Vocal responses to noise reveal the presence of the Lombard effect in a frog

Abstract

Many animal communication systems have evolved signal flexibility depending on environmental conditions. A common strategy of vocal communication is to increase amplitude in response to increasing noise levels. This phenomenon, known as the Lombard effect, is a widespread trait among mammals and birds. Anurans are a major group with many species that rely heavily on acoustic signals for sexual communication. Although these species often communicate in noisy environments, the presence of the Lombard effect in frogs remains unclear. We exposed male túngara frogs (Physalaemus pustulosus) to different noises with and without playback of conspecific calls. Males increased call amplitude, call rate, and call complexity in response to low-frequency noise (overlapping the species call range) but not to high-frequency (nonoverlapping) noise. Vocal amplitude increased linearly with noise level demonstrating that túngara frogs exhibit the Lombard effect, and we discuss why different frog species may differ in their control over vocal amplitudes. Furthermore, we found the overall effect of noise to be similar to the effect of conspecific call playback. We speculate that vocal amplitude control may have evolved primarily as a response to increased competition at the cocktail party, similar to the way humans raise their voice when in a heated debate, and subsequently as a strategy to deal with background noise more generally.