Biological ornament
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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 characteristic Secondary sex characteristics are features that appear during puberty in humans, and at sexual maturity in other animals. These characteristics are particularly evident in the sexual dimorphism, sexually dimorphic phenotypic traits that distinguis ...
s, and others appear on young birds during the period when they are dependent on being fed by their parents. Ornaments are used in displays to attract
mates Mates is an English surname, and may refer to: * Mates (born 1964), British newsreader and journalist * Michael Mates (born 1934), British politician * Frederick S. Mates, founded the Mates Investment Fund in 1967 that crashed in the bear market o ...
, which may lead to the evolutionary process known as
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 ...
. An animal may shake, lengthen, or spread out its ornament in order to get the attention of the opposite sex, which will in turn choose the most attractive one with which to
mate Mate may refer to: Science * Mate, one of a pair of animals involved in: ** Mate choice, intersexual selection ** Mating * Multi-antimicrobial extrusion protein, or MATE, an efflux transporter family of proteins Person or title * Friendship ...
. Ornaments are most often observed in males, and choosing an extravagantly ornamented male benefits females as the
gene In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a ba ...
s that produce the ornament will be passed on to her
offspring In biology, offspring are the young creation of living organisms, produced either by a single organism or, in the case of sexual reproduction, two organisms. Collective offspring may be known as a brood or progeny in a more general way. This ca ...
, increasing their own reproductive fitness. As
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 ...
noted, the male offspring will inherit the ornament while the female offspring will inherit the preference for said ornament, which can lead to a
positive feedback Positive feedback (exacerbating feedback, self-reinforcing feedback) is a process that occurs in a feedback loop which exacerbates the effects of a small disturbance. That is, the effects of a perturbation on a system include an increase in the ...
loop known as a
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 fem ...
. These structures serve as cues to
animal sexual behaviour Animal sexual behaviour takes many different forms, including within the same species. Common mating or Reproduction, reproductively motivated systems include Monogamous pairing in animals, monogamy, Polygyny in animals, polygyny, Polyandry ...
, that is, they are sensory signals that affect mating responses. Therefore, ornamental traits are often selected by mate choice.


Sexual selection

There are several evolutionary explanations for the presence of ornaments. Darwin was the first to correctly hypothesize that
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 ...
by female choice was responsible for the evolution of elaborate plumage and remarkable displays in male birds such as the quetzal and the
sage grouse Sage-grouse are grouse belonging to the bird genus ''Centrocercus.'' The genus includes two species: the Gunnison grouse (''Centrocercus minimus'') and the greater sage-grouse (''Centrocercus urophasianus''). These birds are distributed throug ...
. Sexual selection is selection acting on variation among individuals in their ability to obtain access to mating partners. In his 1871 book '' The Descent of Man, and Selection in Relation to Sex'', Darwin was perplexed by the elaborate ornamentation that males of some species have because they appeared to be detrimental to survival and have negative consequences for reproductive success. Darwin proposed two explanations for the existence of such traits: these traits are useful in male-male combat or they are preferred by females. More recently, many alternative theories of sexual selection have been proposed, many of them centered around the idea that elaborate male ornaments allow females to assess the 'quality' of a male's genes so that she can ensure that her offspring get the best genes (health, physical vigor, etc.). 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 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 ...
, which is the idea that elaborate male ornaments are actually a handicap and that males with such ornaments are demonstrating their physical quality by showing that they can survive despite having such a handicap. Potential mates know that the ornament indicates quality because inferior mates could not afford to produce such wastefully extravagant ornaments. More specifically, ornaments may indicate the underlying genetic quality of the male, for example, in peafowls their tail size and symmetry is largely dictated by genetics. In other words, each peafowl grows the best tail they are able to and only those with the highest genetic quality can produce the most impressive tails. The tail of a peafowl is an
honest signal Within evolutionary biology, signalling theory is a body of theoretical work examining communication between individuals, both within species and across species. The central question is when organisms with conflicting interests, such as in sex ...
for the female in determining the health status of a potential mate. In 1982, William Hamilton and Merlene Zuk proposed that male ornaments may enable healthy males to advertise the fact that they are free of diseases and parasites, a theory that is now known as the "Bright Male" hypothesis. According to this hypothesis, if an animal was diseased, it would not be able to grow such beautifully colored plumage. Since disease is a major source of juvenile mortality, females would choose the males with the most elaborate ornaments to ensure that they will have healthy offspring.


