![Springbok pronk](https://upload.wikimedia.org/wikipedia/commons/4/47/Springbok_pronk.jpg)
Within
evolutionary biology
Evolutionary biology is the subfield of biology that studies the evolutionary processes (natural selection, common descent, speciation) that produced the diversity of life on Earth. It is also defined as the study of the history of life fo ...
, signalling theory is a body of
theoretical
A theory is a rational type of abstract thinking about a phenomenon, or the results of such thinking. The process of contemplative and rational thinking is often associated with such processes as observational study or research. Theories may be s ...
work examining
communication between individuals, both within species and across species. The central question is when organisms with conflicting interests, such as 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 ...
, should be expected to provide honest signals (no presumption being made of conscious intention) rather than
cheating
Cheating generally describes various actions designed to subvert rules in order to obtain unfair advantages. This includes acts of bribery, cronyism and nepotism in any situation where individuals are given preference using inappropriate cr ...
.
Mathematical models
A mathematical model is a description of a system using mathematical concepts and language. The process of developing a mathematical model is termed mathematical modeling. Mathematical models are used in the natural sciences (such as physics, b ...
describe how signalling can contribute to an
evolutionarily stable strategy
An evolutionarily stable strategy (ESS) is a strategy (or set of strategies) that is ''impermeable'' when adopted by a population in adaptation to a specific environment, that is to say it cannot be displaced by an alternative strategy (or set of ...
.
Signals are given in contexts such as mate selection by females, which subjects the
advertising
Advertising is the practice and techniques employed to bring attention to a product or service. Advertising aims to put a product or service in the spotlight in hopes of drawing it attention from consumers. It is typically used to promote a ...
males' signals to selective pressure. Signals thus evolve because they modify the behaviour of the receiver to benefit the signaller. Signals may be honest, conveying information which usefully increases the fitness of the receiver, or dishonest. An individual can cheat by giving a dishonest signal, which might briefly benefit that signaller, at the risk of undermining the signalling system for the whole population.
The question of whether selection of signals works at the level of the individual organism or gene, or at the level of the group, has been debated by biologists such as
Richard Dawkins
Richard Dawkins (born 26 March 1941) is a British evolutionary biologist and author. He is an emeritus fellow of New College, Oxford and was Professor for Public Understanding of Science in the University of Oxford from 1995 to 2008. An ath ...
, arguing that individuals evolve to signal and to receive signals better, including resisting manipulation.
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 ...
suggested that cheating could be controlled by the
handicap principle
The handicap principle is a hypothesis proposed by the biologist Amotz Zahavi to explain how evolution may lead to "honest" or reliable signalling between animals which have an obvious motivation to bluff or deceive each other. It suggests that ...
, where the best horse in a
handicap race is the one carrying the largest handicap weight. According to Zahavi's theory, signallers such as male peacocks have 'tails' that are genuinely handicaps, being costly to produce. The system is evolutionarily stable as the large showy tails are honest signals. Biologists have attempted to verify the handicap principle, but with inconsistent results. The mathematical biologist
Ronald Fisher
Sir Ronald Aylmer Fisher (17 February 1890 – 29 July 1962) was a British polymath who was active as a mathematician, statistician, biologist, geneticist, and academic. For his work in statistics, he has been described as "a genius who a ...
analysed the contribution that having two copies of each gene (
diploid
Ploidy () is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes. Sets of chromosomes refer to the number of maternal and paternal chromosome copies, respectively ...
y) would make to honest signalling, demonstrating that a
runaway effect could occur 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 evolutionary equilibrium depends sensitively on the balance of costs and benefits.
The same mechanisms can be expected in humans, where researchers have studied behaviours including risk-taking by young men, hunting of large game animals, and costly religious rituals, finding that these appear to qualify as costly honest signals.
Sexual selection
When animals choose mating partners,
traits such as signalling are subject to evolutionary pressure. For example, the male
gray tree frog
The gray treefrog (''Dryophytes versicolor'') is a species of small arboreal holarctic tree frog native to much of the eastern United States and southeastern Canada.
It is sometimes referred to as the eastern gray treefrog, northern gray tree ...
, ''Hyla versicolor'', produces a call to attract females. Once a female chooses a mate, this selects for a specific style of male calling, thus propagating a specific signalling ability. The signal can be the call itself, the intensity of a call, its variation style, its repetition rate, and so on. Various hypotheses seek to explain why females would select for one call over the other. The sensory exploitation hypothesis proposes that pre-existing preferences in female receivers can drive the evolution of signal innovation in male senders, in a similar way to the hidden preference hypothesis which proposes that successful calls are better able to match some 'hidden preference' in the female. Signallers have sometimes evolved
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 ...
, and receivers have sometimes evolved multiple trait preferences.
Honest signals
![2012-06-09 16-04-22 Switzerland Kanton Graubünden Sagliaz cropped](https://upload.wikimedia.org/wikipedia/commons/6/6a/2012-06-09_16-04-22_Switzerland_Kanton_Graub%C3%BCnden_Sagliaz_cropped.JPG)
In biology, signals are traits, including structures and behaviours, that have evolved specifically because they change the behaviour of receivers in ways that benefit the signaller. Traits or actions that benefit the receiver exclusively are called "cues". For example, when an alert bird deliberately gives a warning call to a stalking predator and the predator gives up the hunt, the sound is a "signal". But when a foraging bird inadvertently makes a rustling sound in the leaves that attracts predators and increases the risk of predation, the sound is not a signal, but a cue.
