Handicap principle

Origins The handicap principle was proposed in 1975 by the Israeli biologist Amotz Zahavi. He argued that mate choice involving what he called "signal selection" would lead to "honest" or reliable signalling between male and female animals, even though they have an interest in bluffing or deceiving each other. The handicap principle asserts that secondary sexual characteristics are costly signals, which are reliable indicators of the signaller's quality, since they cost the signaller resources that lower-quality individuals could not afford. The generality of the phenomenon is a matter of some debate and disagreement, and Zahavi's views on the scope and importance of handicaps in biology have not been accepted by 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. Grafen's signaling game model a notable early critic of Zahavi's ideas. However, the handicap principle gained wider acceptance because it is supported by game theory models, most notably the Scottish biologist Alan Grafen's 1990 signalling game model. This was essentially a rediscovery of the Canadian-American economist Michael Spence's job market signalling model, where the job applicant signals their quality by declaring a costly education. In Grafen's model, the courting male's quality is signalled by investment in an extravagant trait—similar to the peacock's tail. The signal is reliable if the marginal cost to the signaller is proportionately lower for higher-quality signallers than for lower-quality ones: the cost can be lower or the benefit higher, or both. This is illustrated in the figures from Johnstone 1997, which show that the optimum signalling levels are different for low- and high-quality signallers. Further game theoretical models demonstrated the evolutionary stability of handicapped signals in nestlings' begging calls, in predator-deterrent signals and in threat-displays. 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). Analysis of some begging models shows that non-communication strategies are not only evolutionarily stable, but lead to higher payoffs for both players. 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. 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 and then in continuous models. Similar results were obtained in conflict models: threat displays need not be handicaps to be honest and evolutionarily stable. 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. 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. == Predictions and interpretations ==

" may be an example of the handicap principle in humans.|alt=Photo of a Rolls-Royce car The handicap principle predicts that a sexual ornament, or any other signal such as visibly risky behaviour, 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, and bowerbird bowers. American scientist 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 gift-giving potlatch ceremony as a human example of the handicap principle in action: the conspicuous generosity is costly. 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 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 that it was a fitter individual than its fellows. French biologist Patrice David showed that in the stalk-eyed fly species Cyrtodiopsis dalmanni, genetic variation underlies the response to environmental stress, such as variable food quality, of a male sexual ornament, eye span. He showed that some male genotypes always develop large eye spans, but others reduce eye span in proportion to environmental worsening. David inferred that female mate choice yields genetic benefits for offspring. Signals to other species , a behaviour that may serve as a pursuit deterrence signal to predators However, the mathematical biologist John Maynard Smith commented that other explanations were possible, such as that it was an honest signal of fitness, and it was hard to see how stotting could be a handicap. 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. Immunocompetence handicaps 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, at the same time augmenting secondary sexual traits and displays. A review of empirical studies into the various aspects of this theory found weak support. == See also ==