Handicap principle
Encyclopedia
Hypothesis
A hypothesis is a proposed explanation for a phenomenon. The term derives from the Greek, ὑποτιθέναι – hypotithenai meaning "to put under" or "to suppose". For a hypothesis to be put forward as a scientific hypothesis, the scientific method requires that one can test it...
originally proposed in 1975 by biologist
Biology
Biology is a natural science concerned with the study of life and living organisms, including their structure, function, growth, origin, evolution, distribution, and taxonomy. Biology is a vast subject containing many subdivisions, topics, and disciplines...
Amotz Zahavi
Amotz Zahavi
Amotz Zahavi is an Israeli evolutionary biologist, a Professor Emeritus at the Zoology Department of Tel Aviv University, and one of the founders of the Society for the Protection of Nature in Israel...
to explain how evolution
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
may lead to "honest" or reliable signaling
Signalling theory
Within evolutionary biology, signalling theory is a body of theoretical work examining communication between individuals. The central question is when organisms with conflicting interests should be expected to communicate "honestly"...
between animals who have an obvious motivation to bluff or deceive each other. The handicap principle suggests that reliable signals
Signalling theory
Within evolutionary biology, signalling theory is a body of theoretical work examining communication between individuals. The central question is when organisms with conflicting interests should be expected to communicate "honestly"...
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
Sexual selection
Sexual selection, a concept introduced by Charles Darwin in his 1859 book On the Origin of Species, is a significant element of his theory of natural selection...
, the theory suggests that animals of greater biological fitness
Fitness (biology)
Fitness is a central idea in evolutionary theory. It can be defined either with respect to a genotype or to a phenotype in a given environment...
signal this status through handicapping behaviour
Ethology
Ethology is the scientific study of animal behavior, and a sub-topic of zoology....
or morphology
Morphology (biology)
In biology, morphology is a branch of bioscience dealing with the study of the form and structure of organisms and their specific structural features....
that effectively lowers this quality. The central idea is that sexually selected
Sexual selection
Sexual selection, a concept introduced by Charles Darwin in his 1859 book On the Origin of Species, is a significant element of his theory of natural selection...
traits function like conspicuous consumption
Conspicuous consumption
Conspicuous consumption is spending on goods and services acquired mainly for the purpose of displaying income or wealth. In the mind of a conspicuous consumer, such display serves as a means of attaining or maintaining social status....
, signalling the ability to afford to squander a resource simply by squandering it. Receivers know that the signal indicates quality because inferior quality signallers 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.
Handicap models
John Maynard Smith
John Maynard Smith,His surname was Maynard Smith, not Smith, nor was it hyphenated. F.R.S. was a British theoretical evolutionary biologist and geneticist. Originally an aeronautical engineer during the Second World War, he took a second degree in genetics under the well-known biologist J.B.S....
being one notable early critic of Zahavi's ideas) it has gained wider acceptance due to supporting game theoretic
Game theory
Game theory is a mathematical method for analyzing calculated circumstances, such as in games, where a person’s success is based upon the choices of others...
models, most notably Alan Grafen
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-editor of the 2006 festschrift Richard Dawkins: How a...
's signalling game model. Grafen's model is essentially a rediscovery of Michael Spence
Michael Spence
Andrew Michael Spence is an American economist and recipient of the 2001 Nobel Memorial Prize in Economic Sciences, along with George A. Akerlof and Joseph E. Stiglitz, for their work on the dynamics of information flows and market development. He conducted this research while at Harvard University...
'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 (Fig. 2).
A series of papers by 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. 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
Signalling theory
Within evolutionary biology, signalling theory is a body of theoretical work examining communication between individuals. The central question is when organisms with conflicting interests should be expected to communicate "honestly"...
. 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
Evolutionarily stable strategy
In game theory and behavioural ecology, an evolutionarily stable strategy , which is sometimes also called an evolutionary stable strategy, is a strategy which, if adopted by a population of players, cannot be invaded by any alternative strategy that is initially rare. An ESS is an equilibrium...
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 (Fig. 3).
Cheap talk
In game theory, cheap talk is communication between players which does not directly affect the payoffs of the game. This is in contrast to signaling in which sending certain messages may be costly for the sender depending on the state of the world...
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.
Generality and empirical examples
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 songBird 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 are distinguished by function from calls.-Definition:The distinction between songs and calls is based upon...
s, the peacock's tail, courtship dances, bowerbird
Bowerbird
Bowerbirds make up the bird family Ptilonorhynchidae. The family has 20 species in eight genera. These are medium-sized passerines, ranging from the Golden Bowerbird to the Great Bowerbird...
's bowers, or even possibly jewellery
Jewellery
Jewellery or jewelry is a form of personal adornment, such as brooches, rings, necklaces, earrings, and bracelets.With some exceptions, such as medical alert bracelets or military dog tags, jewellery normally differs from other items of personal adornment in that it has no other purpose than to...
and humor. Jared Diamond
Jared Diamond
Jared Mason Diamond is an American scientist and author whose work draws from a variety of fields. He is currently Professor of Geography and Physiology at UCLA...
has proposed that certain risky human behaviours, such as bungee jumping
Bungee jumping
Bungee jumping is an activity that involves jumping from a tall structure while connected to a large elastic cord. The tall structure is usually a fixed object, such as a building, bridge or crane; but it is also possible to jump from a movable object, such as a hot-air-balloon or helicopter, that...