Signalling systems are shaped by mutual interests between signallers and receivers. An alert bird such as a
Eurasian jay
The Eurasian jay (''Garrulus glandarius'') is a species of passerine bird in the crow family Corvidae. It has pinkish brown plumage with a black stripe on each side of a whitish throat, a bright blue panel on the upper wing and a black tail. The ...
warning off a stalking predator is communicating something useful to the predator: that it has been detected by the prey; it might as well quit wasting its time stalking this alerted prey, which it is unlikely to catch. When the predator gives up, the signaller can get back to other tasks such as feeding. Once the stalking predator is detected, the signalling prey and receiving predator thus have a mutual interest in terminating the hunt.
Within species, mutual interests increase with kinship. Kinship is central to models of signalling between relatives, for instance when broods of nestling birds beg and compete for food from their parents.
![yellow-banded](https://upload.wikimedia.org/wikipedia/commons/b/b8/Yellow-banded.poison.dart.frog.arp.jpg)
The term honesty in
animal communication
Animal communication is the transfer of information from one or a group of animals (sender or senders) to one or more other animals (receiver or receivers) that affects the current or future behavior of the receivers. Information may be sent int ...
is controversial because in non-technical usage it implies intent, to discriminate deception from honesty in human interactions. However, biologists use the phrase "honest signals" in a direct, statistical sense. Biological signals, like warning calls or resplendent tail feathers, are honest if they ''reliably'' convey useful information to the receiver. That is, the signal trait tells the receiver about an otherwise unobservable factor. Honest biological signals do not need to be perfectly informative, reducing uncertainty to zero; all they need to be useful is to be correct "on average", so that some behavioural response to the signal is advantageous, statistically, compared to the behaviour that would occur in absence of the signal. Ultimately the
value
Value or values may refer to:
Ethics and social
* Value (ethics) wherein said concept may be construed as treating actions themselves as abstract objects, associating value to them
** Values (Western philosophy) expands the notion of value beyo ...
of the signalled information depends on the extent to which it allows the receiver to increase its
fitness.
One class of honest signal is the
aposematic
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 ...
warning signal, generally visual, given by poisonous or dangerous animals such as
wasp
A wasp is any insect of the narrow-waisted suborder Apocrita of the order Hymenoptera which is neither a bee nor an ant; this excludes the broad-waisted sawflies (Symphyta), which look somewhat like wasps, but are in a separate suborder. Th ...
s,
poison dart frog
Poison dart frog (also known as dart-poison frog, poison frog or formerly known as poison arrow frog) is the common name of a group of frogs in the family Dendrobatidae which are native to tropical Central and South America. These species are ...
s, and
pufferfish
Tetraodontidae is a family of primarily marine and estuarine fish of the order Tetraodontiformes. The family includes many familiar species variously called pufferfish, puffers, balloonfish, blowfish, blowies, bubblefish, globefish, swellfis ...
. Warning signals are honest indications of noxious prey, because conspicuousness evolves in tandem with noxiousness (a conspicuous, non-noxious organism gets eaten). Thus, the brighter and more conspicuous the organism, the more toxic it usually is. The most common and effective colours are red, yellow, black and white.
There is an obvious evolutionary advantage to faking a signal like "Don't eat me, I'm poisonous"; the sender scares off predators without the
metabolic cost
In production, research, retail, and accounting, a cost is the value of money that has been used up to produce something or deliver a service, and hence is not available for use anymore. In business, the cost may be one of acquisition, in which ...
of actually being toxic. Such fake I'm-harmful signals are called
Batesian mimicry
Batesian mimicry is a form of mimicry where a harmless species has evolved to imitate the warning signals of a harmful species directed at a predator of them both. It is named after the English naturalist Henry Walter Bates, after his work on bu ...
. There is also an obvious evolutionary advantage to the predator learning to distinguish the real from the fake signal; this sets off an
evolutionary arms race
In evolutionary biology, an evolutionary arms race is an ongoing struggle between competing sets of co-evolving genes, phenotypic and behavioral traits that develop escalating adaptations and counter-adaptations against each other, resembling an a ...
, making such fake signals evolutionarily unstable. An "honest signal", on the other hand, cannot be faked by the sender (the feathers can't look resplendent unless the bird is healthy) or can't be faked without a cost that exceeds the benefit (like incessantly giving warning calls). This unfakability is what makes the signal reliable (and thus "honest"). Hence, "honest" signals are evolutionarily stable.
The mathematical biologist
John Maynard Smith
John Maynard Smith (6 January 1920 – 19 April 2004) was a British theoretical and mathematical evolutionary biologist and geneticist. Originally an aeronautical engineer during the Second World War, he took a second degree in genetics und ...
discusses whether honest signalling must always be costly. He notes that it had been shown that "in some circumstances" a signal is reliable only if it is costly. He states that it had been assumed that parameters such as pay-offs and signalling costs were constant, but that this might be unrealistic. He states that with some restrictions, signals can be cost-free, reliable, and evolutionarily stable. However, if costs and benefits "vary uniformly over the whole range" then indeed honest signals have to be costly.
Dishonest signals
![Uca lactea-male-600-450](https://upload.wikimedia.org/wikipedia/commons/3/31/Uca_lactea-male-600-450.jpg)
Because there are both mutual and conflicting interests in most animal signalling systems, a central problem in signalling theory is dishonesty or
cheating
Cheating generally describes various actions designed to subvert rules in order to obtain unfair advantages. This includes acts of bribery, cronyism and nepotism in any situation where individuals are given preference using inappropriate cr ...
