Túngara Frog: A Model for Sexual Selection and Communication

Abstract

Sexual selection is responsible for the evolution of extreme and elaborate sexual dimorphisms. This is especially true for communication systems involved in mate attraction. Anurans, in general, and tungara frogs (Physalaemus pustulosus), in particular, have emerged as an ideal model for studies of sexual selection and communication. Male tungara frogs gather in choruses and vocally advertise for females with a long-distance advertisement call. The call consists of whine that can be followed by 0–7 chucks; only the sister taxa of the tungara frog are known to produce similarly variably complex calls. Male tungara frogs add chucks in response to the vocalizations of other males, and females are more attracted to calls with chucks than calls without chucks. Females move about the chorus and choose a mate with minimum interference from males. Females are more likely to choose larger males, and this preference results from their preference for the lower-frequency chucks produced by larger males. The preference for lower-frequency chucks, in turn, results from the relationship between the average tuning of the female’s inner ear and the average dominant frequency of the chuck. The female’s basilar papilla is more sensitive to chucks with lower-than-average compared to higher-than-average frequencies. The tuning of this inner ear organ did not evolve in tungara frogs but also characterizes most of its close relatives; this suggests that aspects of the chuck evolved to match preexisting sensory biases in females. The communication system of tungara frogs does not occur in a private channel. As do female tungara frogs, two eavesdroppers, frog-eating bats, Trachops cirrhosus, and blood-sucking flies, Corethrella spp., also use the mating call to localize males, but in these cases the eavesdroppers either eat them or suck their blood. Furthermore, as with the female frogs, the eavesdroppers are attracted preferentially to complex calls over simple calls. This exemplifies the conflict between natural selection and sexual selection highlighted by Darwin.