In frogs, sexual communication is mainly vocal. Males of a species serenade the females, sometimes in large choruses composed of several species. Amongst all the singing and all the background noise, it is hard to imagine how the messages from the males of one species reach their target without getting garbled amidst all the confusion. This is even more remarkable when taking into account the fact that the sound characteristics of different species calling together sometimes overlap. This is the case of Eupsophus roseus and Eupsophus vertebralis, two species of frogs that inhabit the temperate forests of Argentina and Chile, whose calls overlap in parts of the frequency spectrum. Felipe Moreno-Gómez and his colleagues at the Universidad Austral de Chile, the Universidad de Chile and the Muséum National d'Histoire Naturelle in France set out to understand how females of E. roseus are able to tease out the calls of their lovers from those of E. vertebralis males and other noise in the environment.
It has been suggested that in order to improve communication between two organisms, the sensitivity of the receiver needs to be tuned in to the signal characteristics produced by the signaller. Moreno-Gómez and his team decided to put this hypothesis to the test to find out whether this was the case for E. roseus. First, they recorded the calls of males and background noise in the field during the breeding season and analysed their spectral characteristics. Back in the lab, they recorded the acoustic sensitivity of females at different sound frequencies. They played back tones produced by a sound generator and then recorded the neuronal activity from the torus semicircularis (the auditory nucleus in the brain of a frog) in response to the different frequency tones through an electrode implanted in this area of the brain. Then they compared the sound frequencies to which the females were sensitive with the spectra of male vocalizations and background noise.
Just as expected, the auditory spectral sensitivity of females matched the sound characteristics of male vocalizations. Furthermore, the main components of the male calls had frequencies much higher than the frequencies of abiotic noise, which meant that there was no masking effect from background noise. Furthermore, even though there was some overlap in the sound spectra of the calls from male E. roseus and E. vertebralis, this overlap was much lower than the high degree of matching the researchers observed between the auditory sensitivity of E. roseus females and the main components of the calls of their males. This mismatch between the sensitivity of females to abiotic and biotic noise helps their potential mates stand out as the signal to noise ratio is much higher for them.
Moreno-Gómez and his colleagues showed that female frogs of the E. roseus species are tuned in to the serenades of their suitors, filtering out most of the environmental noise including that of a closely related species. This concordance between female sensitivity and male signal helps prevent miscommunication between parties, which could potentially lead to interspecies mating and, therefore, decreased fitness through the production of sterile hybrids. It has been said that love is blind, but in the case of E. roseus females, a more accurate description would be that their love is deaf to most sounds but their mate's.
