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Sexual Diamorphism

Birds of Paradise
Birds of Paradise


Sexual Dimorphism

Sexual Dimorphism is the physical difference between males and females of the same species. A popular example is the bright coloring of skin and feathers known as plumage, on males in several bird species.  

Sexual Dimorphism can be linked to parental investment.  Parental investment increases the reproductive success of the offspring receiving it (Freeman, 2007). The more involved parent tends to be more plain in appearance because it needs camoflauge to protect itself and its offspring. The parent with brighter colors or a "showier" appearance not only attracts mates, it can act as a divergence or distraction. In 90 percent of mammals, females provide substantial parental care while males provide little or none. 

This "showy" plumage can have a significant effect on extinction rate for passerids. McLain, Moulton, and Redfearn (1995) actually found that in their study of extinction rates for birds that dimorphic species may have as much as a 59% extinction rate, compared to a 23% extinction rate for monomorphic species. However, Doherty, Surci, Royle, Hines, Nichols, and Boulinier (2003) found that these increased extinction rates could be balanced by the promotion of speciation caused by sexual selection related to mating preferences for these sexually dimorphic traits. 

Sexual dimorphism can be related to condition dependant ornaments. Plumage and other sexually dimorphic traits can be honest indicators of mate quality and condition. It takes energy and time to grow and display some of these traits. According to Chandler, Ofria, and Dworkin (2012) females stand a better chance of mating with a high quality male if they have a preference for males that are ornamental.  


Sources:

Chandler, C., Ofria, C., Dworkin, I. (2012). Runaway sexual selection leads to good genes. Evolution, 67(1), 110-119.

Doherty, P., Sorci, G., Royle, J., Hines, J., Nichols, J., Boulinier, T. (2003). Sexual selection affects local extinction and turnover in bird communities. Proceedings of the National Academy of Sciences of the United States of America, 100(10), 5858-5862.

Freeman, S. & Herron, J. (2007). Evolutionary Analysis 4th Ed. Sexual Selection, pp 401-441. 

McLain, D., Moulton, M., Redfearn, T. (1995). Sexual selection and the risk of extinction of introduced birds on oceanic islands. OIKOS, 74, 27-34.

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