Handicap principle
The handicap principle is a disputed hypothesis proposed by the Israeli biologist Amotz Zahavi in 1975. It is meant to explain how sexual selection may lead to "honest" or reliable signalling between male and female animals which have an obvious motivation to bluff or deceive each other.[2][3][4] The handicap principle suggests that secondary sexual characteristics are costly signals which must be reliable, as they cost the signaller resources that individuals with less of a particular trait could not afford. The handicap principle further proposes that animals of greater biological fitness signal this through handicapping behaviour, or morphology that effectively lowers overall fitness. The central idea is that sexually selected traits function like conspicuous consumption, signalling the ability to afford to squander a resource. Receivers then know that the signal indicates quality, because inferior-quality signallers are unable to produce such wastefully extravagant signals.[5][6][7][8][9]
The handicap principle is supported by game theory modelling representing situations such as nestlings begging for food, predator-deterrent signalling, and threat displays. However, honest signals are not necessarily costly, undermining the theoretical basis for the handicap principle, which remains unconfirmed by empirical evidence.
History
Origins
The handicap principle was proposed in 1975 by the
Grafen's signaling game model
The handicap principle was initially controversial;[5][6][7][8] The British biologist John Maynard Smith was a notable early critic of Zahavi's ideas.[9][16][17]
However, the handicap principle gained wider acceptance because it is supported by
A series of papers by the American biologist Thomas Getty showed that Grafen's proof of the handicap principle depends on the critical, simplifying assumption that signallers trade off costs for benefits in an additive fashion, analogous to the way humans invest money to increase income in the same currency.[19][20][21][22] This is illustrated in the figures from Johnstone 1997, which show that the optimum signalling levels are different for low- and high-quality signallers.[13] The validity of the assumption that costs and benefits are additive has been contested, in its application to the evolution of sexually selected signals. It can be reasoned that since fitness depends on the production of offspring, this is a multiplicative rather than additive function of reproductive success.[23]
Further game theoretical models demonstrated the evolutionary stability of handicapped signals in nestlings' begging calls,[24] in predator-deterrent signals[25] and in threat-displays.[26][27] In the classic handicap models of begging in game theory, all players are assumed to pay the same amount to produce a signal of a given level of intensity, but differ in the relative value of eliciting the desired response (donation) from the receiver. The hungrier the baby bird, the more food is of value to it, and the higher the optimal signalling level (the louder its chirping).[24]
Cheap talk models without handicaps
Counter-examples to handicap models predate handicap models themselves. Models of signals (such as threat displays) without any handicapping costs show that what biologists call cheap talk may be an evolutionarily stable form of communication.[28] Analysis of some begging models shows that non-communication strategies are not only evolutionarily stable, but lead to higher payoffs for both players.[29][30] In human mate choice, mathematical analyses including Monte Carlo simulations suggest that costly traits ought to be more attractive to the other sex and much rarer than non-costly traits.[31]
It was soon discovered that honest signals need not be costly at the honest equilibrium, even under conflict of interest. This conclusion was first shown in discrete models[32][33] and then in continuous models.[34][35][36] Similar results were obtained in conflict models: threat displays need not be handicaps to be honest and evolutionarily stable.[37]
Unworkable theory lacking empirical evidence
In 2015, Simon Huttegger and colleagues wrote that the distinction between "indexes" (unfakable signals) and "fakable signals", crucial to the argument for the handicap principle, is an artefact of signalling models. They demonstrated that absent that dichotomy, cost could not be the only factor controlling signalling behaviours, and that indeed it was "probably not the most important" factor acting against deception.[38]
Dustin J. Penn and Szabolcs Számadó stated in 2019 that there was still no empirical evidence for evolutionary pressure for wasteful biology or acts, and proposed that the handicap principle should be abandoned.[39]
Predictions and interpretations
The handicap principle predicts that a
The handicap principle gains further support by providing interpretations for behaviours that fit into a single unifying
Signals to members of the same species
Zahavi studied in particular the Arabian babbler, a highly social bird, with a life-length of 30 years, which appears to behave altruistically. Its helping-at-the-nest behavior, where non-parent birds assist in feeding, guarding, and caring for nestlings, often occurs among unrelated individuals. This, therefore, cannot be explained by kin selection, natural selection acting on genes that close relatives share with the altruistic individual. Zahavi reinterpreted these behaviors according to his signalling theory and its correlative, the handicap principle. The altruistic act is costly to the donor, but may improve its attractiveness to potential mates. The evolution of this condition may be explained by competitive altruism.[42][43][44]
French biologist Patrice David showed that in the
Signals to other species
Signals may be directed at
Another example is provided by larks, some of which discourage merlins by sending a similar message: they sing while being chased, telling their predator that they will be difficult to capture.[49]
Immunocompetence handicaps
The theory of
See also
References
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- ^ Zahavi, Amotz (1990). "Arabian Babblers: The quest for social status in a cooperative Breeder". In Stacey, P. B.; Koenig, W. D. (eds.). Cooperative Breeding in Birds. Cambridge University Press. pp. 105–130.
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External links
- Honest Signalling Theory: A Basic Introduction. By Carl T. Bergstrom, University of Washington, 2006.