, may be expressions of instincts that have evolved through the operation of the handicap principle. Zahavi has invoked the potlatch
Potlatch
A potlatch is a gift-giving festival and primary economic system practiced by indigenous peoples of the Pacific Northwest Coast of Canada and United States. This includes Heiltsuk Nation, Haida, Nuxalk, Tlingit, Makah, Tsimshian, Nuu-chah-nulth, Kwakwaka'wakw, and Coast Salish cultures...
ceremony as a human example of the handicap principle in action. This interpretation of potlatch can be traced to Thorstein Veblen
Thorstein Veblen
Thorstein Bunde Veblen, born Torsten Bunde Veblen was an American economist and sociologist, and a leader of the so-called institutional economics movement...
's use of the ceremony in his book Theory of the Leisure Class as an example of "conspicuous consumption
Conspicuous consumption
Conspicuous consumption is spending on goods and services acquired mainly for the purpose of displaying income or wealth. In the mind of a conspicuous consumer, such display serves as a means of attaining or maintaining social status....
".
The handicap principle gains further support by providing interpretations for behaviours that fit into a single unifying gene-centered view of evolution
Gene-centered view of evolution
The gene-centered view of evolution, gene selection theory or selfish gene theory holds that evolution occurs through the differential survival of competing genes, increasing the frequency of those alleles whose phenotypic effects successfully promote their own propagation, with gene defined as...
and making earlier explanations based on group selection
Group selection
In evolutionary biology, group selection refers to the idea that alleles can become fixed or spread in a population because of the benefits they bestow on groups, regardless of the alleles' effect on the fitness of individuals within that group....
obsolete. A classic example is that of stotting
Stotting
Stotting is a gait of quadrupeds, particularly gazelles , involving jumping high into the air by lifting all four feet off the ground simultaneously. This may occur during pursuit by a predator...
in gazelle
Gazelle
A gazelle is any of many antelope species in the genus Gazella, or formerly considered to belong to it. Six species are included in two genera, Eudorcas and Nanger, which were formerly considered subgenera...
s. This behaviour consists in the gazelle initially running slowly and jumping high when threatened by a predator such as a lion
Lion
The lion is one of the four big cats in the genus Panthera, and a member of the family Felidae. With some males exceeding 250 kg in weight, it is the second-largest living cat after the tiger...
or cheetah
Cheetah
The cheetah is a large-sized feline inhabiting most of Africa and parts of the Middle East. The cheetah is the only extant member of the genus Acinonyx, most notable for modifications in the species' paws...
. 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.
Immunocompetence handicaps
The theory of immunocompetenceImmunocompetence
Immunocompetence is the ability of the body to produce a normal immune response following exposure to an antigen. Immunocompetence is the opposite of immunodeficiency or immuno-incompetent or immuno-compromised...
handicaps suggests that androgen
Androgen
Androgen, also called androgenic hormone or testoid, is the generic term for any natural or synthetic compound, usually a steroid hormone, that stimulates or controls the development and maintenance of male characteristics in vertebrates by binding to androgen receptors...
-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
Secondary sex characteristic
Secondary sex characteristics are features that distinguish the two sexes of a species, but that are not directly part of the reproductive system. They are believed to be the product of sexual selection for traits which give an individual an advantage over its rivals in courtship and aggressive...
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.
Directed at members of the same species
Amotz Zahavi
Amotz Zahavi is an Israeli evolutionary biologist, a Professor Emeritus at the Zoology Department of Tel Aviv University, and one of the founders of the Society for the Protection of Nature in Israel...
studied in particular the Arabian Babbler
Arabian Babbler
The Arabian Babbler is a passerine bird belonging to the genus Turdoides, a genus of Old World babblers.It is 26–29 cm long with a wingspan of 31-33.5 cm and a weight of 64-83 grams. It has a fairly long curved bill, a long tail, rounded wings and strong legs and feet. The plumage...
, a very social bird, with a life-length of 30 years, which was considered to have altruist behaviours. 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
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 receives some sort of benefit in the future...
.
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
Geoffrey Miller (evolutionary psychologist)
Geoffrey F. Miller , Associate Professor of Psychology at the University of New Mexico, is an American evolutionary psychologist.Miller is a 1987 graduate of Columbia University, where he earned a B.A. in biology and psychology. He received his PhD in cognitive psychology from Stanford University...
who expressed that Veblen goods such as luxury cars and other forms of conspicuous consumption
Conspicuous consumption
Conspicuous consumption is spending on goods and services acquired mainly for the purpose of displaying income or wealth. In the mind of a conspicuous consumer, such display serves as a means of attaining or maintaining social status....
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.
Directed at other species
Stotting
Stotting is a gait of quadrupeds, particularly gazelles , involving jumping high into the air by lifting all four feet off the ground simultaneously. This may occur during pursuit by a predator...
, 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 bit of energy to show the predator that it has the fitness necessary to avoid capture, it may avoid having to evade the lion in an actual pursuit. The lion, faced with the demonstration of fitness, might decide that it might 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 lark
Lark
Larks are passerine birds of the family Alaudidae. All species occur in the Old World, and in northern and eastern Australia; only one, the Shore Lark, has spread to North America, where it is called the Horned Lark...
s, some of which discourage merlin
Merlin (bird)
The Merlin is a small species of falcon from the Northern Hemisphere. A bird of prey once known colloquially as a pigeon hawk in North America, the Merlin breeds in the northern Holarctic; some migrate to subtropical and northern tropical regions in winter.-European and North American...
by sending a similar message: they sing while being chased, telling their predator that they will be difficult to capture.