Ecological Consequences of Agonistic Interactions in Lizards

It is relatively easy to document direct agonistic interactions among or within species that lead to exclusion of subordinate animals from preferred habitats. It is more difficult to measure the competitive effects of that exclusion on the "loser." Thus, much behavioral research assumes that the intensity of agonistic interactions is correlated with their cost to performance, but does not directly measure such a cost. We examined fitness-associated traits of pregnant viviparous scincid lizards (Eulamprus heatwolei) in outdoor enclosures under three conditions: alone, with a conspecific, or with a larger and more aggressive heterospecific (Egernia saxatilis, a sympatric scincid species). As in previous laboratory studies, Egernia saxatilis attacked Eulamprus heatwolei and excluded individuals of the smaller species from warmer shelter sites. However, Eulamprus heatwolei were able to maintain "normal" schedules of body temperatures and food intake, apparently by modifying their behavior to minimize encounters with their aggressive cage mates. Accordingly, the subordinate animals showed no overt ill effects in spite of strong agonistic interactions: the body condition, locomotor performance, and corticosterone levels of subordinate Eulamprus housed with Egernia were indistinguishable from those of lizards kept in solitary cages or with conspecifics only. Similarly, the offspring of these pregnant females were born at the same time, and with similar phenotypic traits, as the offspring of control females. Thus, even in situations of intense interspecific aggression and consequent habitat exclusion, behavioral flexibility of the subordinate animals may decrease or eliminate the inferred ecological consequences of that interaction.