Biological ornament
A biological ornament is a characteristic of an animal that appears to serve a decorative function rather than a utilitarian function. Many are
Sexual selection
There are several evolutionary explanations for the presence of ornaments. Darwin was the first to correctly hypothesize that
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,
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.[8] 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.
Honest signalling
Females may improve survival of offspring by selecting mates on the basis of ornamentation signals that honestly reveal health. Numerous studies have been carried out to test if sexual selection based on the intensity of the expression of ornamentation in males reflects their level of oxidative stress.[9][10] It is considered that female choice may select for traits in males that reliably indicate level of oxidative stress, as such traits would be a good indicator of male quality[9][10] Elevated oxidative stress can lead to increased DNA damage that can contribute to aging or cancer. Female choice thus may promote the evolution of ornaments in males that reliably reveal the level of oxidative stress in potential mating partners.
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.[11] 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.[12]
Other birds that exhibit ornamentation include
Biological ornamentation is also seen in the common roach fish, Rutilus rutilus. Male roach develop sexual ornaments (breeding tubercles) during the breeding season.[14] 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.[15] Orb-weaver spiders 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.[16] A buck in peak physical health will shed his antlers later than a weaker buck.[17] 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.[18]
Use in courtship displays
Biological ornaments are used in courtship displays in many species, especially insects, fish, and birds. A well known ornament used in courting displays is seen in peafowls. 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,[19] because only the healthiest peacocks can afford to divert energy and nutrients towards growing expensive and cumbersome plumage,[20] 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.[19] 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.[19]
Ornaments that play a role in reproduction develop under the influence of two series of genes.[21] 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.[18] Important studies concerning this have been conducted in the Stalk-eyed fly, 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.[18] 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.[18] 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.[18] 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.[18]
Sexually selected ornaments of males may impose survival costs but advance success in the competition for mates.[19] 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.[1] 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.[22] For example, baby American coots 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.[19] In experiments in which ornaments have been physically altered on baby coots, elaborate ornamentation has been proven to be beneficial to young offspring.[19] 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.[19]
Female ornamentation
Male animals are typically more elaborately ornamented than females.
Female ornamentation has long been overlooked because of the greater prevalence of elaborate displays in males.
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.[24] In empidid dance flies, males frequently provide nuptial gifts and it is usually to only the female that is ornamented.[24] 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.[24]
References and notes
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- ^ a b Darwin, C., ed. (1871). The Descent of Man. Prometheus Books.
- ^ Kappeler, P., ed. (2010). Animal Behavior: Evolution and Mechanisms. Springer.
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- ^ a b Alcock, J., ed. (2005). Animal Behavior (8th ed.). Sinauer Associates. [page needed]
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- ^ Taskinen, J. & R. Kortet. (2002). "Dead and alive parasites: sexual ornaments signal resistance in the male fish, Rutilus rutilus". Evolutionary Ecology Research. 4: 919–929.
- .
- ^ ) Stearns, Stephen C., and Rolf F. Hoekstra. Evolution: an Introduction. Oxford University Press, 2005.
- ^ Robb, Bob. “Why Do Deer Shed Their Antlers?” Grand View Outdoors, 4 June 2015, www.grandviewoutdoors.com/big-game-hunting/why-do-deer-shed-their-antlers/.
- ^ a b c d e f Goss, R.J., ed. (1983). Deer antlers: Regeneration, function, and evolution. Academic Press.
- ^ ISBN 9780878930098.
- ^ Pinker, S., ed. (2002). Animal Behavior: an Evolutionary Approach. Penguin. [dubious ]
- ^ Panafieu, J., ed. (2007). Evolution. Seven Stories.
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- ^ PMID 14561280.
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