Signalling theory

See also signal theory, a concept in engineering

Within evolutionary biology, signalling theory refers to a body of theoretical work examining communication between individuals. The central question is when animals with conflicting interests should be expected to communicate "honestly". The topic is strongly influenced by game theoretical thinking. Mathematical models in which organisms signal their condition to other individuals as part of an evolutionarily stable strategy are the principal form of research in this field.

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Honest signals

Honest signals are ones that are costly enough to make them uneconomical to produce if the true level of need or quality is less than indicated. In other words, when using honest signals, it does not pay for an individual to exaggerate or underplay their qualites or needs. Hard-to-fake signals are favored by natural selection when broad categories of signalers and receivers have conflicting interests but the interests of individual signalers and receivers converge. Receivers want good returns on their investments in others. When making decisions about whether to invest in an offspring, social partner or prospective mate, signal receivers are selected only to pay attention to signals that convey useful information relevant to estimating expected returns.

History

The question of whether individuals of the same species might not be attempting to deceive each other was raised by Richard Dawkins and John Krebs in 1978. This thinking was prompted by the application of a gene-centered view of evolution to the use of threat displays. Dawkins & Krebs criticised previous ethologists, such as Nikolaas Tinbergen and Desmond Morris among others, for supporting the view that such displays were used "for the good of the species". Dawkins and Krebs (and Krebs and Dawkins, 1982) argued that such communication ought to be viewed as an evolutionary arms race in which signallers evolve to become better at manipulating receivers, while receivers evolve to become more resistant to manipulation.

The game theoretical model of the war of attrition was applied to the problem, and appeared to suggest that threat displays ought not to convey any reliable information about intentions (Caryl, 1979). This led to a re-examination of the empirical evidence, and much debate^{[citation needed]}.

Handicapped vs. conventional signals

The handicap principle, first proposed by Amotz Zahavi (1975, 1977), is an influential theory which proposes that honest signals like showy plumage of certain male birds (peacocks tails being an extreme classic example) work to reliably communicate genetic quality by being costly because only high quality individuals can afford to carry them. This theory was initially criticized by many as being either wrong (Kirkpatrick, 1986) or confused^{[citation needed]}. A number of models, most notably Alan Grafen's (1990) game theoretic handicapped courtship display model, have since buttressed the idea. This principle also goes by the name costly signalling theory or simply CST^{[citation needed]}. Debate on the topic deals not with whether such an effect is possible, but whether it is necessarily true of all biological signals. Some, such as John Maynard Smith, believe that signals can be both honest and cost-free (Maynard Smith, 1994). An honest signal may not need to have direct performance costs if, for example, there are socially imposed consequences to dishonesty (eg Enquist, 1985).

Examples

• Sam Brown and W.D. Hamilton ^[1] and Marco Archetti ^[2] proposed the idea that Autumn leaf color is due to the trees signalling to aphids and other pest species that migrate to the trees in autumn. Autumn colour is costly to trees but bright trees might reduce their parasite load; aphids on the other hand might prefer trees with dull leaves because these are the ones with less chemical defenses. Indeed aphids appear to preferentially avoid trees with bright leaves and tree species with bright leaves have more specialist aphid pests than do trees lacking bright leaves. While autumn colours might be a real handicap signal, it is not necessary that the signal is costly to produce. It might also be an index, which is maintained because it is impossible to fake (it would be a signal instead if only strong individuals can afford the cost of displaying it). The topic is still debated.

See also

• Animal communication
• Cheap talk
• Signalling game
• Autumn leaf color

References

• Archetti M. 2000 The origin of autumn colours by coevolution. "J. Theor. Biol." 205: 625-630.
• Caryl, P. G. 1979: Communication by agonistic displays: what can games theory contribute to ethology? Behaviour 68:136-169.
• Dawkins, R. & Krebs, J. R. 1978: Animal signals: information or manipulation? in Behavioural Ecology: an evolutionary approach 1st ed. (Krebs, J. R. & ,Davies, N.B., eds) Blackwell: Oxford, pp 282-309.
• Enquist, M. 1985: Communication during aggressive interactions with particular reference to variation in choice of behaviour. Animal Behaviour 33, 1152-1161.
• Grafen, A. 1990: Biological signals as handicaps. Journal of Theoretical Biology 144: 517-546.
• Hamilton, W.D. & Brown, S.P. 2001 Autumn tree colours as a handicap signal. "Proc. R. Soc. B" 268:1489-1493.
• Kirkpatrick, M 1986: The handicap mechanism of sexual selection does not work. American Naturalist 127, 222-240.
• Krebs, J. R. and Dawkins, R. 1984: Animal signals: mind-reading and manipulation. in Behavioural Ecology: an evolutionary approach, 2nd ed (Krebs, J. R. & ,Davies, N.B., eds), Sinauer: pp 380-402
• Maynard Smith, J. 1994: Must reliable signals always be costly? Animal Behaviour 47, 1115-1120.
• Maynard Smith, J and Harper, D. 2004: Animal Signals
• Zahavi, A. 1975: Mate selection — a selection for a handicap. Journal of theoretical Biology. 53, 205-214
• Zahavi, A. 1977: The cost of honesty (Further remarks on the handicap principle). Journal of theoretical Biology. 67, 603-605

Notes

1. ^ Hamilton and Brown (2001) Autumn tree colours as a handicap signal. PDF Proc. R. Soc. B 268:1489-1493
2. ^ Archetti (2000) The origin of autumn colours by coevolution. PDF J. Theor. Biol. 205: 625-630