Anthropological Research On Sexual Dimorphism

In the study of sexual dimorphism, various body parts of college students were measured and compared to draw conclusions about human variation and sexual dimorphism (size/shape differences).

There is a larger degree of human variation evident in some traits than others, which may be accounted for by phenotypic plasticity of the genotype in question. For example, weight and height have the greatest ranges within the categories of males and females. This can be attributed genetic variation between individuals due to decreased evolutionary pressure toward any specific height. In other words, there is a wide range of heights which are compatible with good reproductive success of an individual. The evolutionary history of humans shows an overall decreased dependence on physical size and strength for survival and reproductive success. There is an evolutionary trend toward more settled living and less strenuous activity required to gain food and mates than people during the environment of evolutionary adaptiveness. Therefore, an individual does not need to be tall, strong, etc. to have good reproductive success, so the range (variation) of heights between men and women is large.

Weight also displays a large range, showing human variation due to differences in diet, exercise, and the genetics of metabolism. Again, a human's reproductive success is not directly linked to his or her weight so variation in this trait is allowed and observed. Weight is limited on the upper and lower extremes by the body's ability to sustain itself upon the excess fat (energy) and nutrition stored in the body. No extremely low weights are observed because the body could not function properly if severely underweight (lacking energy stores), and no outrageously high weights are observed because the body would experience great difficulty supporting excess fat stores and pounds, and modern society discourages obesity.

The range and standard deviations of the eye orbit to eye orbit measure-ments for both men and women show little variation, which can be attributed to binocular vision in humans. Eyes must be in the front of the face for proper focusing, and the distance between them must be maintained in order to support binocular vision. If eyes were too far apart or too close, vision would be detrimentally affected, so the distance from orbit to orbit is very close between men and women. Low ranges and standards of deviation were also seen in the length of nose, which could be attributed to sexual selection pressures. It is possible that throughout evolution, large noses have been perceived as unattractive by the opposite sex, so their size is limited. Restricted nose size is allowable because humans have a very low dependence on smell, which is compensated by a high dependence on sight.



Sexual dimorphism manifests itself in humans by differences in overall body size traits, compared to the larger degree of sexual dimorphism displayed by other primates, especially chimpanzees and bonobos. Less sexual dimorphism often indicates a species with mutual care of the young and monogamy, such as birds, whereas greater sexual dimorphism is often demonstrated by species which practice polygamy and have sharply defined gender roles regarding the rearing of offspring.

The measurements relating to cranium size, which include mandibular breadth, eye orbit to eye orbit, chin to top of skull, and chin to ear aperture show a modest amount of sexual dimorphism. Comparing the measure-ments of cranium size between men and women shows little difference, indicating that cranial capacity is approximately equal between the sexes. This can be viewed in terms of a type of co-evolution within the brain, because as the brain gets bigger (with more folds in the cerebral cortex and development of more advanced regions), there must be a simultaneous compensatory increase in the capacity for blood flow to these new regions. Therefore, the increase in brain size and development is the same for males and females because they both are restricted in their growth rates by nutrient supply.

Males do show slightly greater size of cranial bones and proportionally greater length of hands and feet, which demonstrates sexual dimorphism corresponding to their overall larger body size. The greatest sexual dimorphism is shown in the measurements for height and weight due to the widely practiced gender roles throughout evolution. Greater physical requirements have been placed on men since the age of evolutionary adaptiveness,including hunting and male-male competition. Historically, females reared the children and performed less manual labor, so their size was not pressured by natural selection to increase. Biological factors also explain sexual dimorphism, since males produce hormones (especially testosterone) which contribute to greater muscle mass and growth than the hormones (i.e.: estrogen) produced by females. Thus, sexual dimorphism is shown to some extent in humans, though significantly less than other primates.

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