Editing Handicap principle (section)

== Predictions and interpretations ==

[[File:SC06 2006 Rolls-Royce Phantom.jpg|thumb|Luxury cars and other "[[Veblen goods]]" may be an example of the handicap principle in humans.<ref name="White 2016">{{cite book |last1=White |first1=Richard C. |title=Relational Red Flags: Detecting Undesirable Qualities in Initial Romantic EncountersRomantic Encounters |date=2016 |publisher=Louisiana State University (PhD thesis) |doi=10.31390/gradschool_dissertations.1171 |id=etd-04052016-153947 |url=https://repository.lsu.edu/gradschool_dissertations/1171}}</ref>|alt=Photo of a Rolls-Royce car]]

The handicap principle predicts that a [[sexual ornament]], or any other signal such as visibly risky behavior, 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 song]]s, the peacock's tail, [[courtship dance]]s, 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]]".<ref>{{cite journal |last1=Bliege Bird |first1=R. |last2=Smith |first2=E. A. |year=2005 |title=Signalling theory, strategic interaction, and symbolic capital |journal=[[Current Anthropology]] |volume=46 |issue=2 |pages=221–248 |jstor=427115 |doi=10.1086/427115 |s2cid=13946731 }}</ref>

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 [[gazelle]]s. 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.<ref name="Zahavi Zahavi 1997"/>

=== Signals to members of the same species ===

<!--[[File:Babblers, Ardon Creek, Ramon Makhtesh, Negev, Israel (cropped).jpg|thumb|[[Arabian babbler]] are highly social, helping to care for nestlings of unrelated individuals.<ref name=Zahavi1990/>]]
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Zahavi studied in particular the [[Arabian babbler]], a highly social bird, with a life-length of 30 years, which appears to behave [[altruism|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]].<ref name=Zahavi1974>{{cite journal |last=Zahavi |first=Amotz |author-link=Amotz Zahavi |year=1974 |title=Communal nesting by the Arabian Babbler: A case of individual selection |journal=Ibis |volume=116 |pages=84–87 |doi=10.1111/j.1474-919X.1974.tb00225.x}}</ref><ref>{{cite journal |last1=Anava |first1=A. |last2=Kam |first2=M. |last3=Shkolnik |first3=A. |last4=Degen |first4=A.A. |year=2001 |jstor=4089815 |title=Does group size affect field metabolic rate of Arabian Babbler (''Turdoides squamiceps'') nestlings? |journal=[[The Auk]] |volume=118 |issue=2 |pages=525–528 |url=http://sora.unm.edu/node/131937 |doi=10.1642/0004-8038(2001)118[0525:DGSAFM]2.0.CO;2 |s2cid=38680548 |doi-access=free }}</ref><ref name=Zahavi1990>{{cite book |last=Zahavi |first=Amotz |author-link=Amotz Zahavi |year=1990 |chapter=Arabian Babblers: The quest for social status in a cooperative Breeder |pages=105–130 |title=Cooperative Breeding in Birds |editor1-first=P. B. |editor1-last=Stacey |editor2-first=W. D. |editor2-last=Koenig |publisher=[[Cambridge University Press]]}}</ref>

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 [[Genotype|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.<ref name="david">{{cite journal |first1=P. |last1=David |author2=T. Bjorksten |author3=K. Fowler |author4=A. Pomiankowski |year=2000 |title=Condition-dependent signalling of genetic variation in stalk-eyed flies |journal=[[Nature (journal)|Nature]] |volume=406 |pages=186–188 |doi=10.1038/35018079 |pmid=10910358 |issue=6792|bibcode=2000Natur.406..186D |s2cid=4425172 }}</ref>

=== Signals to other species ===

{{further|Anti-predator adaptation}}

[[File:Stotting gazelle.jpg|thumb|Impala [[stotting]], a behavior that may serve as a [[pursuit deterrence]] signal to predators.<ref name="Caro"/>|alt=Photo of an impala jumping high in the African bush]]

