Sexual prediction has been that females would
Sexual Dimorphism Katie True Abstract: Human subjects were observed to find if sexual dimorphism exists in the form of height, shoulder width, and head circumference. Fifty-one male and sixty-five female subjects were used in the experiment, each with a unique height, shoulder width, and head circumference.
The average shoulder width difference from males to females was five centimeters. The average difference with height between females and males was thirteen centimeters, and in head circumference the average difference was two centimeters.The males were larger in all the cases.
Sexual dimorphism was found to exist strongly in height, males being taller, and slightly in head circumference and shoulder width. Males were only being slightly larger in those areas. Introduction: Sexual dimorphism is the presence of physical differences, besides sex organs, between the sexes (1). These physical differences can be anything from colors and size, to the presence of certain traits. Although sexual dimorphisms are species (and possibly regional) specific, a few general trends exist among most species.
One of these trends is explained using Rensch’s rule, which states that size dimorphism increases as body size increases, with males being the larger sex, but the size dimorphism will decrease when females are the larger sex (2). Although this trend has been recognized, the reasons behind it are still unknown. In our experiment, we examined if sexual dimorphism exists in humans in the form of height, head circumference, and shoulder width.
With the strong presence of sexual dimorphisms in other species, we hypothesized that some form exists in humans as well.The prediction has been that females would be smaller, as in many other species, and as a result, their head circumference, shoulder width, and height would be smaller than males. It was assumed that there will be a correlation between height, head circumference, and shoulder width, as one increases, the other will as well. Methods: We measured the circumference of the heads of both male and female subjects with a measuring tape in centimeters and recorded these measurements. We did the same with the shoulder width of all of the male and female subjects.Then, the known height of each male and female subject was converted from feet and inches to centimeters using the known conversion factor (1 inch = 2. 54 centimeters), and these measurements were recorded as well.
All the resulting measurements were then compiled on a spread sheet using Microsoft Excel and analyzed accordingly using the software. The mean and standard deviation were then calculated for each set of data and graphed to show the results for height, shoulder width, and head circumference. Results: Height: Discussion:We were able to determine whether or not sexual dimorphism exists in humans in the form of height, shoulder width, and head circumference. One-hundred and sixteen subjects were observed and the data collected allowed for the examination of the dimorphic nature of human heights, shoulder widths, and head circumferences. In comparing height, Figure 1 is used to visually represent the average difference between male and female subjects. It was found that the male subjects have a much greater height, proving that sexual dimorphism exists.This dimorphism seems to follow Rensch’s Rule; a large size dimorphism exists with large subjects, when males are the larger sex (2).
This dimorphism could be a result of female’s sexual preference for taller males and male’s preference for shorter females (4). Over time, this preference has turned height into a sexual dimorphism. Further experiments could observe if the height difference between genders has increased over many generations, as the sexual preference is accounted into evolution. In comparing head circumference, Figure 2 is used to visually represent the average difference between male and female subjects.
It was found that male subjects have a slightly greater head circumference, but this difference is not great enough to concretely say that a sexual dimorphism exists. This also goes along with Figure 3, which compared the average difference between male and female subjects with shoulder width. In Figure 3, there was also not a great enough difference to concretely say that a sexual dimorphism exists. The small difference could be a result of sampling error, resulting from the small sample size, which could be confirmed by increasing the sample size.If the difference is proven to be consistent with a larger size, then a slight sexual dimorphism could exist. This could be a result of the fact that head circumference and even shoulder width is not as big of a sexual preference for mating selection (5).
This experiment outlines the fact that a definite sexual dimorphism in humans exists with respect to height, and a slight one exists in head circumference and shoulder width. Further experiments should be conducted on other morphological features as well, such as: weight, foot size, nose size, bust size, ect, to further explore the existence of sexual dimorphism in humans.These dimorphisms exist mostly due to sexual preference as opposed to sexual fitness, because of the fact that humans select mates mainly on appearance (4).
These preferences change over time due to the media and other factors. If these preferences can change over time for humans, what about other species? This experiment only scratches the surface of the existence and importance of sexual dimorphisms. References: 1.
Futuyma, Douglas J. 2005. Evolution. Mahwah: Sinauer Associates, 2005. 2. Dale, James.
2007. “Sexual Selection Explains Rensch’s Rule of Allometry for Sexual Dimorphism. Biological Sciences 274, vol. 1628 3. “Principles of Biology I Lab Manual”, Spring 2010, Department of Biology, Georgia College and State University 4. Pawlowski, Boguslaw, and Grazyna Jasienska. 2005.
“Women’s preferences for sexual dimorphism in height depend on menstrual cycle phase and expected duration of relationship. ” Biological Psychology 70, col. 1: 38-43. 5. Willmore, Katherine E. , Charles C. Roseman, Jeffrey Rogers, Joan T.
Richtsmeier, and James M. Cheverud. 2009. “Genetic Variation in Baboon Craniofacial Sexual Dimorphism.
” Evolution 63, vol. 3: 799-806.