. For example, if foraging birds are safer when they give a warning call, cheats could give false alarms at random, just in case a predator is nearby. But too much cheating could cause the signalling system to collapse. Every dishonest signal weakens the integrity of the signalling system, and so reduces the fitness of the group. An example of dishonest signalling comes from Fiddler crabs such as ''
Uca lactea mjoebergi
''Austruca mjoebergi'' is a species of fiddler crab discovered by and named after the Sweden, Swedish zoology, zoologist Eric Mjöberg (1882–1938), member of a Swedish scientific expedition to Australia in the early 1900s.
The crab is found al ...
'', which have been shown to bluff (no conscious intention being implied) about their fighting ability. When a claw is lost, a crab occasionally regrows a weaker claw that nevertheless intimidates crabs with smaller but stronger claws. The proportion of dishonest signals is low enough for it not to be worthwhile for crabs to test the honesty of every signal through combat.
Richard Dawkins
Richard Dawkins (born 26 March 1941) is a British evolutionary biologist and author. He is an emeritus fellow of New College, Oxford and was Professor for Public Understanding of Science in the University of Oxford from 1995 to 2008. An ath ...
and
John Krebs
John Richard Krebs, Baron Krebs, Kt FRS (born 11 April 1945) is an English zoologist researching in the field of behavioural ecology of birds. He was the principal of Jesus College, Oxford, from 2005 until 2015."Elliott Coues Award, 1999: Si ...
in 1978 considered whether individuals of the same species would act as if attempting to deceive each other. They applied a "
selfish gene" view of evolution to animals'
threat displays
Deimatic behaviour or startle display means any pattern of bluffing behaviour in an animal that lacks strong defences, such as suddenly displaying conspicuous eyespots, to scare off or momentarily distract a predator, thus giving the prey anima ...
to see if it would be in their genes' interests to give dishonest signals. They criticised previous
ethologists
Ethology is the scientific study of animal behaviour, usually with a focus on behaviour under natural conditions, and viewing behaviour as an evolutionarily adaptive trait. Behaviourism as a term also describes the scientific and objectiv ...
, such as
Nikolaas Tinbergen
Nikolaas "Niko" Tinbergen (; ; 15 April 1907 – 21 December 1988) was a Dutch biologist and ornithologist who shared the 1973 Nobel Prize in Physiology or Medicine with Karl von Frisch and Konrad Lorenz for their discoveries concerning the or ...
and
Desmond Morris
Desmond John Morris FLS ''hon. caus.'' (born 24 January 1928) is an English zoologist, ethologist and surrealist painter, as well as a popular author in human sociobiology. He is known for his 1967 book ''The Naked Ape'', and for his televisi ...
, for suggesting that such displays were "
for the good of the species". They argued that such communication ought to be viewed as an
evolutionary arms race
In evolutionary biology, an evolutionary arms race is an ongoing struggle between competing sets of co-evolving genes, phenotypic and behavioral traits that develop escalating adaptations and counter-adaptations against each other, resembling an a ...
in which signallers evolve to become better at manipulating receivers, while receivers evolve to become more resistant to manipulation. The game theoretical model of the
war of attrition
The War of Attrition ( ar, حرب الاستنزاف, Ḥarb al-Istinzāf; he, מלחמת ההתשה, Milhemet haHatashah) involved fighting between Israel and Egypt, Jordan, the Palestine Liberation Organisation (PLO) and their allies from ...
similarly suggests that threat displays ought not to convey any reliable information about intentions.
Sports handicapping metaphor
![Curlin in the Final Turn in the 2008 Stephen Foster Handicap (2972085393)](https://upload.wikimedia.org/wikipedia/commons/b/b7/Curlin_in_the_Final_Turn_in_the_2008_Stephen_Foster_Handicap_%282972085393%29.jpg)
In 1975,
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 ...
proposed a verbal model for how signal costs could constrain
cheating
Cheating generally describes various actions designed to subvert rules in order to obtain unfair advantages. This includes acts of bribery, cronyism and nepotism in any situation where individuals are given preference using inappropriate cr ...
and stabilize an "honest" correlation between observed signals and unobservable qualities, based on an analogy to sports
handicapping
Handicapping, in sport and games, is the practice of assigning advantage through scoring compensation or other advantage given to different contestants to equalize the chances of winning. The word also applies to the various methods by which th ...
systems. He called this idea the
handicap principle
The handicap principle is a hypothesis proposed by the biologist Amotz Zahavi to explain how evolution may lead to "honest" or reliable signalling between animals which have an obvious motivation to bluff or deceive each other. It suggests that ...
. The purpose of a sports handicapping system is to reduce disparities in performance, making the contest more competitive. In a
handicap race, intrinsically faster horses are given heavier weights to carry under their saddles. Similarly, in
amateur golf
Amateur sports are sports in which participants engage largely or entirely without remuneration. The distinction is made between amateur sporting participants and professional sporting participants, who are paid for the time they spend competing ...
, better golfers have fewer strokes subtracted from their raw scores. This creates correlations between the handicap and unhandicapped performance, if the handicaps work as they are supposed to, between the handicap imposed and the corresponding horse's handicapped performance. If nothing was known about two race horses or two amateur golfers except their handicaps, an observer could infer who is most likely to win: the horse with the bigger weight handicap, and the golfer with the smaller stroke handicap. By analogy, if peacock 'tails' (large
tail covert feather
Feathers are epidermal growths that form a distinctive outer covering, or plumage, on both avian (bird) and some non-avian dinosaurs and other archosaurs. They are the most complex integumentary structures found in vertebrates and a premier ...
s) act as a handicapping system, and a peahen knew nothing about two peacocks except the sizes of their tails, she could "infer" that the peacock with the bigger tail has greater unobservable intrinsic quality. Display costs can include extrinsic social costs, in the form of testing and punishment by rivals, as well as intrinsic production costs. Another example given in textbooks is the extinct Irish elk, ''
Megaloceros giganteus
The Irish elk (''Megaloceros giganteus''), also called the giant deer or Irish deer, is an extinct species of deer in the genus '' Megaloceros'' and is one of the largest deer that ever lived. Its range extended across Eurasia
Eurasia ( ...
