Social Facilitation of Reproduction and the Physiological Effects of Vocal Communication in Captive Boreal Chorus Frogs (Pseudacris maculata)

Abstract

Vocalizations are an integral part of anuran breeding ecology. Acoustic signals (calls) are used to defend territories, repel competitors, attract mates, and coordinate spawning. There is a bidirectional relationship between hormonal state and reproductive behaviour, including acoustic signal production, where reception of conspecific calls can influence steroid hormone production and circulating hormone concentrations can affect calling behaviour. We examine the calling behaviour of boreal chorus frogs (Pseudacris maculata), a declining species in northeastern North America, and determine the effects of conspecific signals on reproductive outcomes and gonadal function. We hypothesize that conspecific acoustic signals can be used in anuran conservation to improve captive breeding outcomes as acoustic signals are a potent modulator of reproduction. First, we used autonomous recording units deployed at natural boreal chorus frog breeding locations to quantify diel and seasonal patterns in calling behaviour. We then used these acoustic recordings to produce audio playback files that were broadcasted to spawning pairs in captivity to investigate the effect of conspecific calls on the calling activity of males, reproductive output, and offspring quality in boreal chorus frogs. We found that broadcast of conspecific significantly increased the proportion of viable eggs and survival of subsequent tadpoles compared to the control. To further investigate the mechanism of reproductive enhancement, we used RNA sequencing to examine the gonadal gene expression in groups of male and female boreal chorus frogs exposed to conspecific calls. We found enhanced expression of genes and gene pathways associated with steroidogenesis, gametogenesis, and gonadal function in male boreal chorus frogs, but more limited evidence of these effects in females. Differential gene expression of male frogs exposed to conspecific calls also paralleled expression results of male frogs receiving hormone injections known to stimulate spawning, suggesting similar stimulation of the hypothalamic-pituitary-gonadal axis in both treatment groups. Based on these results, we conclude that conspecific signals are important regulator of reproduction during the breeding season. Our research contributes to our knowledge on anuran breeding ecology as the results provide further evidence for social facilitation of reproduction in this taxon. These results also have implications for amphibian conservation, as recordings of male calling behaviour could be used a tool to enhance reproductive outcomes in captive breeding programs of declining species.