Alarm signal
In animal communication, an alarm signal is an antipredator adaptation in the form of signals emitted by social animals in response to danger. Many primates and birds have elaborate alarm calls for warning conspecifics of approaching predators. For example, the alarm call of the blackbird is a familiar sound in many gardens. Other animals, like fish and insects, may use non-auditory signals, such as chemical messages. Visual signs such as the white tail flashes of many deer have been suggested as alarm signals; they are less likely to be received by conspecifics, so have tended to be treated as a signal to the predator instead.
For devices giving alarm signals, see Alarm device.
Different calls may be used for predators on the ground or from the air. Often, the animals can tell which member of the group is making the call, so that they can disregard those of little reliability.[1]
Evidently, alarm signals promote survival by allowing the receivers of the alarm to escape from the source of peril; this can evolve by kin selection, assuming the receivers are related to the signaller. However, alarm calls can increase individual fitness, for example by informing the predator it has been detected.[2]
Alarm calls are often high-frequency sounds because these sounds are harder to localize.[3][4]
Alarm pheromones[edit]
Alarm signals need not be communicated only by auditory means. For example, many animals may use chemosensory alarm signals, communicated by chemicals known as pheromones. Minnows and catfish release alarm pheromones (Schreckstoff) when injured, which cause nearby fish to hide in dense schools near the bottom.[42] At least two species of freshwater fish produce chemicals known as disturbance cues, which initiates a coordinated antipredator defence by increasing group cohesion in response to fish predators.[43][44] Chemical communication about threats is also known among plants, though it is debated to what extent this function has been reinforced by actual selection. Lima beans release volatile chemical signals that are received by nearby plants of the same species when infested with spider mites. This 'message' allows the recipients to prepare themselves by activating defense genes, making them less vulnerable to attack, and also attracting another mite species that is a predator of spider mites (indirect defence). Although it is conceivable that other plants are only intercepting a message primarily functioning to attract "bodyguards", some plants spread this signal on to others themselves, suggesting an indirect benefit from increased inclusive fitness.[45]
Deceptive chemical alarm signals are also employed. For example, the wild potato, Solanum berthaultii, emits the aphid alarm-pheromone, (E)-β-farnesene, from its leaves, which functions as a repellent against the green peach aphid, Myzus persicae.[46]
Department of Systematics and Ecology, University of Kansas