''. The male Irish elk's enormous
antler
Antlers are extensions of an animal's skull found in members of the Cervidae (deer) family. Antlers are a single structure composed of bone, cartilage, fibrous tissue, skin, nerves, and blood vessels. They are generally found only on male ...
s could perhaps have evolved as displays of ability to overcome handicap, though biologists point out that if the handicap is inherited, its genes ought to be selected against.
![Flickr - lo](https://upload.wikimedia.org/wikipedia/commons/e/e6/Flickr_-_lo.tangelini_-_Tonos_%281%29.jpg)
The essential idea here is intuitive and probably qualifies as
folk wisdom
Folklore is shared by a particular group of people; it encompasses the traditions common to that culture, subculture or group. This includes oral traditions such as tales, legends, proverbs and jokes. They include material culture, ranging fr ...
. It was articulated by
Kurt Vonnegut
Kurt Vonnegut Jr. (November 11, 1922 – April 11, 2007) was an American writer known for his satirical and darkly humorous novels. In a career spanning over 50 years, he published fourteen novels, three short-story collections, five plays, and ...
in his 1961 short story ''
Harrison Bergeron
"Harrison Bergeron" is a satirical dystopian science-fiction short story by American writer Kurt Vonnegut, first published in October 1961. Originally published in ''The Magazine of Fantasy and Science Fiction'', the story was republished in th ...
''. In Vonnegut's futuristic
dystopia
A dystopia (from Ancient Greek δυσ- "bad, hard" and τόπος "place"; alternatively cacotopiaCacotopia (from κακός ''kakos'' "bad") was the term used by Jeremy Bentham in his 1818 Plan of Parliamentary Reform (Works, vol. 3, p. 493). ...
, the Handicapper General uses a variety of handicapping mechanisms to reduce inequalities in performance. A spectator at a ballet comments: "it was easy to see that she was the strongest and most graceful of all dancers, for her handicap bags were as big as those worn by two hundred pound men." Zahavi interpreted this analogy to mean that higher quality peacocks with bigger tails are signalling their ability to "waste" more of some resource by trading it off for a bigger tail. This resonates with
Thorstein Veblen
Thorstein Bunde Veblen (July 30, 1857 – August 3, 1929) was a Norwegian-American economist and sociologist who, during his lifetime, emerged as a well-known critic of capitalism.
In his best-known book, ''The Theory of the Leisure Class'' ...
's idea that
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 ...
and extravagant
status symbol
A status symbol is a visible, external symbol of one's social position, an indicator of economic or social status. Many luxury goods are often considered status symbols. ''Status symbol'' is also a sociological term – as part of social and soci ...
s can signal wealth.
![Irish Elk Side (white background)](https://upload.wikimedia.org/wikipedia/commons/5/5a/Irish_Elk_Side_%28white_background%29.jpg)
Zahavi's conclusions rest on his verbal interpretation of a metaphor, and initially the handicap principle was not well received by evolutionary biologists. However, in 1984, Nur and Hasson used
life history theory
Life history theory is an analytical frameworkVitzthum, V. (2008). Evolutionary models of women's reproductive functioning. ''Annual Review of Anthropology'', ''37'', 53-73 designed to study the diversity of life history strategies used by differen ...
to show how differences in signalling costs, in the form of survival-reproduction tradeoffs, could stabilize a signalling system roughly as Zahavi imagined. Genetic models also suggested this was possible. In 1990 Alan Grafen showed that a handicap-like signalling system was evolutionarily stable if higher quality signallers paid lower marginal survival costs for their signals.
In 1982,
W.D. Hamilton
William Donald Hamilton (1 August 1936 – 7 March 2000) was a British evolutionary biologist, recognised as one of the most significant evolutionary theorists of the 20th century.
Hamilton became known for his theoretical work expounding a ...
proposed a specific but widely applicable handicap mechanism,
parasite
Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson has ...
-mediated
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 ...
. He argued that in the never-ending
co-evolution
In biology, coevolution occurs when two or more species reciprocally affect each other's evolution through the process of natural selection. The term sometimes is used for two traits in the same species affecting each other's evolution, as well ...
ary race between hosts and their parasites, sexually selected signals indicate health. This idea was tested in 1994 in
barn swallow
The barn swallow (''Hirundo rustica'') is the most widespread species of swallow in the world. In fact, it appears to have the largest natural distribution of any of the world's passerines, ranging over 251 million square kilometres globally. ...
s, a species where males have long tail streamers. Møller found that the males with longer tails, and their offspring, did have fewer bloodsucking mites, whereas fostered young did not. The effect was therefore genetic, confirming Hamilton's theory.
