Sexual Dimorphism
Sexual dimorphism involves the differentiation of phenotypic traits between genders. This can include anything from differing reproductive systems and organs to size, color or behavior. Many dimorphic traits are the result of sexual selection. Thus, the appearance of these traits depend heavily on the reproductive system that a species participates in. For example, inter-male competition for mating rights, and promiscuous females cause in higher sperm competition, resulting in the production of small sperm that can be "mass produced" (3). This competition is especially evident in chimpanzees, who have much larger testicles in comparison to body size than either gorillas or orangutans. Since chimpanzee females mate with multiple males during estrus, unlike female gorillas and orangutans, selection favored males who could produce the most sperm and out-compete their rivals (2).
Sexual dimorphism in humans
Unfortunately, as humans participate in a wide array of breeding behaviors, it is not quite as easy to explain our dimorphisms. We can, however, examine the physiological differences between genders and their side-effects, and use this information to make inferences. Sex hormones are a perfect target for such an approach, as their differences between genders are large and the consequences of this are apparent.
The two major sex-hormones present in both genders are testosterone and estrogen. Both are important for males and females, but are produced at much higher levels in one gender. Testosterone is made by the testes in males, the ovaries in females, and in small amounts in the adrenal glands of both genders. Testosterone is responsible for secondary sex characteristics and sperm production in men, maintaining muscle and producing several psychological responses such as libido (4). Estrogen is important in both men and women for bone development. In women it is necessary for ovulation, pregnancy, menstruation, the development of secondary sexual characteristics (1). These hormonal differences are responsible for a huge proportion of the sexual dimorphism in humans, but they also have a number of not so obvious effects on health.
The two major sex-hormones present in both genders are testosterone and estrogen. Both are important for males and females, but are produced at much higher levels in one gender. Testosterone is made by the testes in males, the ovaries in females, and in small amounts in the adrenal glands of both genders. Testosterone is responsible for secondary sex characteristics and sperm production in men, maintaining muscle and producing several psychological responses such as libido (4). Estrogen is important in both men and women for bone development. In women it is necessary for ovulation, pregnancy, menstruation, the development of secondary sexual characteristics (1). These hormonal differences are responsible for a huge proportion of the sexual dimorphism in humans, but they also have a number of not so obvious effects on health.
Works Cited
1. "Estrogen." Wikipedia 2015. Web. 27 Apr 2015.
2. Harcourt, A. H., et al. "Testis Weight, Body Weight and Breeding System in Primates." Nature 293 (1981): 55-57. Web.
3. "Sexual Dimorphism." Wikipedia 2015. Web. 27 Apr 2015.
4. "Testosterone." Wikipedia 2015. Web. 27 Apr 2015.
Images:
Banner: http://www.svihura.com/albums/Tanzania2006/Ndutu%20Day%204/slides/CANON_2006.02.13-0082.html
1. http://pubpages.unh.edu/~jel/512/sexes/primate_sexuality.html
2. Harcourt, A. H., et al. "Testis Weight, Body Weight and Breeding System in Primates." Nature 293 (1981): 55-57. Web.
3. "Sexual Dimorphism." Wikipedia 2015. Web. 27 Apr 2015.
4. "Testosterone." Wikipedia 2015. Web. 27 Apr 2015.
Images:
Banner: http://www.svihura.com/albums/Tanzania2006/Ndutu%20Day%204/slides/CANON_2006.02.13-0082.html
1. http://pubpages.unh.edu/~jel/512/sexes/primate_sexuality.html