Alarm Calls in Birds and Mammals

Abstract

Alarm signals are used by animals to signal the presence of some kind of threat. Such signals can employ a number of different sensory modalities such as vision, audition, tactile, and olfaction. Much of the work with the evolution of alarm signals has been done with alarm calls, which are vocalizations made in response to the presence of a predator or a perceived threat. Alarm calls can have two different forms of acoustic structure. One is a high-pitched pure tone that often has ventriloquistic properties. The other is a call that is frequency-modulated such as a whistle or a call with a series of harmonics. The evolution of alarm calls appears to have progressed from sounds that were made purely from fear to sounds that contain complex information about the predator or source of alarm. Beyond that, there are two hypotheses for how and to whom alarm calls communicate information. One hypothesis suggests that kin-selection played a dominant role, and that such calls evolved to warn relatives of the caller about potential danger. Another hypothesis suggests that alarm calls evolved as a way of communicating with a predator through pursuit-invitation, as a way of informing the predator that an attempt to capture the caller would be futile. Alarm calls can contain referential information, that is, information about some attribute or behavior of the predator. Some alarm calls show response-urgency, in that the calls change in response to how urgently the caller and other animals have to respond to an attack by a predator. Other animals have calls that contain information about the attributes of the predator, such as whether the predator is a terrestrial or aerial hunter, and in some cases, the species, size, shape, and color of the predator.