Socially Mediated Pitch Alteration by Territorial Male Bullfrogs, Rana catesbeiana

Abstract

Acoustic communication signals play important roles in aggressive social interactions between male anuran amphibians (Wells, 1977). For example, a number of studies demonstrate that male frogs behaviorally discriminate between two conspecific signals that differ in size-related spectral properties during conflicts over mates, calling sites, or territories (Davies and Halliday, 1978; Arak, 1983; Ramer et al., 1983; Given, 1987). In anurans, spectral call properties, such as dominant or fundamental frequency, are usually negatively correlated with body size because of morphological constraints on the sound producing apparatus (Martin, 1972). Because larger male frogs typically win fights against smaller males (e.g., Howard, 1978; Arak, 1983; Given, 1988), spectral properties of frog calls are often cited as a classic example of an honest, unbluffable signal of fighting ability (Wiley, 1983; Alcock, 1998; Bradbury and Vehrencamp, 1998). In playback tests, male frogs are often more likely to persistently attack sources of the high-frequency calls of simulated small males, whereas they typically stop calling or abandon their calling sites in response to the low-frequency calls of a simulated large opponent (e.g., Davies and Halliday, 1978; Arak, 1983; Wagner, 1989). There is growing evidence, however, that spectral properties of frog calls may be plastic, especially during social interactions among competing males. Lopez et al. (1988) first demonstrated frequency alteration in white-lipped frogs (Leptodactylus albilabris), in which males increased or decreased the dominant frequency of their calls to match that of a nearby male. Wagner (1989, 1992) and Bee and Perrill (1996) demonstrated, respectively, that male cricket frogs (Acris crepitans) and green frogs (Rana clamitans) reliably decreased the dominant frequency of their calls during simulated aggressive interactions over calling sites. Males of the erspect. Plant Ecology and Evolutio ary System-