Basic Mechanisms and Evolution of Acoustic Communication

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BASIC MECHANISMS AND EVOLUTION OF ACOUSTIC COMMUNICATION HOWARD CARL GERHARDT, JR., PRINCIPAL INVESTIGATOR(Name used as an author: H. Carl Gerhardt)Communication affects the reproductive success of signalers and receivers. Thus, communication plays a critical role in evolution and speciation, and differences in signals are often reliable indicators of the status of populations that have diverged little, if at all, in their physical appearance. Mutual selective pressures exerted by senders and receivers might be expected to generate precise matching between signal properties and the selectivity of the sensory system. The reason is that senders that produce very different signals are unlikely to attract mates, and receivers that have very unique preferences may not find any male of their own species acceptable. However, some exceptional examples have been found in which receivers are more effectively stimulated by signals that are not produced by members of their own species (=conspecific males), but by those of closely related species (=heterospecific males). These novel preferences have been called "hidden preferences," and a phylogenetic analysis sometimes shows that the preference predates signals in evolutionary time and can hence such preferences can be considered "pre-existing biases." This project is a comparative study of the matching of signal properties and receiver preferences in a complex of nine closely related North American treefrogs. Male treefrogs produce simple, stereotyped sounds, and females respond to these sounds or to synthetic, computer-generated sounds by approaching a speaker that emits them. Playback experiments using synthetic calls will identify the key physical properties (=preference criteria) used by females to select among signals. These experiments will determine for each species whether females respond best to signals with properties typical of conspecific males or whether calls or call-elements of other species might be more attractive or might enhance the attractiveness of conspecific calls. It might be found that matching to conspecific call properties is good in all nine species, that mismatches are prevalent in all species, or that matching occurs in some species but not in others. Regardless of the results of these experiments, a phylogenetic analysis - which will rely on morphological and molecular characters of all nine species - will be used to suggest the order of evolution of preferences and signals. This particular group of treefrogs also has two features that could help to explain a pattern in which matches occur in some species and mismatches in others. First, some species in the group routinely form mixed-species choruses with close relatives having similar calls. Here it might be expected that matching of senders and receivers will be strong in order to prevent mating between species. Other species in the group have been isolated from any other species with similar calls for long periods of time whereas others. These are the best candidates for finding mismatches ("hidden preferences" and "pre-existing biases") because these biases might be ancestral traits that can be expressed without the risk of mating with another species. Second, it has been found that some species in the group have twice the number of chromosomes of the other species and that changes in chromosome number affect call properties. Such a change in chromosome number might also affect female preferences in ways that could either track the changes in the calls or result in mismatches of senders and receivers. This hypothesis will be tested by studying the preferences of frogs with experimentally induced extra sets of chromosomes.