Handicap Principle

peacock

The handicap principle is a hypothesis originally proposed in 1975 by biologist Amotz Zahavi to explain how evolution may lead to ‘honest’ or reliable signaling between animals who have an obvious motivation to bluff or deceive each other. The handicap principle suggests that reliable signals must be costly to the signaler, costing the signaler something that could not be afforded by an individual with less of a particular trait.

For example, in the case of sexual selection, the theory suggests that animals of greater biological fitness signal this status through handicapping behavior or morphology that effectively lowers this 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 signalers cannot afford to produce such wastefully extravagant signals.

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

Though the idea was initially controversial (John Maynard Smith being one notable early critic of Zahavi’s ideas) it has gained wider acceptance due to supporting game theoretic models, most notably Alan Grafen’s signalling game model. Grafen’s model is essentially a rediscovery of Michael Spence’s job market signalling model, 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 a costly education. In both cases, it is the decreased cost to higher-quality signallers of producing increased signal that stabilizes the reliability of the signal.

However A series of papers by T. Getty shows that Grafen’s proof of the handicap principle depends on the critical simplifying assumption that signalers trade off costs for benefits in an additive fashion, the way humans invest money to increase income in the same currency. This assumption that costs and benefits trade off in an additive fashion is contested to be valid for the survival cost – reproduction benefit trade-off that is assumed to mediate the evolution of sexually selected signals. It can be reasoned that as fitness depends on the production of offspring, this is therefore a multiplicative function of reproductive success.

Further formal game theoretical signalling models demonstrated the evolutionary stability of handicapped signals in nestlings’ begging calls, in predator-deterrent signals and in threat-displays. 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 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. 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.

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 (a Native American ceremony where property is given away altruistically) 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.’

The handicap principle gains further support by providing interpretations for behaviors 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 behavior 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 behavior might be adapted to alerting other gazelle to a cheetah’s presence or might be part of a collective behavior 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.

The theory of immunocompetence handicaps suggests that androgen-mediated traits accurately signal condition due to the immunosuppressive effects of androgens. 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, or because heightened immune system activity has a propensity to launch autoimmune attacks against gametes, such that suppression of the immune system enhances fertility. 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.

Amotz Zahavi studied in particular the Arabian Babbler, a very social bird, with a life-length of 30 years, which was considered to have altruist behaviors. The helping-at-the-nest behavior often occurs among unrelated individuals, and therefore cannot be explained by kin selection. Zahavi reinterpreted these behaviors according to his signal 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.

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

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. Obesity, a sign of ability to procure or afford plenty of food, comes at the expense of health, agility, and in more advanced cases, even strength, but, before recent cultural changes, exhibited ‘fitness’ to the opposite sex and was also often associated with wealth.

On the opposite end of the spectrum, significantly above average height, stoutness, and lean body mass require vastly higher caloric intake and come at the expense of speed, more than can be offset by the defensive advantages, but are highly attractive to females. Modern bodybuilding with steroids is an especially notable example, where bodybuilders are willing to knowingly forfeit general health to high risk of injury and the immunosuppressant qualities of excess androgens, risk impotence, and grow to sizes that are inconvenient and impractical in the modern world, in exchange for the appearance of supreme physical health and reproductive fitness.

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