![Peacock Flying](https://upload.wikimedia.org/wikipedia/commons/0/0e/Peacock_Flying.jpg)
The handicap principle is a
hypothesis
A hypothesis (plural hypotheses) is a proposed explanation for a phenomenon. For a hypothesis to be a scientific hypothesis, the scientific method requires that one can test it. Scientists generally base scientific hypotheses on previous obse ...
proposed by the biologist
Amotz Zahavi
Amotz Zahavi ( he, אמוץ זהבי) (August 14, 1928 – May 12, 2017) was an Israeli evolutionary biologist, a Professor in the Department of Zoology at Tel Aviv University, and one of the founders of the Society for the Protection of Nature in ...
to explain how
evolution
Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation ...
may lead to "honest" or reliable
signalling
In signal processing, a signal is a function that conveys information about a phenomenon. Any quantity that can vary over space or time can be used as a signal to share messages between observers. The ''IEEE Transactions on Signal Processing'' ...
between animals which have an obvious motivation to bluff or deceive each other.
It suggests that costly signals must be
reliable signals, costing the signaller something that could not be afforded by an individual with less of a particular trait. For example, in
sexual selection
Sexual selection is a mode of natural selection in which members of one biological sex mate choice, choose mates of the other sex to mating, mate with (intersexual selection), and compete with members of the same sex for access to members of t ...
, the theory suggests that animals of greater
biological fitness
Fitness (often denoted w or ω in population genetics models) is the quantitative representation of individual reproductive success. It is also equal to the average contribution to the gene pool of the next generation, made by the same individua ...
signal this status through handicapping
behaviour
Behavior (American English) or behaviour (British English) is the range of actions and mannerisms made by individuals, organisms, systems or artificial entities in some environment. These systems can include other systems or organisms as wel ...
, or
morphology
Morphology, from the Greek and meaning "study of shape", may refer to:
Disciplines
*Morphology (archaeology), study of the shapes or forms of artifacts
*Morphology (astronomy), study of the shape of astronomical objects such as nebulae, galaxies, ...
that effectively lowers this quality. The central idea is that sexually selected traits function like
conspicuous consumption
In sociology and in economics, the term conspicuous consumption describes and explains the consumer practice of buying and using goods of a higher quality, price, or in greater quantity than practical. In 1899, the sociologist Thorstein Veblen co ...
, signalling the ability to afford to squander a resource. Receivers then know that the signal indicates quality, because inferior-quality signallers are unable to produce such wastefully extravagant signals.
History
The handicap principle was proposed in 1975 by
Israeli
Israeli may refer to:
* Something of, from, or related to the State of Israel
* Israelis, citizens or permanent residents of the State of Israel
* Modern Hebrew, a language
* ''Israeli'' (newspaper), published from 2006 to 2008
* Guni Israeli ...
biologist
A biologist is a scientist who conducts research in biology. Biologists are interested in studying life on Earth, whether it is an individual Cell (biology), cell, a multicellular organism, or a Community (ecology), community of Biological inter ...
Amotz Zahavi.
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 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.
Handicap models
![Handicap-signal-of-quality](https://upload.wikimedia.org/wikipedia/commons/9/9b/Handicap-signal-of-quality.png)
Though the handicap principle was initially controversial,—
John Maynard Smith was a notable early critic of Zahavi's ideas—it has gained wider acceptance because it is supported by
game theoretic models, most notably
Alan Grafen
Alan Grafen is a Scottish ethologist and evolutionary biologist. He currently teaches and undertakes research at St John's College, Oxford. Along with regular contributions to scientific journals, Grafen is known publicly for his work as co-edit ...
's
signalling game model. This is essentially a rediscovery of
Michael Spence
Andrew Michael Spence (born November 7, 1943) is a Canadian-American economist and Nobel laureate.
Spence is the William R. Berkley Professor in Economics and Business at the Stern School of Business at New York University, and the Philip H. Kn ...
'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—such as the
peacock
Peafowl is a common name for three bird species in the genera ''Pavo (genus), Pavo'' and ''Afropavo'' within the tribe Pavonini of the family Phasianidae, the pheasants and their allies. Male peafowl are referred to as peacocks, and female pea ...
's tail. In both cases, the signal is reliable if the cost to the signaller of producing it is proportionately lower for higher-quality signallers than it is for lower-quality signallers (Fig. 2). Having read Grafen's mathematical modeling, Smith accepted the validity of the Handicap Principle.