Another example is Lozano's hypothesis that
carotenoids
Carotenoids (), also called tetraterpenoids, are yellow, orange, and red organic pigments that are produced by plants and algae, as well as several bacteria, and fungi. Carotenoids give the characteristic color to pumpkins, carrots, parsnips, ...
have dual but mutually incompatible roles in
immune
In biology, immunity is the capability of multicellular organisms to resist harmful microorganisms. Immunity involves both specific and nonspecific components. The nonspecific components act as barriers or eliminators of a wide range of pathogens ...
function and signalling. Given that animals cannot synthesize carotenoids ''de novo'', these must be obtained from food. The hypothesis states that animals with carotenoid-depended sexual signals are demonstrating their ability to "waste" carotenoids on sexual signals at the expense of their immune system.
The handicap principle has proven hard to test empirically, partly because of inconsistent interpretations of Zahavi's metaphor and Grafen's marginal fitness model, and partly because of conflicting empirical results: in some studies individuals with bigger signals seem to pay higher costs, in other studies they seem to be paying lower costs. A possible explanation for the inconsistent empirical results is given in a series of papers by Getty, who shows that Grafen's proof of the handicap principle is based on the critical simplifying assumption that signallers trade off costs for benefits in an additive fashion, the way humans invest money to increase income in the same currency. But the assumption that costs and benefits trade off in an additive fashion is true only on a logarithmic scale; for the survival cost – reproduction benefit tradeoff is assumed to mediate the evolution of sexually selected signals. Fitness depends on producing offspring, which is a multiplicative function of reproductive success given an individual is still alive times the probability of still being alive, given investment in signals.
Costly signalling and Fisherian diploid dynamics
The effort to discover how costs can constrain an "honest" correlation between observable signals and unobservable qualities within signallers is built on strategic models of signalling games, with many simplifying assumptions. These models are most often applied to
sexually selected signalling in
diploid
Ploidy () is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes. Sets of chromosomes refer to the number of maternal and paternal chromosome copies, respectively ...
animals, but they rarely incorporate a fact about diploid sexual reproduction noted by the mathematical biologist
Ronald Fisher
Sir Ronald Aylmer Fisher (17 February 1890 – 29 July 1962) was a British polymath who was active as a mathematician, statistician, biologist, geneticist, and academic. For his work in statistics, he has been described as "a genius who a ...
in the early 20th century: if there are "preference genes" correlated with choosiness in females as well as "signal genes" correlated with display traits in males, choosier females should tend to mate with showier males. Over generations, showier sons should also carry genes associated with choosier daughters, and choosier daughters should also carry genes associated with showier sons. This can cause the evolutionary dynamic known as
Fisherian runaway
Fisherian runaway or runaway selection is a sexual selection mechanism proposed by the mathematical biologist Ronald Fisher in the early 20th century, to account for the evolution of ostentatious male ornamentation by persistent, directional fe ...
, in which males become ever showier.
Russell Lande
Russell Scott Lande (born 1951) is an American evolutionary biologist and ecologist, and an International Chair Professor at Centre for Biodiversity Dynamics at the Norwegian University of Science and Technology (NTNU). He is a fellow of the Roy ...
explored this with a
quantitative genetic model, showing that Fisherian diploid dynamics are sensitive to signalling and search costs. Other models incorporate both costly signalling and Fisherian runaway. These models show that if fitness depends on both survival and reproduction, having
sexy sons and choosy daughters (in the stereotypical model) can be adaptive, increasing fitness just as much as having healthy sons and daughters.
Examples
![Autumn](https://upload.wikimedia.org/wikipedia/commons/9/92/Autumn.westonbirt.750pix.jpg)
Sam Brown and
W. D. Hamilton
William Donald Hamilton (1 August 1936 – 7 March 2000) was a British evolutionary biologist, recognised as one of the most significant evolutionary theorists of the 20th century.
Hamilton became known for his theoretical work expounding a ...
, and Marco Archetti, proposed that
autumn leaf colour
Autumn leaf color is a phenomenon that affects the normal green leaves of many deciduous trees and shrubs by which they take on, during a few weeks in the autumn season, various shades of yellow, orange, red, purple, and brown. The phenomenon i ...
is a signal from trees to
aphid
Aphids are small sap-sucking insects and members of the superfamily Aphidoidea. Common names include greenfly and blackfly, although individuals within a species can vary widely in color. The group includes the fluffy white woolly aphids. A t ...
s and other pest species that migrate in autumn to the trees. In their theory, bright autumn coloration with pinks and yellows is costly to trees because pigments require energy to synthesize, but the investment may help them to reduce their parasite load.
Stotting
Stotting (also called pronking or pronging) is a behavior of quadrupeds, particularly gazelles, in which they spring into the air, lifting all four feet off the ground simultaneously. Usually, the legs are held in a relatively stiff position. Ma ...
, as in
Thomson's gazelle
Thomson's gazelle (''Eudorcas thomsonii'') is one of the best known species of gazelles. It is named after explorer Joseph Thomson and is sometimes referred to as a "tommie". It is considered by some to be a subspecies of the red-fronted gazelle a ...
, is cited as an example of signalling: the gazelles jump close to a predator instead of escaping, in what could be a signal of strength.
Fecal chemical signaling in
Hamiltons Frog
The Hamilton's frog ''(Leiopelma hamiltoni)'' is a primitive frog native to New Zealand, one of only four extant species belonging to the family Leiopelmatidae. New Zealand's frog species all are in the family Leiopelmatidae. The male remains with ...
is an example of honest signaling. The frog's feces signals to others their size and strength warding off predators and other potential competitors.
Human honest signals
Human behaviour may also provide examples of costly signals. In general, these signals provide information about a person's phenotypic quality or cooperative tendencies. Evidence for costly signalling has been found in many areas of human interaction including risk-taking, hunting, and religion.
