Floating frogs sound larger: environmental constraints on signal production drives call frequency changes.

In animal communication, receivers benefit from signals providing reliable information on signalers' traits of interest. Individuals involved in conflicts, such as competition between rivals, should pay particular attention to cues that are "unfakeable" by the senders due to the intrinsic properties of the production process. In bioacoustics, the best-known example of such "index signals" is the relationship between a sender's body size and the dominant frequency of their vocalizations. Dominant frequency may, however, not only depend on an animal's morphology but also on the interaction between the sound production system and its immediate environment. Here, we experimentally altered the environment surrounding calling frogs and assessed its impact on the signal produced. Our results show that frogs that are floating are able to inflate their vocal sacs fully and that this change in inflation level is correlated with a decrease of call dominant frequency.