantly longer territorial residency, were recaptured more often, and were more often found under multiple, adjacent cover objects than unstriped salamanders. In the laboratory, we examined territorial behavior of the two morphs. As residents, striped salamanders were more aggressive and less submissive than were unstriped residents. When compared to intruders, resident salamanders of both morphs behaved more aggressively, but significant differences between resident and intruder behavior were only detected for the striped phenotype. These differences in aggression and cover object use may help to explain how striped males gain access to larger females and may be important in the interpretation of sympatric niche divergence and assortative mating by color in this species.