Costly signalling in hunting
![Kalina hunter gatherer](https://upload.wikimedia.org/wikipedia/commons/c/cc/Kalina_hunter_gatherer.jpg)
Large game hunting has been studied extensively as a signal of men's willingness to take physical risks, as well as showcase strength and coordination. Costly signalling theory is a useful tool for understanding food sharing among
hunter gatherers
A traditional hunter-gatherer or forager is a human living an ancestrally derived lifestyle in which most or all food is obtained by foraging, that is, by gathering food from local sources, especially edible wild plants but also insects, fungi, ...
because it can be applied to situations in which
delayed reciprocity is not a viable explanation. Instances that are particularly inconsistent with the delayed reciprocity hypothesis are those in which a hunter shares his kill indiscriminately with all members of a large group. In these situations, the individuals sharing meat have no control over whether or not their generosity will be reciprocated, and
free riding becomes an attractive strategy for those receiving meat. Free riders are people who reap the benefits of group-living without contributing to its maintenance. Costly signalling theory can fill some of the gaps left by the delayed reciprocity hypothesis. Hawkes has suggested that men target large game and publicly share meat to draw social attention or to show off. Such display and the resulting favorable attention can improve a hunter's reputation by providing information about his phenotypic quality. High quality signallers are more successful in acquiring mates and allies. Thus, costly signalling theory can explain apparently wasteful and
altruistic
Altruism is the principle and moral practice of concern for the welfare and/or happiness of other human beings or animals, resulting in a quality of life both material and spiritual. It is a traditional virtue in many cultures and a core asp ...
behaviour.
In order to be effective, costly signals must fulfill specific criteria. Firstly, signallers must incur different levels of cost and benefit for signalling behaviour. Secondly, costs and benefits must reflect the signallers'
phenotypic
In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology or physical form and structure, its developmental processes, its biochemical and physiological proper ...
quality. Thirdly, the information provided by a signal should be directed at and accessible to an audience. A receiver can be anyone who stands to benefit from information the signaller is sending, such as potential mates, allies, or competitors. Honesty is guaranteed when only individuals of high quality can pay the (high) costs of signalling. Hence,
costly signals make it impossible for low-quality individuals to fake a signal and fool a receiver.
Bliege Bird et al. observed turtle hunting and spear fishing patterns in a
Meriam community in the
Torres Strait
The Torres Strait (), also known as Zenadh Kes, is a strait between Australia and the Melanesian island of New Guinea. It is wide at its narrowest extent. To the south is Cape York Peninsula, the northernmost extremity of the Australian mai ...
of Australia, publishing their findings in 2001. Here, only some Meriam men were able to accumulate high caloric gains for the amount of time spent turtle hunting or spear fishing (reaching a threshold measured in kcal/h). Since a daily catch of fish is carried home by hand and turtles are frequently served at large feasts, members of the community know which men most reliably brought them turtle meat and fish. Thus, turtle hunting qualifies as a costly signal. Furthermore, turtle hunting and spear fishing are actually less productive (in kcal/h) than foraging for shellfish, where success depends only on the amount of time dedicated to searching, so shellfish foraging is a poor signal of skill or strength. This suggests that energetic gains are not the primary reason men take part in turtle hunting and spear fishing. A follow-up study found that successful Meriam hunters do experience greater social benefits and reproductive success than less skilled hunters.
The
Hadza people
The Hadza, or Hadzabe (''Wahadzabe'' in Swahili), are a Tanzanian indigenous ethnic group mostly based in southwest Karatu District of Arusha Region. They live around Lake Eyasi in the central Rift Valley and in the neighboring Serengeti Platea ...
of
Tanzania
Tanzania (; ), officially the United Republic of Tanzania ( sw, Jamhuri ya Muungano wa Tanzania), is a country in East Africa within the African Great Lakes region. It borders Uganda to the north; Kenya to the northeast; Comoro Islands and ...
also share food, possibly to gain in reputation. Hunters cannot be sharing meat mainly to provision their families or to gain reciprocal benefits, as teenage boys often give away their meat even though they do not yet have wives or children, so costly signalling of their qualities is the likely explanation. These qualities include good eyesight, coordination, strength, knowledge, endurance, or bravery. Hadza hunters more often pair with highly fertile, hard-working wives than non-hunters. A woman benefits from mating with a man who possesses such qualities as her children will most likely inherit qualities that increase fitness and survivorship. She may also benefit from her husband's high social status. Thus, hunting is an honest and costly signal of phenotypic quality.
Frank W. Marlowe’s ''The Hadza: Hunter-Gatherers of Tanzania'' showed that this data confirms that this is also true within the Hazda, based on the documentation on the
!Kung, in
Megan Biesele
Megan is a Welsh language, Welsh feminine given name, originally a diminutive form of Margaret. Margaret is from the Greek μαργαρίτης (''margarítēs''), Latin ''margarīta'', "pearl". Megan is one of the most popular Welsh-language name ...
’s book about !Kung folklore called
Women Like Meat.
