Preferences of female barking treefrogs, Hyla gratiosa, for larger males: univariate and composite tests

Body size is an important determinant of male mating success, and acoustic signals often contain information about size. Most playback experiments that have tested for female preferences for larger males have manipulated only single characteristics, even though size is often correlated with multiple signal attributes. We first determined which characteristics of the advertisement calls of male barking treefrogs were correlated with body size, then we used two-choice playback experiments to determine whether female choice for larger males was influenced by these characteristics. In univariate tests, we varied only one of three size-correlated call parameters (fundamental frequency, call amplitude, call duration) at a time. In composite tests, we varied all three characteristics simultaneously, using univariate regressions of each parameter on size to predict the values expected for each characteristic for small, medium and large males. In univariate tests, females preferred large over small males for call amplitude and duration, medium over small males for amplitude and fundamental frequency, and large over medium males for call amplitude. In composite playbacks, females preferred large males over medium and small males, and medium males over small males. Females thus showed clear preferences for larger males, especially in the composite tests. Comparisons between the results of the univariate and composite tests indicated that female preferences for larger males are mediated by a combination of call characteristics rather than any single trait. Reliance on a combination of traits may function to increase the probability that females select larger males despite weak correlations between body size and individual call characteristics.