Geographic Variation in Animal Communication Systems

Intraspecific communication is fundamental to most social behavior. It is also a special problem in animal behavior because it necessarily involves the interaction of two systems within a species, a sender and a receiver (Walker 1957, Blair 1964, Capranica 1966, Schneider 1974, Hoy et al. 1977, Hopkins and Bass 1981, Gerhardt 1988, Brenowitz 1994). Sender and receiver components are almost always separable morphologically, physiologically, and behaviorally. Each may be under different mechanistic and developmental control, and, especially in those cases in which the senders and receivers are segregated by sex, the impact of selection pressures and constraints can be very different (Brenowitz 1986, Ryan 1986, 1988; Wilczynski 1986, Endler 1983, 1993). The presence of two different but necessarily interacting components make the evolution of communication systems a particularly challenging problem in behavioral biology. In any communication system, the interaction between senders and receivers dictates some degree of matching such that the signal emitted by one member of the communicating pair is effectively received, recognized, and assessed by the other member (Blair 1964, Gerhardt 1982, 1988; Capranica and Moffat 1983, Littlejohn 1988, Ryan 1988, 1991; Endler 1993). Effective coupling of senders and receivers is crucial when communication underlies mate choice. Communication systems that accurately discriminate between heterospecifics and conspecifics, while effectively linking conspecifics to each other, are important for ensuring mating with genetically compatible conspecifics. As such, communication systems can be integral parts of speciation and the maintenance of species isolation (Blair 1958, Mayr 1963, Paterson 1985, 1993; Littlejohn 1981, 1988; Butlin 1987, Coyne and Orr 1989, Claridge 1993, Moore 1993, Wood 1993). The natural variation among and within species in both signals and receivers provides a means for examining the factors contributing to the evolution of communication systems (Templeton 1981, Ryan and Keddy-Hector 1992, Paterson 1993). Among the different levels of variation observed, geographic variation provides the best material for disentangling the myriad factors shaping the evolution and divergence of communication systems and for testing fundamental ideas about the evolution of behavior (Endler 1983, Baker and Cunningham 1985, Nevo and Capranica 1985, Ryan and Wilczynski 1991, Loftus-Hills and Littlejohn 1992).