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Character Displacement and Aggression in Two Species ofTerrestrial Salamanders

Abstract Two species of plethodontid salamanders (Plethodon cinereus and Plethodon hoffmani) exhibit character displacement with respect to body size and head morphology in Pennsylvania. In areas of allopatry, the two species do not differ in these traits while in an area of sympatry, P. cinereus is smaller (and eats smaller prey) and P. hoffmani is larger (and eats larger prey) in body size and head morphology. We tested the hypothesis that this form of character displacement is a consequence of alpha selection, in which natural selection favors increased aggression of sympatric species relative to their allopatric conspecifics. We found little evidence that allomonal response to or aggressive behavior toward heterospecifics differed significantly between allopatric and sympatric populations of either P. cinereus or P. hoffmani. Thus our data do not support the hypothesis of alpha selection shaping the morphology of the two species in sympatry. However, we found that both allopatric and sympatric populations of P. cinereus were significantly more aggressive and significantly less submissive than either allopatric or sympatric populations of P. hoffmani. These results suggest either of two conflicting interpretations concerning interspecific competition between the two species. First, the boundary of contact between the two species is static because although P. cinereus is aggressively superior, P. hoffmani is exploitatively superior (eats larger prey) in competition in areas of sympatry. Alternatively, P. cinereus is slowly encroaching on the geographic distribution of P. hoffmani because the former species is not only aggressively superior but also exploitatively superior (eats the more profitable, although smaller, soft-bodied prey types). Our results also support inferences drawn from previous research that P. cinereus competes strongly when geographically in contact with other confamilial species (e.g., Plethodon hubrichti, Plethodon shenandoah, juveniles of Plethodon glutinosus, and Eurycea cirrigera).