Sexual Dimorphism

Introduction:
In Charles Darwin’s theory of evolution, it states that individuals will experience mutations when their hereditary information is transcribed and copied, known as DNA. These mutations in the DNA could give the organism an advantage in its ecosystem, helping it to survive and giving it an increased chance to reproduce, where the mutation will be passed to its offspring. This creates variation in species where individuals in a population will have an evolution advantages over others of its species due to its fitness, which is its ability to function in its environment. (Hedrick and Temeles, 1989) These changes, if favorable to the organism will become more common in the species due to the ones that have this trait it can breed more, increasing the amount of individuals with this trait through their offspring, slowly changing the species. These traits that give advantages can be anything that gives one an advantage, such as better eyesight to see predators, or larger claws to defend its young. In many species, different sexes will develop different traits to better suit to their needs to survive, known as sexual dimorphism. If a male or female develops a mutation that helps it in its ecological niche or to survive allowing it to outcompete others of its gender in caring for its young, or fighting for better mates, it will be able to pass this on due to it being more fit, causing a difference in between the two sexes. (Hedrick and Temeles, 1989) It can also occur if there is a need to compete for food in between sexes. This occurs due to the evolutionary advantage that the individual has, and is extremely common in nature, such as male deer having a large set of antlers to fight with other males for chances to male, which passes on this trait, making it more common. The male deer with the larger set of points will be able to fight better and mate more, giving his male offspring larger antlers, furthering the difference between male and females in the species. Females do not need this trait because they do not fight, and uses up energy while not posing as an advantage for a female to have, not increasing the probability of its passing on. (Hedrick and Temeles, 1989) Evolution biologist study this because it just an example of evolution and how the species interacts with each other. Another reason for the study of sexual dimorphism is it gleams information on why species might have developed the way they have, in some species, away from what evolution would have predicted such as red canaries. In accordance with evolution, it would be predicated that red cardinals would be picked off by predators due to the red ones being more visible but due to sexual dimorphism, the red cardinal is more dominate. This is because the female is more attracted to the red colour in the male which indicates it is healthier, giving it more breeding chances, passing on its red colouring trait to its offspring. Red cardinals also will have larger territories and better feeding structures, resulting in its access to more resources for it to survive. (Wolfenbarger, 1997) Sexual dimorphism is most common when there is competition in between males and females for feeding opportunities. Feeding opportunities is extremely important for the animal because it requires it to survive and has to compete with other species and its own species for sustenance. (Walker el al, 2002) If an animal is able to feed on a larger variety of sources then it will have a greater chance of surviving and reproducing. This drives sexual dimorphism because often males fill different roles in different species and will require different amounts of energy, such as the humming bird, where the female has the longer beak and can feed on a greater amount of flowers than the males because she cares for the babies, needing that larger amount of energy. (Temeles el al, 2000) In this lab, using bite force and canine width, we will investigate Artic foxes to see if sexual dimorphism exists in their species. My hypothesis is that males will have a larger canine width and will have a resulting stronger bite force than females, proving that sexual dimorphism does exist in artic foxes. We are preforming this experiment because in other species of mammals, such as gorillas, sea lions, and humans, sexual dimorphism does exist. In sea lions and humans, males are on average larger than the female counterpart. This experiment is being performed to observe the differences between the male and female artic foxes and reason if sexual dimorphism does occur in this species. ( Ralls, 1977)

Methods:
In the lab, skull length was recorded by measuring from the front of the skull right before the teeth to the back lobe with a ruler. The skulls were handled with care due to their extreme fragile nature. Thirty one measurements of skull length were made with roughly even male and female skulls. The width of the canines was also recorded with the measurements of the skulls. Left and right upper canines were measured. The left canine on the skull corresponds with your left canine with the same for the right. The canines were measured at the widest point on the teeth with a ruler. The identification number for every measurement was recorded with the values to avoid confusion. In this section of the lab, we measured bite force on the pictures of the skulls by following the method developed by Thomason in 1984 to measure bite force. This was done by first finding the conversion factor of the image to actual measurements by measuring one centimetre on the ruler in the picture. Then measurements of the lever arm of the temporalis(TL) was taken by tracing this onto paper with a grid on it with a constant area per unit which allowed for the tracing to be converted into units of centimetres. The lever arm of the masseter (ML), distance from jaw joint to the caudal border of canine (Oc), the area of the masseter (MT) and the temporalis (TT) were all taken in the same way. These measurements were all converted to actual size using the conversion factor and area. These values were then using in junction with the formula developed by Thomason to determine bite force in Newtons, Bite force= (2(MTxML)+(TTxTL)x0.3)/Oc The measurements from the skulls were then statistically analyzed using Excel with T tests. We used T tests to compare the means of male and female skulls of average canine width and average bite force.

Literature Cited:
Haqq, M. C. and P. K. Donahoe. 1998. Regulation of sexual dimorphism in Mammals. Physiological Reviews 78: 1-33
Hedrick, A. V. and E. J. Temeles. 1989. The evolution of sexual dimorphism in animals: hypotheses and tests. Trends in Ecology and Evolution 4:136-138.
Ralls, K. 1977. Sexual dimorphism in Mammals: Avian Models and Unanswered Questions The American Society of Naturalists. 111:917-938
Temeles, E. J., Pan, I.L., Brennan, J. L., and J. N Horwitt. 2000. Evidence for ecological causation of sexual dimorphism in a hummingbird. Science 289:441-443
Wolfenbarger, L. L. 1997. Red coloration of male northern cardinals correlates with mate quality and territory quality. Behavioral Ecology 10:80-90

Cited: Haqq, M. C. and P. K. Donahoe. 1998. Regulation of sexual dimorphism in Mammals. Physiological Reviews 78: 1-33 Hedrick, A. V. and E. J. Temeles. 1989. The evolution of sexual dimorphism in animals: hypotheses and tests. Trends in Ecology and Evolution 4:136-138. Ralls, K. 1977. Sexual dimorphism in Mammals: Avian Models and Unanswered Questions The American Society of Naturalists. 111:917-938 Temeles, E. J., Pan, I.L., Brennan, J. L., and J. N Horwitt. 2000. Evidence for ecological causation of sexual dimorphism in a hummingbird. Science 289:441-443 Wolfenbarger, L. L. 1997. Red coloration of male northern cardinals correlates with mate quality and territory quality. Behavioral Ecology 10:80-90