A series of papers by
Thomas 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 is illustrated in the figures to the right, from Johnstone 1997.
The validity of this assumption has been contested, in its application to 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, not an additive 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. The hungrier the baby bird, the more food is of value to it, and the higher the optimal signalling level (the louder its chirping).
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. 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. Mathematical analyses including
Monte Carlo simulations
Monte Carlo methods, or Monte Carlo experiments, are a broad class of computational algorithms that rely on repeated random sampling to obtain numerical results. The underlying concept is to use randomness to solve problems that might be determi ...
suggest that costly traits used in
mate choice
Mate choice is one of the primary mechanisms under which evolution can occur. It is characterized by a "selective response by animals to particular stimuli" which can be observed as behavior.Bateson, Paul Patrick Gordon. "Mate Choice." Mate Choic ...
by humans should be generally less common and more attractive to the other sex than non-costly traits.
Generality and empirical examples
The theory predicts that a
sexual ornament
A biological ornament is a characteristic of an animal that appears to serve a decorative function rather than a utilitarian function. Many are secondary sexual characteristics, and others appear on young birds during the period when they are depen ...
, 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
Bird vocalization includes both bird calls and bird songs. In non-technical use, bird songs are the bird sounds that are melodious to the human ear. In ornithology and birding, songs (relatively complex vocalizations) are distinguished by func ...
s, the
peacock
Peafowl is a common name for three bird species in the genera ''Pavo (genus), Pavo'' and ''Afropavo'' within the tribe Pavonini of the family Phasianidae, the pheasants and their allies. Male peafowl are referred to as peacocks, and female pea ...
's tail,
courtship dance
A courtship display is a set of display behaviors in which an animal, usually a male, attempts to attract a mate; the mate exercises choice, so sexual selection acts on the display. These behaviors often include ritualized movement ("dances"), ...
s, and
bowerbird
Bowerbirds () make up the bird family Ptilonorhynchidae. They are renowned for their unique courtship behaviour, where males build a structure and decorate it with sticks and brightly coloured objects in an attempt to attract a mate.
The family ...
bowers.
Jared Diamond has proposed that certain risky human behaviours, such as
bungee jumping
Bungee jumping (), also spelled bungy jumping, is an activity that involves a person jumping from a great height while connected to a large elastic cord. The launching pad is usually erected on a tall structure such as a building or crane, a ...
, may be expressions of instincts that have evolved through the operation of the handicap principle. Zahavi has invoked the
potlatch
A potlatch is a gift-giving feast practiced by Indigenous peoples of the Pacific Northwest Coast of Canada and the United States,Harkin, Michael E., 2001, Potlatch in Anthropology, International Encyclopedia of the Social and Behavioral Science ...
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
''The Theory of the Leisure Class: An Economic Study of Institutions'' (1899), by Thorstein Veblen, is a treatise of economics and sociology, and a critique of conspicuous consumption as a function of social class and of consumerism, which are s ...
'' as an example of "
conspicuous consumption
In sociology and in economics, the term conspicuous consumption describes and explains the consumer practice of buying and using goods of a higher quality, price, or in greater quantity than practical. In 1899, the sociologist Thorstein Veblen co ...
".
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
Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill th ...
such as a
lion
The lion (''Panthera leo'') is a large Felidae, cat of the genus ''Panthera'' native to Africa and India. It has a muscular, broad-chested body; short, rounded head; round ears; and a hairy tuft at the end of its tail. It is sexually dimorphi ...
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.
This may communicate to the cheetah to avoid chasing this fitter individual, or the message may be oriented toward the other gazelles, which would indicate its fitness to potential mates.
Immunocompetence handicaps
The theory of
immunocompetence
In immunology, immunocompetence is the ability of the body to produce a normal immune response following exposure to an antigen. Immunocompetence is the opposite of immunodeficiency (also known as ''immuno-incompetence'' or being ''immuno-compro ...
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.
Examples
Directed at members of the same species
![SC06 2006 Rolls-Royce Phantom](https://upload.wikimedia.org/wikipedia/commons/e/e2/SC06_2006_Rolls-Royce_Phantom.jpg)
Zahavi studied in particular the
Arabian babbler
The Arabian babbler (''Argya squamiceps'') is a passerine bird until recently placed in the genus ''Turdoides''. It is a communally nesting resident bird of arid scrub in the Middle East which lives together in relatively stable groups with stric ...
