- Alarm signal
In the field of
animal communication, an alarm signal is an antipredator adaptationreferring 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.Fact|date=July 2007
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.Fact|date=September 2007
This cost/benefit tradeoff of alarm calling behaviour has sparked many interest debates among
evolutionary biologists seeking to explain the occurrence of such apparently "self-sacrificing" behaviour. The central question is this: "If the ultimate purpose of any animal behaviour is to maximize the chances that an organism's own genes are passed on, with maximum fruitfulness, to future generations, why would an individual deliberately risk destroying themselves (their entire genome) for the sake of saving others (other genomes)?".
Some scientists have used the evidence of alarm-calling behaviour to challenge the theory that "evolution works only/primarily at the level of the
geneand 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)Fact|date=July 2007, 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.Fact|date=July 2007 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.Fact|date=July 2007
Another theory suggests that alarm signals function to attract further predators, which fight over the prey organism, giving it a better chance of escape. [Chivers, D. P.; G. E. Brown; R. J. F. Smith. 1996. The Evolution of Chemical Alarm Signals: Attracting Predators Benefits Alarm Signal Senders. "The American Naturalist". 148:649-659.] 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 principleand stotting). [Woodland, D. J.; Z. Jaafar; M. Knight. 1980. The "Pursuit Deterrent" Function of Alarm Signals. "The American Naturalist" 115:748-753.]
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.
False alarm calls
Deceptive alarm calls are used by male swallows ("
Hirundo rustica"). [Møller, A. P. (1990) Deceptive use of alarm calls by male swallows "Hirundo rustica": A new paternity guard. "Behavioral Ecology" 1:1-6.] Males give these false alarm calls when females leave the nest area during the matingseason, 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. [Arnqvist, G. & Rowe, L. (2005) "Sexual conflict". Princeton University Press, Princeton ISBN 0691122172]
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. [Wickler, W. (1968) "Mimicry in Plants and Animals" (Translated from the German) McGraw-Hill, New York. ISBN 0070701008. p. 108.]
Other alarm signals
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. [Campbell, N. & Reece, J. 2004. "Biology" 7th Edition - Benjamin Cummings ISBN 080537146X] Animals are not the only organism to communicate threats to conspecifics either; some plants are able to perform a similar trick. 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. [Kobayashi, Y. & N. Yamamura. 2007. Evolution of signal emission by uninfested plants to help nearby infested relatives "Evolutionary Ecology". 21:281-294.]
* [http://physorg.com/news4716.html Chickadees' alarm-call carry information about size, threat of predator]
* [http://www.cmiae.org/trek-pika.htm The Trek of the Pika] "A story complete with sounds of pika and marmot calls"
* [http://www.springerlink.com/content/p808mw6u7035w650/ Characteristics of arctic ground squirrel alarm calls]
OecologiaVolume 7, Number 2 / June, 1971
* [http://www.eeb.ucla.edu/Faculty/Blumstein/pdf%20reprints/Blumstein&Armitage1998_AB.pdf Why do Yellow-bellied Marmots Call?] DANIEL T. BLUMSTEIN & KENNETH B. ARMITAGEDepartment of Systematics and Ecology, University of Kansas
* [http://www.springerlink.com/content/h81112342070564u/ Alarm calls of Belding's ground squirrels to aerial predators: nepotism or self-preservation?]
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