SOCIAL INTERACTIONS IN 2 SYMPATRIC SALAMANDERS - EFFECTIVENESS OF A HIGHLY AGGRESSIVE STRATEGY

In terrestrial plethodontid salamanders, aggressive behaviour is thoug ht to function in the spacing of territorial residents among contested cover objects on the forest floor. Such behaviour, when exhibited tow ard heterospecifics, plays an important role in the competitive intera ctions between species. We compared levels of aggressive behaviour in intra- and interspecific contexts in two species of sympatric salamand ers (Plethodon ouachitae and P. albagula) that have similar ecological requirements but differ in adult size. We also tested the effectivene ss of such behaviour in holding cover objects (territorial foci) in th e laboratory and on the forest floor. We predicted that if one species were more aggressive than the other, then that species would have gre ater success in obtaining and holding cover objects. In laboratory tri als, residents of P. ouachitae (the smaller species) were extremely ag gressive in both intra- and interspecific contexts. Individuals of P. ouachitae delivered bites at a rate 14 times that of previously studie d species of Plethodon and were significantly more likely to escalate to biting when tested as territorial residents (in intra- and interspe cific trials) and as intruders (in interspecific trials). Plethodon al bagula exhibited a lower level of aggression, similar to other species of Plethodon. In laboratory trials, in which salamanders competed for cover objects of differing quality, residents of P. ouachitae were ef fective in expelling conspecific intruders, and they were marginally e ffective at expelling intruding P. albagula. Residents of P. albagula were less effective in expelling conspecific intruders and did not exp el intruding P. auachitae. We conclude that the extreme aggression exh ibited by P. ouachitae enabled it to expel intruders from artificial c over objects and to invade cover objects held by larger heterospecific residents. Field data supported intraspecific defence of cover object s by P. ouachitae, but results for Il albagula were inconclusive. Thes e results are consistent with the geographic distributions of these sp ecies (P. ouachitae typically outnumbers P. albagula in the Ouachita M ountains) and provide an example of a behavioural mechanism overcoming a size-related disadvantage.