Handicap principle

  The handicap principle is a hypothesis originally proposed in 1975 by biologist Amotz Zahavi^[1][2][3] to explain how evolution may lead to "honest" or reliable communication between animals who have an obvious motivation to bluff or deceive each other. The handicap principle suggests that reliable signals must be burdensomely costly to the signaller, costing the signaller in the trait being signalled in a manner that an individual with less of that trait could not afford. For example, in the case of sexual selection, the theory suggests that animals of greater quality communicate this status through handicapping behaviour or morphology that effectively lowers their quality. The central idea is that sexually selected traits function like conspicuous consumption, signalling the ability to afford to squander a resource simply by squandering it. Receivers know that the signal indicates quality because inferior quality signallers cannot afford to produce such wastefully extravagant signals.

The generality of the phenomenon is the matter of some debate and disagreement, Zahavi's views on the scope and importance of handicaps in biology remain outside the mainstream.^[4] Nevertheless, the idea has been very influential,^[5][6][7] with most researchers in the field believing that the theory explains some aspects of animal communication.

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Handicap models

  Though the idea was initially controversial^{[8][9][10][11]} (John Maynard Smith being one notable early critic of Zahavi's ideas^[12][13][14]) it has gained wider acceptance due to supporting game theoretic models, most notably Alan Grafen's signalling game model.^[15] Grafen's model is essentially a rediscovery of Michael Spence's job market signalling model,^[16] where the signalled trait was conceived as a courting male's quality, signalled by investment in an extravagant trait -such as the peacock's tail- rather than an employee signalling their quality by way of an expensive education. In both cases, it is the decreased cost to higher quality signallers of producing increased signal that stabilizes the reliability of the signal (Fig. 2). Further formal game theoretical signalling models demonstrated the evolutionary stability of handicapped signals in nestling begging calls^[17] predator deterrent signals^[18] and threat displays.^[19][20] In the classic handicapped models of begging, 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 (Fig. 3).

  Counter-examples to handicap models predate handicap models themselves. Models of signals, such as threat displays, without any handicapping costs show that conventional signalling may be evolutionarily stable in biological communication^[21] Further, analysis of some begging models also shows that, in addition to the handicapped outcomes, non-communication strategies are not only evolutionarily stable, but lead to higher payoffs for both players.^[22][23]

Generality and empirical examples

The theory predicts that a sexual ornament, or any other signal, must be costly if it is to accurately advertise a trait of relevance to an individual with conflicting interests. Typical examples of handicapped signals include bird songs, the peacock's tail, courtship dances, bowerbird's bowers, or even possibly jewellery and humor. Jared Diamond has proposed that certain risky human behaviours, such as bungee jumping, may be expressions of instincts that have evolved through the operation of the handicap principle. Zahavi has invoked the potlatch ceremony as a human example of the handicap principle in action. This interpretation of potlatch can be traced to Thorstein Veblen's use of the ceremony in his book Theory of the Leisure Class as an example of "conspicuous consumption".^[24]

The handicap principle gains further support by providing interpretations for behaviours that fit into a single unifying gene-centered view of evolution and making earlier explanations based on group selection obsolete. A classic example is that of stotting in gazelles. This behaviour consists in the gazelle initially running slowly and jumping high when threatened by a predator such as a lion or cheetah. The explanation based on group selection was that such behaviour might be adapted to alerting other gazelle to a cheetah's presence or might be part of a collective behaviour pattern of the group of gazelle to confuse the cheetah. Instead, Zahavi proposed that each gazelle was communicating to the cheetah that it was a fitter individual than its fellows and that the predator should avoid chasing it.

Immunocompetence handicaps

The theory of immunocompetence handicaps suggests that androgen-mediated traits accurately signal condition due to the immunosuppressive effects of androgens^[25]. This immunosuppression may be either because testosterone alters the allocation of limited resources between the development of ornamental traits and other tissues, including the immune system^[26], or because heightened immune system activity has a propensity to launch autoimmune attacks against gametes, such that suppression of the immune system enhances fertility^[27] Healthy individuals can afford to suppress their immune system by raising their testosterone levels, which also augments secondary sexual traits and displays. A review of empirical studies into the various aspects of this theory found weak support^[28].

Examples

The tail of a peacock makes the peacock more vulnerable to predators, and is therefore a handicap. But the message that the tail carries to the potential mate peahen is 'I have survived in spite of this huge tail, hence I am fitter and more attractive than others'.

Another example is stotting a sort of hopping that certain gazelles do when they sight a lion. As this behavior gives no evident benefit and would seem to waste resources, it was a puzzle until handicap theory offered an explanation. According to this analysis, the gazelle invests of bit of energy to show the lion that it had the fitness necessary to avoid capture, thus avoiding spending the energy required to evade actual pursuit. The lion might recognize that it could not catch this gazelle and so avoid a wasted pursuit.

An example in humans was suggested by Geoffrey Miller who expressed that Veblen goods such as luxury cars and other forms of conspicuous consumption are manifestations of the handicap principle, being used by men to advertise their "fitness" to women.

See also

• Aposematism

References