Signals may be directed at [[Predation|predators]], with the function of showing that pursuit will probably be unprofitable. [[Stotting]], for instance, is a form of energetic jumping that certain [[Gazelle|gazelles]] do when they sight a predator. As this behavior gives no evident benefit and would seem to waste resources (diminishing the gazelle's head start if chased by the predator), it appeared likely to be selected against. However, it made sense when seen as a [[pursuit deterrence]] signal to predators. By investing a little energy to show a lion that it has the fitness necessary to avoid capture, a gazelle reduces the likelihood that it will have to evade the lion in an actual pursuit. The lion, faced with the demonstration of fitness, might decide that it would fail to catch this gazelle, and thus choose to avoid a probably wasted pursuit. The benefit to the gazelle is twofold. First, for the small amount of energy invested in the stotting, the gazelle might not have to expend the tremendous energy required to evade the lion. Second, if the lion is in fact capable of catching this gazelle, the gazelle's bluff may lead to its survival that day (in the event the bluff succeeds).<ref name="Caro">{{cite journal |last=Caro |first=Tim M. |author-link=Tim Caro |title=The functions of stotting in Thomson's gazelles: Some tests of the predictions |journal=Animal Behaviour |year=1986 |volume=34 |issue=3 |pages=663–684 |doi=10.1016/S0003-3472(86)80052-5 |s2cid=53155678 }}</ref> However, the mathematical biologist [[John Maynard Smith]] commented that [[Stotting#Possible explanations|other explanations were possible]], such as that it was an honest signal of fitness,<ref name="Maynard Smith 2003 p61"/> or an honest signal that the predator had been detected,<ref>{{cite journal |last1=FitzGibbon |first1=C. D. |last2=Fanshawe |first2=J. H. |title=Stotting in Thomson's gazelles: an honest signal of condition |journal=Behavioral Ecology and Sociobiology |date=August 1988 |volume=23 |issue=2 |pages=69–74 |doi=10.1007/bf00299889 |bibcode=1988BEcoS..23...69F |s2cid=2809268 }}</ref> and it was hard to see how stotting could be a handicap.<ref name="Maynard Smith 2003 p61">{{cite book |last1=Maynard Smith |first1=John |author1-link=John Maynard Smith |last2=Harper |first2=David |title=Animal Signals |publisher=Oxford University Press |year=2003 |pages=61–63 |isbn=978-0-19-852685-8 |url=https://books.google.com/books?id=SUA51MeG1lcC&pg=PA61}}</ref>

Another example is provided by [[lark]]s, some of which discourage [[merlin (bird)|merlins]] by sending a similar message: they [[Bird vocalization|sing]] while being chased, telling their predator that they will be difficult to capture.<ref>{{cite journal |last=Cresswell |first=Will |title=Song as a pursuit-deterrent signal, and its occurrence relative to other anti-predation behaviours of skylark (Alauda arvensis) on attack by merlins (Falco columbarius) |journal=Behavioral Ecology and Sociobiology |date=March 1994 |volume=34 |issue=3 |pages=217–223 |doi=10.1007/BF00167747 |bibcode=1994BEcoS..34..217C |s2cid=25608814}}</ref>

=== Immunocompetence handicaps ===

The theory of [[immunocompetence]] handicaps suggests that [[androgen]]-mediated traits accurately signal condition due to the [[Immunosuppression|immunosuppressive]] effects of androgens.<ref>{{cite journal |last1=Folstad |first1=I. |last2=Karter |first2=A. K. |year=1992 |title=Parasites, bright males, and the immunocompetence handicap |journal=American Naturalist |volume=139 |issue=3 |pages=603–622 |jstor=2462500 |doi=10.1086/285346|s2cid=85266542 }}</ref>  This immunosuppression may be either because [[testosterone]] alters the allocation of limited resources between the development of [[Secondary sex characteristic|ornamental traits]] and other tissues, including the [[immune system]],<ref>{{cite journal |last1=Wedekind |first1=C. |last2=Folstad |first2=I. |year=1994 |title=Adaptive or non-adaptive immunosuppression by sex hormones? |journal=American Naturalist |volume=143 |issue=5 |pages=936–938 |jstor=2462885 |doi=10.1086/285641|s2cid=84327543 }}</ref> or because heightened immune system activity has a propensity to launch autoimmune attacks against [[gametes]], such that suppression of the immune system enhances [[fertility]].<ref>{{cite journal |last1=Folstad |first1=I. |last2=Skarstein |first2=F. |year=1996 |title=Is male germ line control creating avenues for female choice? |journal=[[Behavioral Ecology (journal)|Behavioral Ecology]] |volume=8 |issue=1 |pages=109–112 |doi=10.1093/beheco/8.1.109 |doi-access=free }}</ref> 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.<ref>{{cite journal |last1=Roberts |first1=M. L. |last2=Buchanan |first2=K. L. |last3=Evans |first3=M. R. |year=2004 |title=Testing the immunocompetence handicap hypothesis: a review of the evidence |journal=Animal Behaviour |volume=68 |issue=2 |pages=227–239 |doi=10.1016/j.anbehav.2004.05.001 |s2cid=9549459 }}</ref>