ALBERT

Description: Interspecific aggression has important consequences for ecological processes and the evolution of behavioral strategies. We examined interspecific aggressive interactions in the 2 gray tree frog species, Hyla chrysoscelis and Hyla versicolor . These species call side by side in the same ponds, and acoustic interference occurs because of the similar spectral characteristics of their vocalizations. The aggressive calls of these 2 species, although very similar, were statistically distinguishable: calls of H. chrysoscelis were more strongly amplitude modulated than those of H. versicolor . We used playbacks and staged interactions to characterize the behaviors of males exposed to heterospecific competitors or their signals. Males readily responded with aggression to heterospecific individuals and playbacks of their calls. These behaviors were qualitatively similar in both intraspecific and interspecific interactions, but there were some significant differences. First, males of H. chrysoscelis were more aggressive toward playbacks of conspecific advertisement calls than toward those of H. versicolor . There were no significant differences in this respect in H. versicolor . Second, interspecific interactions were usually more escalated than intraspecific interactions and more likely to end with the loser moving away from its opponent. Although neither species had an advantage in staged interactions, behavioral responses were asymmetrical because H. versicolor was more likely than H. chrysoscelis to initiate physical contact. Previous studies showed that H. versicolor suffers from a greater reduction in attractiveness than H. chrysoscelis in the presence of heterospecific call overlap. Thus, the asymmetries in aggressive behavior between the 2 species may be related to differential costs of heterospecific competition.

Description: Animal contests can greatly impact individual fitness; therefore, the behavioral strategies used during contests have received much interest in both theoretical and empirical studies. Recent reviews of animal contest behavior have clarified the predictions for contest dynamics when individuals’ behavioral strategies are based on influences of self, opponent, or both. These predictions mostly apply to natural or staged interactions between 2 competing animals, but studies of aggressive signaling behavior often use other methodologies. In particular, playback tests have been heavily utilized because they provide the investigator with powerful control over experimental conditions. My aims in this paper are 2-fold. First, I discuss how playback tests can be used to study animal contest dynamics. Specifically, I develop predictions for individuals’ responses to playback tests that will allow investigators to discriminate between different contest strategies. By monitoring individual persistence, resource-holding potential (or its correlates as reflected in signal structure), and other behaviors in appropriately designed playback tests, I show that strategies based on influences of self, opponent, and both self and opponent can be discriminated. Second, I illustrate these methods with data from a playback experiment on male responses to acoustic signals in the gray tree frog, Hyla versicolor . Males’ responses to playbacks of synthetic advertisement and aggressive calls that varied in call frequency indicated that both self and opponent characteristics influenced persistence in contests.