The role of body size on the outcome, escalation and duration of contests in the grey treefrog, Hyla versicolor

Abstract

Aggressive interactions in animals are often resolved in favour of the individual with superior fighting ability, or resource-holding potential (RHP). A recent revival of studies of aggressive behaviour has focused on the assessment strategies used in animal contests. Strategies of dispute resolution through mutual or self-assessment of RHP differ in the predicted relationship between interaction duration and each competitor’s relative and absolute RHP. We studied potential components of RHP (mass, length, body condition) and their relationship to contest duration and the level of escalation in the grey treefrog, Hyla versicolor, using a novel method to stage aggressive interactions in the laboratory. Overall, large males were more likely to win than small males, but they only had an advantage in less escalated interactions and were not more successful in physical fights. There was limited evidence for an effect of body size on interaction duration or the level of escalation. Specifically, the body condition of both the smaller and larger contestant was weakly negatively related to the duration and level of escalation of contests. This relationship is the opposite of what would be expected under any assessment strategy. Given these data, coupled with the lack of relationships between other size measures and interaction duration, we conclude that assessment of body size does not occur in contests in H. versicolor. Other unmeasured components of RHP may play a role in determining interaction duration, and the relatively weak and ineffective fighting abilities of this species may limit the dominance of larger individuals. Relatively little is known about aggressive behaviour in frogs. Our method for staging aggressive interactions allows us to address predictions of game theory models in an important group for studies of animal communication.