In the field of animal communication, an alarm signal is an antipredator adaptation referring to various 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 blackbird’s characteristic alarm call is a familiar sound in many gardens. Other animals like fish and insects may use other non-auditory signals, such as chemical messages. While visual signs have been suggested as alarm signals, they are easier to pinpoint by predators and less likely to be received by conspecifics, so have tended to be treated as a signal to the predator instead.
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.
Evidently, alarm signals promote survival by allowing the receivers of the alarm to escape from the source of peril, but this ecological safety system may come at a cost to the signaller. When an animal calls attention to itself by signalling, it may be more likely to be eaten by a predator than if it had kept quiet. This intuition has been verified by experimental data on ground squirrel predation rates and the connection between this and the noisy chirping or whistling alarm calls.
Some scientists have used the evidence of alarm-calling behaviour to challenge the theory that "evolution works only/primarily at the level of the gene and of the gene's "interest" in passing itself along to future generations." If alarm-calling is truly an example of altruism, then our understanding of natural selection becomes more complicated than simply "survival of the fittest gene". This feature of alarm-calling behaviour is taken to suggest that evolution by natural selection is not capable of explaining some behaviours (creationism), or else that evolution has the capacity not just to select at the genetic level, but also at the species level.
Other researchers, generally those who support the selfish gene theory, question the authenticity of this "altruistic" behaviour. For instance, it has been observed that yellow-bellied marmots sometimes emit calls in the presence of a predator, and sometimes do not. Studies show that these marmots may call more often when they are surrounded by their own offspring and by other relatives who share many of their genes. Other researchers have shown that some forms of alarm calling, for example, "aerial predator whistles" produced by Belding's Ground Squirrels, do not increase the chances that a caller will get eaten by a predator; the alarm call is advantageous to both caller and recipient by frightening and warding off the predator.
Another theory suggests that alarm signals function to attract further predators, which fight over the prey organism, giving it a better chance of escape. Others still suggest they are a deterrent to predators, communicating the animals alertness to the predator. One such case is the Eastern Swamphen (Porphyrio porphyrio), which gives conspicuous visual tail flicks (see also aposematism, handicap principle and stotting).
Considerable research effort continues to be directed toward the purpose and ramifications of alarm-calling behaviour, because, to the extent that this research has the ability to comment on the occurrence or non-occurrence of altruistic behaviour, we can apply these findings to our understanding of altruism in human behaviour.
Deceptive alarm calls are used by male swallows (Hirundo rustica). Males give these false alarm calls when females leave the nest area during the mating season, and are thus able to disrupt extra-pair copulations. As this is likely to be costly to females, it can be seen as an example of sexual conflict.
Counterfeit alarm calls are also used by thrushes to avoid intraspecific competition. By sounding a bogus alarm call normally used to warning of aerial predators, they can frighten other birds away, allowing them to eat undisturbed.
Department of Systematics and Ecology, University of Kansas