Among the men of
Ifaluk
Ifalik (also spelled Ifaluk) is a coral atoll of four islands in the central Caroline Islands in the Pacific Ocean, and forms a legislative district in Yap State in the Federated States of Micronesia. Ifalik is located approximately east of Wole ...
atoll, costly signalling theory can also explain why men torch fish. Torch fishing is a ritualized method of fishing on Ifaluk whereby men use torches made from dried coconut fronds to catch large
dog-toothed tuna. Preparation for torch fishing requires significant time investments and involves a great deal of organization. Due to the time and energetic costs of preparation, torch fishing results in net caloric losses for fishers. Therefore, torch fishing is a handicap that serves to signal men's productivity. Torch fishing is the most advertised fishing occupation on Ifaluk. Women and others usually spend time observing the canoes as they sail beyond the reef. Also, local rituals help to broadcast information about which fishers are successful and enhance fishers’ reputations during the torch fishing season. Several ritual behaviors and dietary constraints clearly distinguish torch fishers from other men. First, males are only permitted to torch fish if they participate on the first day of the fishing season. The community is well informed as to who participates on this day, and can easily identify the torch fishers. Second, torch fishers receive all of their meals at the canoe house and are prohibited from eating certain foods. People frequently discuss the qualities of torch fishermen. On Ifaluk, women claim that they are looking for hard-working mates. With the distinct sexual division of labor on Ifaluk, industriousness is a highly valued characteristic in males. Torch fishing thus provides women with reliable information on the work ethic of prospective mates, which makes it an honest costly signal.
In many human cases, a strong reputation built through costly signalling enhances a man's social status over the statuses of men who signal less successfully. Among northern
Kalahari
The Kalahari Desert is a large semi-arid sandy savanna in Southern Africa extending for , covering much of Botswana, and parts of Namibia and South Africa.
It is not to be confused with the Angolan, Namibian, and South African Namib coastal de ...
foraging groups, traditional hunters usually capture a maximum of two or three antelopes per year. It was said of a particularly successful hunter:
:"It was said of him that he never returned from a hunt without having killed at least a wildebeest, if not something larger. Hence the people connected with him ate a great deal of meat and his popularity grew."
Although this hunter was sharing meat, he was not doing so in the framework of reciprocity. The general model of costly signalling is not reciprocal; rather, individuals who share acquire more mates and allies. Costly signalling applies to situations in Kalahari foraging groups where giving often goes to recipients who have little to offer in return. A young hunter is motivated to impress community members with daughters so that he can obtain his first wife. Older hunters may wish to attract women interested in an extramarital relationship, or to be a
co-wife. In these northern Kalahari groups, the killing of a large animal indicates a man who has mastered the art of hunting and can support a family. Many women seek a man who is a good hunter, has an agreeable character, is generous, and has advantageous social ties. Since hunting ability is a prerequisite for marriage, men who are good hunters enter the marriage market earliest. Costly signalling theory explains seemingly wasteful foraging displays.
Physical risks as a costly signal
![Motorcycle racing](https://upload.wikimedia.org/wikipedia/commons/b/b1/Motorcycle_racing.jpg)
Costly signalling can be applied to situations involving physical strain and risk of physical injury or death. Research on physical risk-taking is important because information regarding why people, especially young men, take part in high risk activities can help in the development of prevention programs. Reckless driving is a lethal problem among young men in western societies. A male who takes a physical risk is sending the message that he has enough strength and skill to survive extremely dangerous activities. This signal is directed at peers and potential mates. When those peers are criminals or gang members, sociologists
Diego Gambetta
Diego Gambetta (; born 1952) is an Italian-born social scientist. He is a professor of social theory at the European University Institute in Florence, a Carlo Alberto Chair at the Collegio Carlo Alberto in Turin, and an official fellow at Nuffie ...
and
James Densley
James Densley (born April 13, 1982) is a British-American sociologist and Professor of Criminal Justice at Metropolitan State University. He is best known as co-founder of The Violence Project and as co-author of the bestselling book, ''The Viol ...
find that risk-taking signals can help expedite acceptance into the group.
In a study of risk-taking, some types of risk, such as physical or heroic risk for others' benefit, are viewed more favorably than other types of risk, such as taking drugs. Males and females valued different degrees of heroic risk for mates and same-sex friends. Males valued heroic risk-taking by male friends, but preferred less of it in female mates. Females valued heroic risk-taking in male mates and less of it in female friends. Females may be attracted to males inclined to physically defend them and their children. Males may prefer heroic risk-taking by male friends as they could be good allies.
In western societies, voluntary
blood donation
A blood donation occurs when a person voluntarily has blood drawn and used for blood transfusion, transfusions and/or made into biopharmaceutical medications by a process called Blood fractionation, fractionation (separation of whole blood com ...
is a common, yet less extreme, form of risk-taking. Costs associated with these donations include pain and risk of infection. If blood donation is an opportunity to send costly signals, then donors will be perceived by others as generous and physically healthy. In a survey, both donors and non-donors attributed health, generosity, and ability to operate in stressful situations to blood donors.
Religion as a costly signal
Costly religious rituals such as male and female
circumcision
Circumcision is a surgical procedure, procedure that removes the foreskin from the human penis. In the most common form of the operation, the foreskin is extended with forceps, then a circumcision device may be placed, after which the foreskin ...
, food and water deprivation, and
snake handling look paradoxical in evolutionary terms. Devout religious beliefs wherein such traditions are practiced appear maladaptive. Religion may have arisen to increase and maintain intragroup cooperation. Cooperation leads to altruistic behaviour, and costly signalling could explain this. All religions may involve costly and elaborate rituals, performed publicly, to demonstrate loyalty to the religious group. In this way, group members increase their allegiance to the group by signalling their investment in group interests. However, as group size increases among humans, the threat of free riders grows. Costly signalling theory accounts for this by proposing that these religious rituals are costly enough to deter free riders.
