According to Webster’s dictionary, sexual dimorphism is “the condition in which differences in structure exist between males and females of the same species”. The idea of sexual dimorphism has been around for centuries. Aristotle noted differences in the sexes in his book “The History of Animals,” as did R.A. Fisher in his book “The Genetical theory of Natural Selection”. Perhaps the one of the greatest views on sexual dimorphism was proposed by Charles Darwin in his book, “The Descent of Man and Selection in Relation to Sex”. Darwin thought that sexual dimorphism arose from sexual selection. He states, “sexual selection depends on the success of certain individuals over others of the same sex, in relation to the propagation of the species […]” (Darwin, 1871). He noticed that species where the male differed greatly from the female were polygamous and able to reproduce more of that species. Since the polygamous species reproduced more, they were favored by natural selection.
Many think that sexual differences in the genders are easy to view, but in truth, most physically distinguishing features arise after the male or female in a species has reached puberty or adulthood. The three species used in our study showed examples of sexual dimorphism on a physical level. The fiddler crab, Uca longisignalis, males are best known for their body’s asymmetry. The male has a minor claw used for feeding and a major claw used for display and combat (Kaplan 1988). The human, Homo sapiens, male is generally larger in height and heavier in weight than the human female. The sailfin mollie, Poecllia latipinna, males have large dorsal fins that have orange edges and black spots and wavy lines (Page and Burr 1991). All of these features attract females to the males.
Sexual selection is considered to be one of the most important evolutionary forces and it is used to explain sexual dimorphism.
Methods
Animals Used
In order to test out hypothesis, we used three species: Uca longisignalis, Poecilia latipinna, and Homo sapiens.
Experimental Protocol
We separated the fiddler crabs according to their sexual orientation as described in Kaplan (1988). We then recorded the mass of the crabs by using a top loading electronic device. This process was repeated until all of the crabs were weighed. Next, we separated the humans into groups according to their gender and measured their height from head to toe in centimeters with a yard stick. The results were recorded. Last, we took a bucket of sailfin mollies and place them, six at a time, into a bucket of water that was laced with MS-222 (Argent Chemicals, St. Louis) at a concentration of 0.1 g/ L in order to sedate them. We then separated them according to external characteristics (Page and Burr, 1991). Standard length was measured and rounded to the nearest millimeter.
Results
We measured male and female humans and found that the average height of females was 165.7+/- 3.4 (S.D.) and the males were 182.1+/-8.1 (S.D.) This information is shown in Figure 1. Statistical analysis was carried out to test whether this was a significant difference (Table 1)
We weighed the male and female fiddler crabs and found that the average for the females was 4.0 +/- 1.1 (S.D) and the average for the males were 2.3 +/- 0.6 (S.D). This data is graphed in Figure 2. Calculations that were completed in order to determine if there was a significant difference between the genders is shown in Table 1.
We measured the standard length of male and female sailfin mollies. The average length in the females were 43.0+/- 4.3 (S.D) and the average for the males were 50.0 +/- 6.2 (S.D). This data is graphed in Figure 3. Calculations that were completed in order to determine if there was a significant difference between the genders is shown in Table 1.
Table 1: A summary of calculated data.
Discussion
Gender is a major factor in determining sexual dimorphism in order to test our hypothesis, mass, standard length, and height does not differ in males and females of Uca longisignalis, Pocilia latipinna, and Homo sapiens.
We found that sexual dimorphism in regards to size occurred in the sample of fiddler crabs that we measured. Although the males had larger appendages, the females had a larger body mass. This could be a result of hormones, the female may produces a hormone that allows her to carry more weight in order to protect her eggs, or it could be a result of the males being younger adults. Others have found an absence of sexual dimorphism in regards to enzyme activities (Zou and Fingerman, 1999) suggesting that our external characteristics may not extrapolated to internal physiological parameters.
We found that the sailfin mollies showed sexual dimorphism in regards to standard length. The males were longer in size. A possible reason for this is that the males have to travel a longer distance and be faster in order to feed. A study at the University of Vermont showed that male sailfin mollies had a higher level of a gelatin-like peptide, GAL-LI, in the spinal cord that was oriented around the longitudinal plane (Cornbrooks and Parson 1991) proposing that this peptide may have an effect on the length difference in this particular species of fish.
We found that there was a significant difference in the height of human males and females. The human males were taller than the females. Before the modern ages, males had to be stronger, taller, and fitter in order to survive. This tall trait could have been a part of natural selection and that is why most males are generally taller than females today.
In conclusion, sexual dimorphism in physical appearance is more pronounced in the male of these three species.
References
Cornbrooks EB and Parson RL (1991) Sexually dimorphic distribution of a gelatin-like peptide in the central nervous system of the teleost fish Peocilia latipinna. J Comp Neurol 304(4):639-657.
Darwin C (1885) The Descent of Man and Selection in Relation to Sex. London, John Murray.
Kaplan EH (1988) In: A Field Guide to Southeastern and Caribbean seashores. Houghton Mifflin Co., NY. Pp 334-338.
Page LM and Burr BM (1991) In: A field guide to Freshwater Fishes. Houghton Mifflin Co., NY. Pp 232-233.
Zou E and Fingerman M (1999) Patterns of N-acetyl-beta-glucosaminidase isoenzymes in the epidermis and hepatopancreas and induction of N-acetyl-beta-glucosaminidase activity by 20-hydroxyecdysone in the fiddler crab, Uca pugilator. Comp Biochem Physiol C Pharmacol Toxicol Endocrinol 124: 345-349
