Acoustic Behaviour of Birds and Mammals in the Predator Context; I. Factors Affecting the Structure of Alarm Signals. II. The Functional Significance and Evolution of Alarm Signals

Abstract and Summary I. 7 vocalizations emitted in the predator context are defined in terms of their function. The physical and physiological constraints on the evolution of the physical structure of alarm calls with respect to detectability and localizability are discussed. Detection of various calls depends on signal amplitude, environmental attenuation, signal‐to‐noise ratio, discrimination of the receiver against background noise, and absolute auditory sensitivity of the receiver. The combined effect of these factors is discussed for an exemplary predator‐prey system, in which the hearing of both, predator and prey is known. Localizability of an alarm call is determined by its frequency, bandwidth, and possibly its amplitude relative to the auditory threshold of the receiver. Crude differentiation between localizable and non‐localizable signals is not possible, and localizability of particular sounds varies between species. In some cases the question of detectability may render the problem of localizability unimportant. Besides detectability and localizability, other factors such as the acoustic background formed by the alarm calls of sympatric species and by the species' own repertoire of calls are discussed. II. Requisite conditions and available evidence for the evolution of alarm calls through individual selection and kin selection are described. Five types of alarm calls are discussed individually: The occurrence of mobbing calls indicates that a major function of these calls is predator deterrence (“move on”), although the calls also alert other prey and promote cultural transmission of the predator's characteristics. Alarm calls associated with evasive actions of the prey cause the predator to give up the hunt or diminish its hunting success by warning other prey, which only in some cases are closely related to the caller. Distress calls of a seized prey either attract other prey which then mob the predator, or attract other predators, which presumably attack the first predator. In both cases the chances to escape are enhanced because the predator's attention is diverted. Defence calls are used to threaten a predator. These calls often mimic sounds of other predators. Distraction calls may enhance the effect of distraction display. Although the different functions of various alarm calls are treated individually, certain of the calls may have more than one function and may be employed in nonpredator contexts as well.