"Silent" signals: Selective forces acting on ultrasonic communication systems in terrestrial vertebrates.

Through technological innovations, humans have attained the unique ability to observe and study perceptual phenomena occurring outside the limits of our own sensory capabilities. In the study of acoustic communication, one such realm is the frequency channel above the upper limit of human hearing (ca. ≥20 kHz), which is defined in human terms as “ultrasound”. The ultrasonic frequency boundary is biologically somewhat arbitrary; the ability to hear ultrasound is widespread among terrestrial vertebrates, and is thought to be an ancestral trait of mammals (Masterton et al. 1968; Sales & Pye 1974). However, within the context of acoustic communication, distinguishing between audible (i.e., within the human audible range, ca. 20 Hz – 20 kHz) and ultrasonic communication provides a foundation upon which to explore intriguing questions regarding the adaptive utility of animal communication systems. The ever-expanding selection and quality of ultrasonically-sensitive microphones and recorders have provided the opportunity to address such questions with increasing sophistication. For effective acoustic communication to occur, an emitted signal must reach a receiver with enough clarity to allow an appropriate behavioral and/or physiological decision to be made. After Endler (1993), we can therefore consider clear reception to be the minimum requirement of a successful communication system. With this in mind, it becomes apparent that factors inherent in the transmission properties of ultrasonic frequencies place some restrictions on the environments and social conditions in which they are useful as information-bearing elements (sensu Suga 1972). Higher-frequency sounds attenuate more rapidly with distance (Morton 1975; Lawrence & Simmons 1982; Surlykke 1988; Romer & Lewald 1992) and are more directional than low-frequencies (Kinsler & Frey 1962). In addition, the short wavelengths of high-frequency sounds make them susceptible to reflection and scattering by relatively small objects, such as twigs, leaves, and blades of grass (Marler 1955; Sales & Pye 1974). Thus, in the interest of signal clarity, we may expect animals engaging in long-distance communication, or communicating in environments with small-scale scattering properties, to focus their vocal efforts in the audible frequency range; in this way, they can ensure that their calls have a higher probability of maintaining fidelity during transmission from emitter to receiver. However, additional environmental and behavioral complexities can place further, and in some cases competing, selective constraints on signal frequency. In addition, certain ecological scenarios may encourage the exploitation of ultrasound’s distinctive transmission properties. Herein, we will discuss some biological and ecological scenarios in which evolution may favor the development of an ultrasonic communication system, and provide examples of terrestrial vertebrates that have apparently been subject to such selection pressures. We focus on terrestrial vertebrates because the study of ultrasonic communication in this group is a rapidly expanding field. By defining biophysical, behavioral, and environmental conditions under which ultrasonic communication is likely to evolve in terrestrial vertebrates, we are well situated to identify additional organisms that may use similar strategies. The discovery of ultrasonic communicators among this group that run the evolutionary spectrum from amphibians to mammals provides an opportunity to gain more complete understanding of the behavioral foundations of high-frequency communication. In addition, comparative studies of ultrasonically-communicating taxa may give insight into the fundamental physiological mechanisms that underlie the ability to both produce and perceive extraordinarily high-frequency sounds.