Competition between closely related species can affect behavioral interactions. Intense interspecific competition may favor the evolution of elevated levels of interspecific aggression, termed alpha selection. Salamanders of the genus Plethodon exhibit territorial aggression, and in some cases interference mechanisms may have evolved under the process of alpha selection. Throughout Ohio, two closely related and ecologically similar species of salamander, Plethodon cinereus (red-backed salamander) and P. electromorphus (northern ravine salamander), occur in similar habitats and can be found in sympatry. However, the occurrence of syntopic areas is infrequent compared to the range overlap of each species and seems to be limited by factors other than broad geographic factors. Here, we used salamanders from allopatric and sympatric locations to examine the behavior of both species toward interspecific intruders. We hypothesized that animals in sympatry would exhibit heightened aggression compared to salamanders from allopatric areas. We found support for this hypothesis, indicating that intense interference competition may occur between P. cinereus and P. electromorphus in sympatry. Further, our results are consistent with the hypothesis of alpha selection at the level of behavioral aggression. These results suggest that sympatry may represent an unstable equilibrium where neither species can gain a competitive advantage.
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Vol. 65 • No. 2