Noun: a physical characteristic (as the breasts of a female mammal or the breeding plumage of a male bird) that appears in members of one sex at puberty or in seasonal breeders at the breeding season and is not directly concerned with reproduction —called also secondary sexual characteristic (Merriam Webster Dictionary: http://www.merriam-webster.com/dictionary/secondary%20sex%20characteristic)
The role of secondary sexual characteristics
Secondary sexual characteristics advertise the fitness of an individual to a potential mate, and can occur in males, females or both ,usually appearing at puberty. While these attributes do not contribute directly to reproduction they play an important role in mate selection. They can appear as adornements used in displays, or characteristics which play a role in combat and competition. Secondary sexual characterisics can appear in populations as exaggerated traits as a result of mating preferences for individuals who manifest these traits, leading to their magnification in the population over time (Lande, 1981).
The tusk of a narwhal,
the feathers of a peacock, the bright facial coloration of mandrills, coloration in fish, size differentiation between the sexes, male facial hair in humans are all examples of secondary sexual characteristics.
Lande, R. (1981). Models of speciation by sexual selection on polygenic traits. Proceedings of the National Academy of Sciences, 78(6), 3721-3725.
Peacock photo: Adrian Pingstone
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Beard photo: Wen-Yan King (NGO [http://www.medapt.org medapt)]