Environmental and morphological constraints interact to drive the evolution of communication signals in frogs

Abstract

Animals show a rich diversity of signals and displays. Among the many selective forces driving the evolution of communication between individuals, one widely recognized factor is the structure of the environment in which signals are produced, transmitted and received. In particular, animals communicating by sounds often emit acoustic signals from specific locations, such as high up in the air, from the ground or in the water. The properties of these different display sites will impose different constraints on sound production and transmission and may therefore drive signal evolution. Here, we used comparative phylogenetic analyses to assess the relationship between the display site properties and the structure of reproductive calls from 161 frog species from the frog families Ranidae, Leptodactylidae and Hylidae. Specifically, we compared the dominant frequency of species that vocalize from aquatic versus non-aquatic sites, and its relation with body size. We found that the dominant frequency of frogs calling from the water was lower than that of species calling outside of the water, a trend that was consistent across the three families studied. Furthermore, phylogenetic path analysis revealed that the call site had both direct and indirect effects on the dominant frequency. Indirect effects were mediated by call site influencing male body size, which in turn was negatively associated to call dominant frequency. Our results suggest that properties of display sites can drive signal evolution, most likely through morphological constraints, in particular the ones imposed on the sound production mechanism. Also, variation in body size between calling sites explained some of the differences we found in call frequency, highlighting the relevance of the interplay between morphological adaptation and signal evolution. Changes of display site may therefore have important evolutionary consequences, as it may influence sexual selection processes and ultimately may even promote speciation. Impact summaryTo attract or impress mates, animals have evolved a great diversity of communication signals, such as song and dance, or brightly colored body parts. Whether these sexual signals are successful depends to a large extent on the environment in which they are produced, transmitted and perceived. For acoustic signals, such as the mating calls of frogs, the environment is well known to influence both their transmission and perception. The impact of the environment on the production of sounds is however far less understood. Here we studied the relation between the environment and signal design across a wide range of frog species, specifically comparing calls of aquatic versus non-aquatic species. Frogs that called from water were found to call at lower pitch, which was partly explained by the fact that they were also larger. Our results point towards an important environmental driver of signal evolution, namely morphological constraints on signal production. We argue that the environment can impose limits on morphological traits that are either directly or indirectly involved in signal production. Such a mechanism would in particular be important when species move into new habitats, as rapid changes to display sites may lead to rapid changes in sexual signaling and sexual attractiveness.