, a very social bird, with a life-length of 30 years, which was considered to have
altruistic behaviors. The helping-at-the-nest behavior often occurs among unrelated individuals, and therefore cannot be explained by kin selection. Zahavi reinterpreted these behaviours 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
Competitive altruism is a possible mechanism for the persistence of cooperative behaviors, specifically those that are performed unconditionally. The theory of reciprocal altruism can be used to explain behaviors that are performed by a donor who ...
.
[ Zahavi, Amotz (1990). "Arabian Babblers: The quest for social status in a cooperative Breeder", pp. 105–130 in ''Cooperative Breeding in Birds'', P. B. Stacey and W. D. Koenig (eds.), Cambridge University Press]
Research by Patrice David on the
stalk-eyed fly species ''
Cyrtodiopsis dalmanni
}
''Teleopsis dalmanni'', synonym ''Cyrtodiopsis dalmanni'', also known as the Malaysian stalk-eyed fly, is a species of fly in the family Diopsidae. ''T. dalmanni'' flies possess lateral elongations on their head capsules called eyestalks. These ...
'' has demonstrated that genetic variation underlies the response to environmental stress, such as variable food quality, or of male sexual ornaments, such as increased eye span.
David demonstrated that some male genotypes develop large eye spans under all conditions, whereas other genotypes progressively reduce eye spans as environmental conditions deteriorate. Several non-sexual traits, including female eye span and male and female wing length, also show condition-dependent expression, but their genetic response is entirely explained by scaling with body size. Unlike these characteristics, male eye span still reveals genetic variation in response to environmental stress after accounting for differences in body size. Thus, David inferred that these results strongly support the conclusion that female mate choice yields genetic benefits for offspring as eye span acts as a truthful indicator of male fitness. Eye span is therefore not only selected on the basis of attractiveness, but also because it demonstrates good genes in mates.
Directed at other species
![Stotting gazelle](https://upload.wikimedia.org/wikipedia/commons/f/f6/Stotting_gazelle.jpg)
The signal receiver need not be a
conspecific of the sender, however. Signals may also be directed at
predator
Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill th ...
s, with the function of showing that pursuit will probably be unprofitable.
Stotting, for instance, is a sort of hopping that certain 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 was a puzzle until handicap theory offered an explanation. According to this analysis, if the gazelle simply invests a little energy to show a lion that it has the fitness necessary to avoid capture, it may not have to evade the lion in an actual pursuit. The lion, faced with the demonstration of fitness, might decide that it will not catch this gazelle, and thus, avoid a 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 leads to its survival that day.
Another example is provided by
larks, some of which discourage
merlins by sending a similar message: they
sing
Singing is the act of creating musical sounds with the voice. A person who sings is called a singer, artist or vocalist (in jazz and/or popular music). Singers perform music (arias, recitatives, songs, etc.) that can be sung with or without ...
while being chased, telling their predator that they will be difficult to capture.
See also
*
Aposematism
Aposematism is the advertising by an animal to potential predators that it is not worth attacking or eating. This unprofitability may consist of any defences which make the prey difficult to kill and eat, such as toxicity, venom, foul taste or ...
*
Game theory
Game theory is the study of mathematical models of strategic interactions among rational agents. Myerson, Roger B. (1991). ''Game Theory: Analysis of Conflict,'' Harvard University Press, p.&nbs1 Chapter-preview links, ppvii–xi It has appli ...
*
Multiple sexual ornaments
Many species have multiple sexual ornaments, whereby females select mating partners using several cues instead of only one cue. Whereas this phenomenon is self-evident and hence long recognized, adaptive explanations of why females use several in ...
*
Parasite-stress theory
Parasite-stress theory, illustrated by researchers Corey Fincher and Randy Thornhill, is a theory of human evolution proposing that parasites and diseases encountered by a species shape the development of species' values and qualities. The differe ...
*
Sacrifice
Sacrifice is the offering of material possessions or the lives of animals or humans to a deity as an act of propitiation or worship. Evidence of ritual animal sacrifice has been seen at least since ancient Hebrews and Greeks, and possibly exi ...
*
Signaling game
*
Signaling theory
References
External links
''Honest Signalling Theory: A Basic Introduction'' By Carl T. Bergstrom, University of Washington, 2006.
{{DEFAULTSORT:Handicap Principle
Animal communication
Ethology
Selection
Sexual selection