Sources of variability in advertisement and aggressive calling in competitive interactions in the grey treefrog, Hyla versicolor

Abstract

Understanding the sources and magnitudes of variation in signal traits is an important first step towards understanding the forces that may act on signal evolution. In anuran amphibians, acoustic communication plays a major role in the processes of mate attraction and aggression. The significance of variation in characteristics of the mate-attracting advertisement calls is relatively well understood, whereas another call type, the aggressive call, has received much less attention. I recorded both advertisement and aggressive calls given by male grey treefrogs, Hyla versicolor, in staged competitive interactions. From these recordings, I assessed the effects of physical and abiotic influences on signal production by measuring the correlations between the call characteristics of both call types and variables associated with size, body condition and body temperature. I also estimated within- and between-male variation in call characteristics. Temperature affected diverse characteristics of both call types, whereas the effects of morphology were limited primarily to call-frequency variables. Spectral characteristics of both call types were largely static within males, whereas gross-temporal characteristics were much more variable. Within each call type, many of the call characteristics were correlated with one another, whereas between call types, there were relatively few strong correlations between call characteristics. I discuss these results in terms of their implications for signal evolution, mate choice and assessment in contests.