Energetics of calling in the male treefrog Hyla arborea: when being large means being sexy at low cost

Abstract

Female mate choice is often thought to be based on signals that honestly reflect male quality as a potential mate. However, while particularly costly acoustic signals have often been studied, the existence of differences in energetic costs among males has rarely been considered. These differences may be decisive in many ‘income breeding’ species in which males face a trade-off between calling in order to attract females and foraging to renew their energetic reserves. We thus examined calling energetic costs in an income breeder, Hyla arborea (L.), known to produce costly energetic calls, in order to explore the sources of inter-individual variability. After an arginine vasotocin injection, we determined the relationships between energetic costs (via oxygen consumption), call characteristics and male morphology. We found a strong variation in male calling energetic costs. For the same call rate, some males consumed oxygen at a rate four times greater than others. This difference was mainly explained by male size, bigger males consuming less oxygen per gram of tissues than smaller ones during calling. In addition, an acoustic parameter, within bout call rate, also influenced the energetic cost of emitted signals. These findings highlight the importance of calling energetic cost in income breeding species which have strong consequences on sexual selection processes. Indeed, energetic constraints are key parameters to understand the inter-individual variation in call characteristics. Males with a reduced cost may increase their chorus tenure and/or the attractiveness of their calls. Regarding female mate choice, choosing a big male may involve indirect benefits linked both to male size and to calling cost.