Marathon callers: acoustic variation during sustained calling in treefrogs

Abstract

We investigated the effects of within-night sustained calling on call acoustic properties in a typical lek-breeding anuran (Hyla intermedia), in which males engage in intense acoustic competitions to attract females. We continuously recorded the calling of randomly selected males over a large portion of the nightly chorus activity and investigated the pattern of temporal variation in both fine-scale (pulse rate, call duration and rise-time) and gross-scale (call rate, call-group duration) properties. Despite the strong within-individual correlations that we observed between both fine- and gross-scale properties, only the fine-scale properties showed a consistent pattern of temporal variation: during sustained calling, call duration and rise-time tended to increase and pulse rate to decrease, whereas call rate and call-group duration were as likely to decrease as to increase. We hypothesised that vocal fatigue from sustained calling was responsible for the observed pattern of within-night temporal variation. As with some marathon champions who change the technique but not the speed of their running to remain competitive in a race, male treefrogs, to be sexually competitive, might mitigate vocal fatigue by modifying the call fine-scale structure but not the quantity of their calling.