Echolocation by Insect-Eating Bats

B (order Chiroptera) are ecologically more diverse than any other group of mammals. Numerous morphological, physiological, and behavioral adaptations of sensory and motor systems permit bats access to a wide range of habitats and resources at night. The more than 750 species of the suborder Microchiroptera occupy most terrestrial habitats and climatic zones and exploit a great variety of foods, ranging from insects and other arthropods, small vertebrates, and blood to fruit, leaves, nectar, flowers, and pollen. Echolocation is one of the adaptations that make bats so successful. Echolocating animals emit signals of high frequency (mostly ultrasonic) and analyze the returning echoes to detect, characterize, and localize the reflected objects. Sophisticated echolocation systems have evolved only in the bat suborder Microchiroptera and in dolphins. Less efficient systems have been reported for a few species of the bat suborder Megachiroptera and for some birds (Henson and Schnitzler 1980). Bats use echolocation for orientation in space, that is, for determining their position relative to the echo-producing environment. In addition, many bats, especially those that hunt for flying insects, use echolocation to detect, identify, and localize prey. Bats use a wide variety of species-specific signal types differing in frequency structure, duration, and sound pressure level (SPL). In addition, signal structure varies depending on the echolocation task confronting the bat. Search signals that are emitted when bats search for prey differ from approach signals that are emitted when they approach prey. The echolocation signals and hearing systems of bats are well adapted for gathering behaviorally relevant information (e.g., Schnitzler and Henson 1980, Neuweiler 1989, Fenton 1990, Denzinger et al. forthcoming). In this article we describe the echolocation behavior of insect-eating bats and show how differing circumstances such as habitat type, foraging mode, and diet favor different signal types. To demonstrate relationships between echolocation and ecological conditions, we outline the perceptual tasks that must be performed by foraging bats and discuss the suitability of typical elements of echolocation signals for solving such problems. We then define habitat types according to the problems they impose on bats and relate the observed variability in signal structure to ecological constraints set by habitat type and foraging mode. Perceptual problems for foraging bats Foraging bats confront a multitude of problems when flying to their hunting grounds and searching for prey. These problems differ depending on where bats hunt, what they eat, and how they acquire their food. For example, bats hunting for insects in the open encounter conditions different from those that search for prey near the edges of vegetation, in vegetation gaps, in dense forest, or near the ground. The problems also differ depending upon whether they capture moving prey in flight (aerial mode) or mostly stationary prey from surfaces such as leaves or ground (gleaning mode) or water (trawling mode). Foraging bats must detect, classify, and localize an insect and discriminate between echoes of prey and echoes of unwanted targets such as twigs, foliage, or the ground, referred to as clutter echoes, or simply “clutter. ”For many bats echolocation delivers all of the information they need to catch an insect.