Examples

Ornamentation is a common biological trait seen in birds. The male quetzal has elaborate ornamentation to aid in mating. Male quetzals have iridescent green wing coverts, back, chest and head, and a red belly. During mating season, male quetzals grow twin tail feathers that form an amazing train up to three feet long (one meter) with vibrant colors. Most female quetzals have no ornamentation and are drab. Coloration and tail feather length in quetzals help determine mate choice because the females choose the more elaborately ornamented males. Other birds that exhibit ornamentation include
sage grouse Sage-grouse are grouse belonging to the bird genus ''Centrocercus.'' The genus includes two species: the Gunnison grouse (''Centrocercus minimus'') and the greater sage-grouse (''Centrocercus urophasianus''). These birds are distributed throug ...
s and
widowbird ''Euplectes'' is a genus of passerine bird in the weaver family, Ploceidae. It contains the bishops and widowbirds. They are all native to Africa south of the Sahara. It is believed that all birds in the genus are probably polygynous. The gen ...
s. Sage grouse birds gather in a
lek Lek or LEK may refer to: * Lek mating, mating in a lek, a type of animal territory in which males of a species gather * Albanian lek, the currency of Albania * Lek (magazine), a Norwegian softcore pornographic magazine * Lek (pharmaceutical comp ...
, or a special display area, and strut and display their plumage to attract a mate. Whereas, the extraordinary tail feathers of the male
long-tailed widowbird The long-tailed widowbird (''Euplectes progne''), also known as the "sakabula", is a species of bird in the family Ploceidae.Mackworth-Praed, C.W., and C.H. Grant. (1960). ''Birds of Eastern and North Eastern Africa''. Longmans, Green and Co LTD. ...
are displayed to choosy females while the male flies above his grassland territory. Biological ornamentation is also seen in the '' common roach'' fish, ''Rutilus rutilus''. Male roach develop sexual ornaments (breeding tubercles) during the breeding season. Roach display lek-like spawning behavior, whereby females choose between males, usually choosing the more elaborately ornamented ones.


Alternate functions


Lures

There are many instances in which decorations may appear ornamental but actually serve other functions. For example, some species of spiders decorate their webs with shimmering ornaments in order to lure prey.
Orb-weaver spider Orb-weaver spiders are members of the spider family Araneidae. They are the most common group of builders of spiral wheel-shaped webs often found in gardens, fields, and forests. The English word "orb" can mean "circular", hence the English name ...
s use elaborate, ultraviolet coloured web ornaments to attract bees that specialize in taking nectar from similarly coloured flowers. In turn, the spider captures the bee in its nest and reaps the food benefits. In this case, what may seem as an ornament to attract mates is actually used as a lure to trap food.


Armaments

Armaments are anatomical weapons which have evolved amongst species whose males compete intra-sexually for access to females. Armaments are used in direct contests for the opportunity to mate or for the resources needed to attract mates. These weapons such as tusks, antlers, horns, spurs, and lips increase success in rivalry among competitors to gain or maintain dominance, control a harem, or obtain access to territories Examples of animals that use armaments to compete in battle against rival males include deer and antelope; scarabid, lucanid, and cerambycid beetles; certain fish, and narwhales. A buck in peak physical health will shed his antlers later than a weaker buck. Antlers harden just before the breeding season and drop off afterward, and they only occur in males (except in caribou). Antlers are used extensively for fighting and ritualized antler to antler shoving matches.


Use in courtship displays

Biological ornaments are used in
courtship display 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 in many species, especially insects, fish, and birds. A well known ornament used in courting displays is seen in
peafowl Peafowl is a common name for three bird species in the genera '' 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 peafowl are refe ...
s. Male peacocks spread and shake their tails to attract and impress potential mates. Peahens choose the peacocks with the largest number of eyespots on their tails, because only the healthiest peacocks can afford to divert energy and nutrients towards growing expensive and cumbersome plumage, as explained by the handicap principle. More elaborate ornamentation increases the likelihood that a male will mate and has been shown to affect survival of their offspring. The offspring of males with larger eyespots on their ornamented tails have been shown to weigh more and were more likely to be alive after 2 years than the progeny of males with fewer eyespots. Ornaments that play a role in reproduction develop under the influence of two series of genes. First, it develops from genes in males that determine the presence and characteristics of the ornament, and second it develops from genes in females that draw her to this kind of ornamentation. Important studies concerning this have been conducted in the
Stalk-eyed fly Stalk-eyed flies are insects of the fly family Diopsidae. The family is distinguished from most other flies by most members of the family possessing "eyestalks": projections from the sides of the head with the eyes at the end. Some fly species fr ...
, showing that females are attracted to mates that share characteristics with their fathers. Therefore, sexual selection is a mechanism that differently affects both sexes. Initially, an ornament may have been selected for reasons not linked to reproduction, but over time, the characteristic may become exaggerated due to sexual selection. The females will select for more and more elaborate ornamentation, which represents better survival skills because the male with those characteristics must be physically fit enough to handle the unwanted predator attention that comes with the ornament. Therefore, the males with the most extreme ornamentation will have more offspring, and the gene for "showiness" will be passed on. This evolution can then lead to organs of excessive size that may become troublesome for the males, such as large, bushy tails, bright feathers, etc. The point of equilibrium is reached when their ornamentation becomes too much of a handicap on the male's survival, and the "vital" natural selection goes to work, altering the exaggerated characteristic until it reaches an equilibrium point. Sexually selected ornaments of males may impose survival costs but advance success in the competition for mates. The interesting thing about sexual ornaments is that they impede the male's chances for survival, yet they continue to be passed on from generation to generation. The larger the male peacock's tail feathers are, or the brighter the birds feathers are, the harder it is for them to escape predators and maneuver through trees, and the more food they will need to eat to develop the ornament. A peacock's tail almost certainly reduces survival of the peacock as they reduce maneuverability, power of flight, and make the bird more conspicuous to predators. Ornaments, therefore, have a great effect on the fitness of the animals that carry them, but the benefits of having an ornament must outweigh the costs for them to be passed on.