Irons proposed that costly signalling theory could explain costly religious behaviour. He argued that hard-to-fake religious displays enhanced trust and solidarity in a community, producing emotional and economic benefits. He showed that display signals among the
Yomut
The Yomut or Yomud is a Turkmen tribe that lives in Western and Central Asia, including Gorgan, Iran; Turkmenbashi, Turkmenistan; the eastern Caspian shores; Khiva, Uzbekistan; and Dashoguz, Turkmenistan.
The Yomut carpet is a type of rug t ...
Turkmen of northern
Iran
Iran, officially the Islamic Republic of Iran, and also called Persia, is a country located in Western Asia. It is bordered by Iraq and Turkey to the west, by Azerbaijan and Armenia to the northwest, by the Caspian Sea and Turkmeni ...
helped to secure trade agreements. These "ostentatious" displays signalled commitment to Islam to strangers and group members. Sosis demonstrated that people in religious communities are four times more likely to live longer than their secular counterparts, and that these longer lifespans were positively correlated with the number of costly requirements demanded from religious community members. However, confounding variables may not have been excluded. Wood found that religion offers a subjective feeling of well-being within a community, where costly signalling protects against free riders and helps to build self-control among committed members. Iannaccone studied the effects of costly signals on religious communities. In a self-reported survey, as the strictness of a church increased, the attendance and contributions to that church increased proportionally. In effect, people were more willing to participate in a church that has more stringent demands on its members. Despite this observation, costly donations and acts conducted in a religious context does not itself establish that membership in these clubs is actually worth the entry costs imposed.
Despite the experimental support for this hypothesis, it remains controversial. A common critique is that devoutness is easy to fake, such as simply by attending a religious service. However, the hypothesis predicts that people are more likely to join and contribute to a religious group when its rituals are costly. Another critique specifically asks: why religion? There is no evolutionary advantage to evolving religion over other signals of commitment such as nationality, as Irons admits. However, the reinforcement of religious rites as well as the intrinsic reward and punishment system found in religion makes it an ideal candidate for increasing intragroup cooperation. Finally, there is insufficient evidence for increase in fitness as a result of religious cooperation. However, Sosis argues for benefits from religion itself, such as increased longevity, improved health, assistance during crises, and greater psychological well-being, although both the supposed benefits from religion and the costly-signaling mechanism have been contested.
Some scholars view the emergence of language as the consequence of some kind of social transformation
that, by generating unprecedented levels of public trust, liberated a genetic potential for linguistic creativity that had previously lain dormant.
"Ritual/speech coevolution theory" views rituals as costly signals that ensures honesty and reliability of language communication.
Scholars in this intellectual camp point to the fact that even
chimpanzee
The chimpanzee (''Pan troglodytes''), also known as simply the chimp, is a species of great ape native to the forest and savannah of tropical Africa. It has four confirmed subspecies and a fifth proposed subspecies. When its close relative th ...
s and
bonobo
The bonobo (; ''Pan paniscus''), also historically called the pygmy chimpanzee and less often the dwarf chimpanzee or gracile chimpanzee, is an endangered great ape and one of the two species making up the genus '' Pan,'' the other being the comm ...
s have latent symbolic capacities that they rarely—if ever—use in the wild.
Objecting to the sudden mutation idea, these authors argue that even if a chance mutation were to install a language organ in an evolving bipedal primate, it would be adaptively useless under all known primate social conditions. A very specific social structure—one capable of upholding unusually high levels of public accountability and trust—must have evolved before or concurrently with language to make reliance on "cheap signals" (words) an
evolutionarily stable strategy
An evolutionarily stable strategy (ESS) is a strategy (or set of strategies) that is ''impermeable'' when adopted by a population in adaptation to a specific environment, that is to say it cannot be displaced by an alternative strategy (or set of ...
. The animistic nature of early humans language could serve as the handicap-like cost that helped to ensure the reliability of communication. The attribution of spiritual essence to everything surrounding early humans served as a built-in hard-to-fake mechanism that provided instant verification and ensured
the inviolability of one's speech.
See also
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Alarm signal
In animal communication, an alarm signal is an antipredator adaptation in the form of signals emitted by social animals in response to danger. Many primates and birds have elaborate alarm calls for warning conspecifics of approaching predators. ...
*
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 ...
*
Dramaturgy (sociology)
Dramaturgy is a sociological perspective commonly used in micro-sociological accounts of social interaction in everyday life.
The term was first adapted into sociology from the theatre by Erving Goffman, who developed most of the related termin ...
*
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 ...
*
Green-beard effect
The green-beard effect is a thought experiment used in evolutionary biology to explain selective altruism among individuals of a species.
The idea of a green-beard gene was proposed by William D. Hamilton in his articles of 1964, and got the ...
*
Origin of language
The origin of language (spoken and signed, as well as language-related technological systems such as writing), its relationship with human evolution, and its consequences have been subjects of study for centuries. Scholars wishing to study th ...
*
Signalling (economics)
In contract theory, signalling (or signaling; see spelling differences) is the idea that one party (the agent) credibly conveys some information about itself to another party (the principal).
Although signalling theory was initially developed ...
*
Virtue signalling
Virtue signalling is the expression of a moral viewpoint with the intent of communicating good character.
Definition
"Virtue signalling", according to the ''Cambridge Dictionary'', is "an attempt to show other people that you are a good person, ...
*
Zoosemiotics
Zoosemiotics is the semiotic study of the use of signs among animals, more precisely the study of semiosis among animals, i.e. the study of how something comes to function as a sign to some animal. It is the study of animal forms of knowing.
Consi ...
Notes
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Further reading
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External links
Animal behavior online: Deceit
{{DEFAULTSORT:Signalling Theory
Animal communication
Evolutionary biology
Sexual selection