The cost of secondary sexual characters and the evolution of cost‐reducing traits

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Abstract
Secondary sexual traits are characterized by their exaggerated expression relative to homologous nonsexual characters in other species. All models of sexual selection assume that sex traits are costly to produce and maintain, and individuals with reduced costs of production and maintenance of secondary sexual characters would be at a selective advantage. A number of morphological, physiological and behavioural traits may have evolved as a result of their cost‐reducing properties: (1) body size, which does not change throughout life, that allows certain individuals to develop exaggerated sex traits, (2) cost‐reducing traits, such as muscle size, that improve with practice and (3) actual cost‐reducing traits, such as wing size in birds with song flight, which are produced in advance of or simultaneously with the sex trait. Cost‐reducing traits may coevolve with secondary sexual characters and allow more extreme sexual signalling than would otherwise have been possible in their absence or in reduced versions.