The evolution of female mate choice for complex calls in túngara frogs

As discussed in the text and in class, túngara frogs have provided a classic example in support of the sensory exploitation hypothesis of receiver bias. This model suggests that male traits evolve to take advantage of latent female preferences (sensory biases). Previous work on túngara frogs has indicated that the female preference for calls including a chuck in additional to the standard whine existed in a common ancestor before males of either of the two extant species who incorporate chucks had evolved the signal. A complementary hypothesis, sensory-matching, suggests that simple calls (whines) stimulate only of the two inner-ear organs of frogs, while complex calls (whines plus chucks) stimulate both and thus are preferred by females.

More recent work (Ron, 2008) does not support the classic sensory exploitation interpretation of the túngara frog system. The author used several methods to reassess the evolution of mating call and female preference. First, mate choice experiments were conducted on additional taxa not previously included in ancestral character reconstructions. Females were exposed to a pair of antiphonally broadcast calls (an unaltered conspecific call and a conspecific call with a digitally added chuck) in an arena, and their resulting phonotaxis was monitored. Additionally, an updated phylogenetic analysis was utilized for character correlations and ancestral character constructions. All known species of Engystomops (formerly Physalaemus) as well as all species groups in the outgroup (Physalaemus) were included in the sampling of calls used in character reconstructions, while female preference trials were more limited in coverage due to logistical difficulties (although still representing double the species sampling of previous studies). The effect of incomplete taxon sampling on the reconstruction of female preference was tested by randomly assigning character states at an important node in one analysis. Under Fisherian sexual selection, the origin of the trait was expected to be correlated with the origin of the preference for it, while under sensory exploitation, a lack of correlation was expected. Finally, the sensory-matching hypothesis was tested by comparing the frequency allocation of simple and complex calls to the sensitivity of the two frog inner ear organs.

In female preference tests, none of the three species tested showed increased attraction to conspecific calls with appended chucks; the author suggests that previous studies may have suffered from a methodological flaw drastically increasing the energy content of modified calls. Character reconstruction indicated that the most recent common ancestor of the in-group lacked a preference for chucks; rather, chuck preference has evolved twice independently. A test of correlated evolution indicated that the preference and the trait evolved in concert. Measurements of calls showed no difference between simple and complex calls in the energy content at frequencies relevant to the inner ear organ sensitivities, rejecting the sensory-matching hypothesis. Together, these data suggest a reinterpretation of the evolution of signal preference and trait in this system.

Ron, S. R. 2008. The evolution of female mate choice for complex calls in tungara frogs. Animal Behaviour. 76:1783-1794.