The structure, function, and evolution of aggressive signals in anuran amphibians

Date of Completion

January 2003


Biology, Ecology




Advertisement and aggressive signals influence fitness in male animals. Advertisement signals increase fitness through mate attraction, and aggressive signals indirectly increase fitness through warding off competitors. Aggressive signals are conspicuous, repetitive, and stereotyped, qualities that improve their detectability to receivers. However, males may modify or grade their aggressive signal structure in a continuous fashion. Hypotheses include varying aggressive content and attractiveness to females, signaling changes in aggressive motivation, and signaling fighting ability. ^ Both green frogs, Rana clamitans, and chorus frogs of the genus Pseudacris have aggressive calls that are very similar in structure to their advertisement calls. In green frogs, aggressive calls are lower in frequency than advertisement calls. In contrast, the aggressive calls that I have described in five species of Pseudacris do not differ in frequency and are longer in duration and contain more pulses than the advertisement calls. I examined how aggressive calls are constructed and modified in both species groups, and I found that both green frogs and several species of Pseudacris grade their aggressive calls in a continuous fashion. For these species, modifications of aggressive calls appear to signal changes along a motivational continuum. I also demonstrated that green frogs are able to discriminate fine-scale differences in aggressive call frequency indicating that graded aggressive calls in this species may encode information useful to receivers. ^ Comparisons among closely related species suggest that aggressive signals are more similar than advertisement signals. This may be due to the lack of directional selection on aggressive calls or to convergent evolution. Chorus frogs in the genus Pseudacris have a well supported phylogeny that makes them an excellent group for studying this aspect of communication. Distance and phylogenetic contrast methods both suggest that advertisement and aggressive calls are evolving at similar rates in this group and are not evolving independently. In each species, the aggressive call may be independently derived from the advertisement call, and this may be directed by a neural program common to the clade. This pattern of signal evolution has been observed in other animals that communicate acoustically such as crickets. ^