Wood frogs (Lithobates sylvaticus) use water surface waves in their reproductive behaviour

The ability to sense water surface waves has been described in only a few species, but across a wide taxonomic range. Water surface waves are typically used to localize prey or to avoid predators, and in some cases also for sexual communication. Here we add to the sparse knowledge of the use of this sensory modality by reporting observational and experimental evidence that wood frogs (Lithobates sylvaticus) respond to water surface waves generated by conspecifics; that there are pronounced differences in response between males and females; and that they use surface waves in a behavioural context not previously reported for anuran reproductive behaviour: sexual eavesdropping. Because the water waves that elicit the described responses are incidental by-products of calling and locomotion behaviour, we consider this an example of sexual eavesdropping rather than sexual communication. Males quickly and accurately approach a surface wave source, thus aiding in mate acquisition which in this species is mainly achieved by scramble competition. By contrast, females move away from a surface wave source. This may help them avoid sexual harassment by mate-searching males. Because it assures that only the fastest, strongest, and potentially fittest males can amplex them, it may also be a strategy for indirect mate choice by females.