How female barking treefrogs, Hyla gratiosa, use multiple call characteristics to select a mate

Female mate choice is an important mechanism of sexual selection, and in many species, females assess multiple characteristics of potential mates. Numerous studies have documented individual traits that females use to select a mate, but few have investigated how females use multiple male sex traits to choose a mate. In this study, we tested four hypotheses to determine how female barking treefrogs use multiple call characteristics concurrently to select a mate: (1) females may perceive as a single trait characteristics that researchers have defined as separate; (2) females may assess traits hierarchically, using the lower-level trait only when the higher-level trait is difficult to distinguish between males; (3) one trait may not be directly assessed by females, but may amplify another trait that is directly assessed; and (4) females may assess several traits at once, weighting them either equally or differently. We tested these hypotheses with two-speaker playback experiments that varied call rate, duration, or both. We also compared preference functions for rate and duration using a logistic regression model. Playback experiments supported simultaneous assessment and refuted the other three hypotheses. Females weighted duration more heavily than rate when percentage difference between stimuli was 29%, but not when percentage difference was 100%. Preference functions revealed that, for a given percentage difference between stimuli, percentage preferences were greater for higher call rates than for longer calls. Thus, female barking treefrogs appear to treat rate and duration as two separate traits and combine preferences for these traits nonadditively.