A biological ornament is a structure of an animal that appears to serve a decorative function rather than an ostensible, utilitarian function. Ornaments are used in displays to attract mates. 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. Ornaments are most often observed in males and picking an extravagantly ornamented male benefits females because those “good genes” will be passed on to her offspring, increasing their survival or reproductive fitness. These genes are considered “good” solely due to the fact that it will increase the likelihood that a female will be attracted to the male that carries them. These structures usually serve as sexual cues, which are sensory signals used lead to mating responses. Therefore, ornamental traits are often selected by mate choice.[1]
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There are several evolutionary explanations for the presence of ornaments. Darwin was the first to correctly hypothesize that sexual selection 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.[2] Sexual selection is selection acting on variation among individuals in their ability to obtain access to mating partners.[3] In his book, The Descent of Man, and Selection in Relation to Sex written in 1871, 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.[2]
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 proposed the handicap principle, 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.[4] Potential mates know that the ornament indicates quality because inferior mates could not afford to produce such wastefully extravagant ornaments. 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.[5] 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.
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.[6] 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.[7]
Other birds that exhibit ornamentation include sage grouses and widowbirds. Sage grouse birds gather in a lek, or a special display area, and strut and display their plumage to attract a mate.[8] Whereas, the extraordinary tail feathers of the male long-tailed widowbird are displayed to choosy females while the male flies above his grassland territory.[7]
Biological ornamentation is also seen in the lekking fish, Rutilus rutilus (roach). Male roach develop sexual ornaments (breeding tubercles) during the breeding season.[9] Roach display lek-like spawning behavior, whereby females choose between males, usually choosing the more elaborately ornamented ones.[10]
There are many instances in which decorations appear ornamental but are actually used as other functions. For example, there are some species of spiders that decorate their webs with shimmering ornaments in order to lure prey.[10] Orb-weaver spiders use elaborate, ultraviolet web ornaments to attract ultraviolet-detecting bees that specialize in taking nectar from ultraviolet-detecting 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.
Deer use their antlers as weapons to battle rival males for access to breeding females although they may seem to be used as ornaments to attract females. 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.[11] Besides their use as weapons, antlers are thought to also serve as visual cues to potential rivals.[9] Since the size of the antlers depends on age, nutrition, and genetics; the larger the antlers, the older, better nourished, and genetically superior the body which produced them. [9] With antlers as an index to health, females can also select a visibly superior individual to mate with and perpetuate her genes but the primary purpose of antlers is not to be used as biological ornaments in attracting mates.
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,[12] because only the healthiest peacocks can afford to divert energy and nutrients towards growing expensive and cumbersome plumage,[13] 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.[12]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.[12]
Ornaments that play a role in reproduction develop under the influence of two series of genes.[14] 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.[11] 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.[11] 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.[11] 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.[11] 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.[11]
Sexually selected ornaments of males may impose survival costs but advance success in the competition for mates.[12] 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 affect 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.
Biological ornamentation has been shown to affect parental favoritism in nestlings. This can be observed in a species of water birds known as the coot. Baby 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.[12] In experiments in which ornaments have been physically altered on baby coots, elaborate ornamentation has been proven to be beneficial to young offspring.[12] 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.[12] An under-ornamented coot has a shorter life expectancy than its ornamented siblings, so it’s more beneficial to the parents’ genes being passed on if they choose the ornamented individuals as favorites.
Male animals are typically more elaborately ornamented than females.[15] The classic sexual selection 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.[16] Therefore, sexual selection typically results in male-biased sex differences in secondary sexual characteristics, which are non-reproductive body parts that help distinguish between sexes in a species.[16] Female ornamentation has long been overlooked because of the greater prevalence of elaborate displays in males.[16] However, the circumstances under which females would benefit from honestly signaling their quality are limited.[16] Females are not expected to invest in ornamentation unless the fitness benefits of the ornament exceed those from investing the resources directly into offspring.[16] 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.[16] In empidid dance flies, males frequently provide nuptial gifts and it is usually to only the female that is ornamented.[16] 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.[16]