THE EVOLUTION OF VOCALIZATION IN FROGS AND TOADS

The most commonly heard vocalizations of frogs are advertisement calls , which attract gravid females and mediate aggressive interactions bet ween males. Frog vocalizations are energetically costly to produce, an d body size often constrains the dominant frequency and intensity of v ocalizations; propagation and degradation of these signals are affecte d by diverse physical and biotic factors. Behaviors and auditory mecha nisms that mitigate these problems are discussed. With some exceptions , female preferences based on dominant frequency are intensity-depende nt and mediate stabilizing selection within populations. Female prefer ences based on dynamic, gross-temporal properties typically mediate st rong directional selection. The high values of these properties prefer red by females increase a male's detectability in dense choruses and a re a reliable predictor of his energetic investment in courtship. Fema le preferences based on fine-temporal properties (e.g. pulse rate) are often in tensity-independent and usually mediate stabilizing selectio n within populations. The overall attractiveness of a signal depends o n variation in more than one of these acoustic properties; their relat ive importance differs between species. Parsimony analysis supports th e idea that auditory biases preceded the evolutionary appearance of ca ll elements that enhance the attractiveness of advertisement calls in one species group of neotropical frogs. A more specific claim that the bias has not been modified by selection after the establishment of th e new signal has little empirical support. Indeed, the selective conse quences of positive phonotaxis to any ''new'' stimulus, whether or not there is a sensory bias, must play a critical role in its establishme nt and maintenance as a mate-attraction signal and on the further evol ution of the female preference. The hypothesis that present-day select ive consequences of mate choice have also acted in the past evolution of call structure and preferences is supported by a few examples of re productive character displacement. However, evolutionary divergence in signals and preferences will have multiple causes, most of which do n ot involve interactions between species. Phylogenetic analyses and stu dies of selection and other evolutionary forces in contemporary popula tions are complementary approaches to gaining insights about the evolu tion of frog vocalizations and animal communication in general.