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Biological ornament
A biological ornament is a secondary sexual characteristic of an animal that appears to serve a decorative function rather than an ostensible, utilitarian function. Ornaments are used in displays to attract mates in a 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 in order to sexually reproduce. Ornaments are most often observed in males and choosing an extravagantly ornamented male benefits females because the ornament displays “good genes” 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 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.
==Sexual selection==
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. Sexual selection is selection acting on variation among individuals in their ability to obtain access to mating partners. 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.〔
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. 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. 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.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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