THE SENSORY BASIS OF SEXUAL SELECTION FOR COMPLEX CALLS IN THE TÚNGARA FROG,<i>PHYSALAEMUS PUSTULOSUS</i>(SEXUAL SELECTION FOR SENSORY EXPLOITATION)

Male túngara frogs (Physalaemus pustulosus) vocalize to attract females, and enhance the attractiveness of their simple, whine-only call by adding chucks to produce complex calls. Complex calls contain more total energy and are of longer duration. By virtue of the greater frequency range of the chuck, complex calls also simultaneously stimulate both the amphibian papilla and the basilar papilla of the frog's inner ear. Female phonotaxis experiments using synthetic stimuli demonstrate that an increase in the call's acoustic energy is not sufficient to account for the enhanced attractiveness of the complex call. However, the stimulation of either or both of the female's sound-sensitive inner-ear organs is sufficient to elicit her preference. We suggest that the female's sensory system generates selection that equally favors at least three evolutionary alternatives for enhancing call attractiveness and that historical constraints imposed by the male's morphology determined which of the alternatives was more likely to evolve. These data are consistent with our hypothesis of sensory exploitation, which states that selection favors those traits that elicit greater stimulation from the female's sensory system and which emphasizes the nonadaptive nature of female preference.