Parental favoritism in nestlings

Biological ornamentation has been shown to affect parental favoritism in nestlings. This can be observed several species of water birds. For example, baby
American coot The American coot (''Fulica americana''), also known as a mud hen or pouldeau, is a bird of the family Rallidae. Though commonly mistaken for ducks, American coots are only distantly related to ducks, belonging to a separate order. Unlike the w ...
s hatch out with long, orange-tipped plumes on their backs and throats which provide signals to parents used to determine which individuals to feed preferentially. In experiments in which ornaments have been physically altered on baby coots, elaborate ornamentation has been proven to be beneficial to young offspring. Ornamented individuals received more frequent feedings from parents. Therefore, the relative growth rates of ornamented chicks were much higher compared with the experimentally altered chicks.


Female ornamentation

Male animals are typically more elaborately ornamented than females. The classic
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 ...
theory notes that because sperm are cheaper to produce than eggs, and because males generally compete more intensely for reproductive opportunities and invest less in parental care than females, males can obtain greater fitness benefits from mating multiple times. Therefore, sexual selection typically results in male-biased sex differences in
secondary sexual characteristics Secondary sex characteristics are features that appear during puberty in humans, and at sexual maturity in other animals. These characteristics are particularly evident in the sexually dimorphic phenotypic traits that distinguish the sexes of a sp ...
, which are non-reproductive body parts that help distinguish between sexes in a species. Female ornamentation has long been overlooked because of the greater prevalence of elaborate displays in males. However, the circumstances under which females would benefit from honestly signaling their quality are limited. Females are not expected to invest in ornamentation unless the fitness benefits of the ornament exceed those from investing the resources directly into offspring. An example of a scenario in which female ornamentation occurs is in the
pipefish Pipefishes or pipe-fishes (Syngnathinae) are a subfamily of small fishes, which, together with the seahorses and seadragons (''Phycodurus'' and ''Phyllopteryx''), form the family Syngnathidae. Description Pipefish look like straight-bodied seah ...
. Within this species of fish, sex roles are reversed so the male is responsible for the postzygotic care. Accordingly, females must compete for access to male parental care. Thus, females have been selected for ornamentation as the benefit of producing the ornamentation, namely male paternal care, outweighs the costs, such as the energy requirements. Most notably, their venter becomes more colourful during breeding season. Males tend to prefer females with more colourful venters, with research showing that there is a positive association between this ornament and the condition of the female. As this ornamentation reliably indicates mate quality, it can be considered an
honest signal Within evolutionary biology, signalling theory is a body of theoretical work examining communication between individuals, both within species and across species. The central question is when organisms with conflicting interests, such as in sex ...
. In the
phalaropes __NOTOC__ A phalarope is any of three living species of slender-necked shorebirds in the genus ''Phalaropus'' of the bird family Scolopacidae. Phalaropes are close relatives of the shanks and tattlers, the '' Actitis'' and Terek sandpipers, ...
and the
Eurasian dotterel The Eurasian dotterel (''Charadrius morinellus''), also known in Europe as just dotterel, is a small wader in the plover family of birds. The dotterel is a brown and black streaked bird with a broad white eye-stripe and an orange-red chest ban ...
, females are more brightly colored than males, and it is the males who are primarily responsible for parental care. It has been proposed that when females gain direct benefits from mating, females may instead be selected for ornamentation that deceives males about their reproductive state. In empidid dance flies, males frequently provide
nuptial gift A nuptial gift is a nutritional gift given by one partner in some animals' sexual reproduction practices. Formally, a nuptial gift is a material presentation to a recipient by a donor during or in relation to sexual intercourse that is not simpl ...
s and it is usually to only the female that is ornamented. Female traits in empidids, such as abdominal sacs and enlarged pinnate leg scales, have been suggested to 'deceive' males into matings by disguising egg maturity.


References and notes

{{Reflist Animal sexuality Mating