Production of Biosonar Signals: Structure and Form

Abstract

Even though they live in quite different habitats, many bats, a few birds, and a number of marine mammals, including dolphins, rely on biosonar for navigation and foraging for food. Despite the fact that bats are aerial echolocators, whereas dolphins use underwater sonar, both groups depend on their sonar signals to detect, discriminate, locate, track, and catch small moving prey. The large differences in the physical characteristics of air and water guarantee that vastly different processes are required in the type of biosonar signals that are used and in the means by which they are generated. Water is approximately 850 times denser than air and the speed of sound is approximately 4.5 times faster in water. Absorption of sound in water is considerably less than in air. The relative impedance between objects in air and in water has caused both the echoic process and the mechanism by which biosonar pulses are produced to differ between these media. The sonar signals of most bats are produced in their larynx. Exceptions include a few bats and echolocating birds that produce sonar clicks with their tongue or syrinx, respectively. This is quite different than dolphins, which produce biosonar signals exclusively through their nasal system. The large size and marine environment of dolphins has favored research on the site of sound production and the propagation of sonar signals through the head and into the water. Research on echolocating bats and birds, on the other hand, has tended to focus on the neuromuscular physiology and biomechanics of sonar pulse production. In this chapter we review the current understanding of how each of these vertebrate groups produce and control their biosonar signals.