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 secondary sexual characteristics, 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, which may lead to the evolutionary process known as sexual selection. 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. Ornaments are most often observed in males, and choosing an extravagantly ornamented male benefits females as the genes that produce the ornament will be passed on to her offspring, increasing their own reproductive fitness. As Ronald Fisher 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 loop known as a Fisherian runaway. These structures serve as cues to animal sexual behavior, that is, they are sensory signals that affect mating responses. Therefore, ornamental traits are often selected by mate choice.

There are several evolutionary explanations for 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 quetzals and the sage grouse (Centrocercus sp.). 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 that they are preferred by females.

More recently, alternative theories of sexual selection have been proposed, many of them focused on how male ornaments allow females of the species to assess the "quality" of the male's genes, such that she can ensure that her offspring get the best genes (and thereby health, physical vigor, and other qualities). In 1975, Amotz Zahavi proposed the handicap principle, which states that elaborate male ornaments are actually a handicap, and that males with such ornaments are demonstrating their physical fitness 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 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 ornaments like 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 have healthy offspring.

Females may improve survival of their 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. 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 Elevated oxidative stress can lead to increased DNA damage that can contribute to aging or cancer. Female choice may thus promote the evolution of ornaments in males that reliably reveal the level of oxidative stress in potential mating partners.

Ornamentation is a common biological trait seen in birds. The male quetzals has elaborate ornamentation to aid in mating. Male resplendent quetzal (Pharomachrus mocinno) 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. Conversely, female quetzals have no ornamentation and are drably colored. 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 grouses (Centrocercus sp.) and widowbirds (Euplectes sp.). Sage grouse gather in a lek, or a special display area, and strut and display their plumage to attract a mate. The extraordinary tail feathers of the male long-tailed widowbird (Euplectes progne) 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 fish 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.