Aggression and territoriality among sympatric western plethodontid salamanders

Territoriality has been documented among plethodontid salamanders in eastern North America, but aggressive interactions among western plethodontids are largely unknown. I examined the responses of three sympatric western Plethodon species (P. vehiculum, P. dunni, and P. vandykei) toward conspecific and congeneric salamanders in two laboratory experiments. In experiment 1, I investigated intra- and inter-specific aggression by the three species, and in experiment 2, I examined aggression and dispersion of the salamanders when fed either a high or a low level of food. In experiment 1, P. dunni were more aggressive than P. vehiculum and P. vandykei, which rarely bit their opponents. In experiment 2, two different food levels had no detectable effect on the aggressiveness of residents. Resident P. dunni (males, females, and juveniles) were equally likely to bite conspecific and congeneric salamanders introduced into their cages. However, their dispersion under cover objects was uniform (suggesting territoriality) only in pairings between conspecific individuals. In contrast, P. vehiculum showed little aggression and were randomly dispersed under cover objects. The variety of responses exhibited by the three sympatric species toward conspecific and congeneric salamanders suggests that their populations are only superficially similar in ecology and may be subjected to different intensities of intraspecific competition.