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 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).
A series of papers by Getty shows that Grafen’s proof of the handicap principle is based 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 is illustrated in the Figures to the right, from Johnstone 1997. This assumption that costs and benefits trade off in an additive fashion is not valid for the survival cost – reproduction benefit tradeoff that is assumed to mediate the evolution of sexually selected signals. Fitness depends on the production of offspring and this is a multiplicative function of reproductive success given an individual is still alive times the probability of still being alive, given investment in signals . Multiplicative survival-reproduction tradeoffs do not correspond to sports handicaps in any simple, useful way. Zahavi’s intuition was correct in the very general sense that “differences in costs” can stabilize the evolution of an “honest” signaling system, but in sexually selected signaling, “differences in costs” are properly decreasing proportional (or log) marginal costs . This is discussed in more detail in the entry for signalling theory.
Further formal game theoretical signalling models demonstrated the evolutionary stability of handicapped signals in nestling begging calls predator deterrent signals and 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 (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 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 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.
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'.
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.
Another example is provided by larks, some of which discourage merlin by sending a similar message: they sing while being chased, telling their predator that they will be difficult to capture.
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 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).
A series of papers by Getty shows that Grafen’s proof of the handicap principle is based 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 is illustrated in the Figures to the right, from Johnstone 1997. This assumption that costs and benefits trade off in an additive fashion is not valid for the survival cost – reproduction benefit tradeoff that is assumed to mediate the evolution of sexually selected signals. Fitness depends on the production of offspring and this is a multiplicative function of reproductive success given an individual is still alive times the probability of still being alive, given investment in signals . Multiplicative survival-reproduction tradeoffs do not correspond to sports handicaps in any simple, useful way. Zahavi’s intuition was correct in the very general sense that “differences in costs” can stabilize the evolution of an “honest” signaling system, but in sexually selected signaling, “differences in costs” are properly decreasing proportional (or log) marginal costs . This is discussed in more detail in the entry for signalling theory.
Further formal game theoretical signalling models demonstrated the evolutionary stability of handicapped signals in nestling begging calls predator deterrent signals and 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 (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 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 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.
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'.
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.
Another example is provided by larks, some of which discourage merlin by sending a similar message: they sing while being chased, telling their predator that they will be difficult to capture.
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