‘Silent’ signals: selective forces acting on ultrasonic communication systems in terrestrial vertebrates

Summary and conclusion: For many terrestrial vertebrates, acoustic communication is essential for establishing and maintaining territories, attracting mates, evading predators and locating prey. Successful acoustic communication requires that signals maintain sufficient integrity during transmission to convey the emitted information reliably from signaller to receiver. Thus, the inherent transmission limitations of ultrasonic frequencies resulting from their high rate of atmospheric attenuation, directionality and vulnerability to scattering place restrictions on the ecological conditions in which they represent useful communication signals. We have discussed some of the ecological and physiological factors that may select for the use of high-frequency communication channels. These scenarios suggest that additional ultrasonic communicators may be discovered among species that produce noncommunicatory ultrasounds correlated with specific behaviours that may be exapted for communication. In addition, social animals living in close quarters, and alarm-calling species, may take advantage of the limited transmission of ultrasounds to ‘beam’ communicatory signals selectively to nearby receivers without alerting competitors or predators. Finally, organisms subject to persistent acoustic interference from broadband, low-frequency environmental noise might gain a significant increase in the signal-to-noise ratio of their calls by shifting signal frequency up into a comparatively noise-free ultrasonic frequency channel. The communicatory advantage of this shift may be sufficient to offset the transmission disadvantages of high-frequency sounds. It is our hope that recognizing some of the conditions under which ultrasonic communication may be evolutionarily advantageous will enhance our ability to identify additional ultrasonically communicating species among terrestrial vertebrates. Pursuing the study of ultrasonic communication in the field offers an opportunity to understand more fully the adaptive utility of differential frequency use in communication within complex environments. In addition, comparative studies of ultrasonic communicators offer behaviourists and physiologists an intriguing opportunity to explore the fundamental mechanisms underlying the use of extraordinarily high frequencies in acoustic communication.
Published Mar. 6, 2012 2:21 PM