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Izabella Plotnick ENGL 3134 Module 3 Assignment July 9th, 2015 Differences between Males and Females due to Sexual Dimorphism within the Red Swamp Crayfish (Procambarus clarkii) Population Abstract: Sexual dimorphism is the differences between genders in different species. However, it does not occur in all species. One specific species it does occur in is crayfish, Procambarus clarkii. Seeing as though crayfish exhibit sexual dimorphism, a question arises about what specific differences females and males have in chela size and cephalothorax size, whether or not males exhibit the larger chela size, and what effects these differences have on relations and interactions among male and females. We focused on these two factors, chela size and cephalothorax size, and we conducted our experimentation with hopes to see how the sizes between males and females differ while testing our hypothesis, which is that males have the larger chela. To test these two factors we measured five female and five male’s chela length and cephalothorax length using a ruler, recording our results. After performing the necessary calculations and measurements, charting out the results, we found that when it comes to male chela size, they are larger than females and it does not deal with their body sizes because males and females body sizes don’t vastly differ. This fact, that a male crayfish holds the larger chela, plays an important role in many different social aspects. Introduction: The species Procambarus clarkii is a red swamp crayfish. Male and female crayfish differentiate in chela size, which affect interaction in social aspects, more specifically mating, predation, competition and gathering food. Chela is the claw of crayfish and takes up most of their dry weight, about 35-50% total (Garvey and Stein 1993). While chela is costly to grow, large chela can be beneficial in a vast amount of ways. For instance, crayfish with larger chela are less susceptible to predators (Garvey and Stein 1993). This correlation isn’t solely found in Crayfish, but among many other species as well. One such species is the Sand Fiddler Crab, Uca pugilator. Based on a study done where seven white ibises, natural predators of the Sand Fiddler Crab, were given the option of choosing either a male or female Sand Fiddler Crab to invade, or choosing a male with his major claw removed versus a complete male, Sand Fiddler Crab claw size/presence directly affected the ibises’ choice of predation (Bildstein Sand Fiddler Crab et. al. 1989). More female Sand Fiddler Crabs were chosen in the case of male versus female, and more male Sand Fiddler Crabs without their major claw were chosen in the case of whole male versus clawless male (Bildstein et. al. 1989). This study stands as an example of how much of an impact claw size can play in daily life, like when dealing with predators. For crayfish, a larger sized chela allows a crayfish to hold down prey easier, allowing them to capture their prey more easily and efficiently (Garvey and Stein 1993). It’s also beneficial for males to have larger body chela because they can hold female crayfish better while mating, which is seen as the most important benefit upon male crayfish (Garvey and Stein 1993). Chela size is considered most important for the purpose of reproduction (Stein 1975). Besides the chela size, female crayfish greatly consider a male’s body size in determining if it wants to mate with it, indicating that a larger body size, without mentioning chela size, generally specified a higher quality mating partner (Aquiloni and Gherardi 2008). Apart from mating and predatory issues, chela size, and overall size in general, affects social dominance among crayfish. Crayfish often challenge one another over scarce resources like shelter and food, and these challenges include pushing, hitting, and striking one another with the claw (LePine et. al. 2012). Being victorious in one battle betters a crayfish’s chance of winning another, thus asserting itself as dominant (LePine et. al. 2012). Once a crayfish reaches a dominant position in society, it is then able to access scarce resources with more ease (LePine et. al. 2012). A dominant crayfish will be able to easier access shelter, and securing shelter is key for a crayfish’s survival (Blank et. al. 1999). There is even evidence to conclude that a dominant crayfish male will easier attract a female crayfish (LePine et. al. 2012). Indubitably, there is much emphasis on the size of a crayfish, especially the size of a chela, and interactions among males and females. Generally, male crayfish are dominant over female crayfish (Blank et. al. 1999). It’s important to know what leads to males’ dominance over females. In turn, an experiment has been conducted comparing both chela and cephalothorax lengths for males and females in order to determine if males have a larger chela, a larger cephalothorax, and a larger ratio. Materials and Methods: Using tongs and being provided gloves, we pulled ten crayfish out of a preserved bucket and placed them in our specimen tray. We examined our ten crayfish to make sure we had five females and five males. After doing so, we were provided a ruler and measured each crayfish chela length and cephalothorax length and reported our results in centimeters. Results: When analyzing the data, it is important to notice the difference among male and female chela sizes, cephalothorax sizes, and chela-cephalothorax ratio. The mean male chela length is 4.495 cm and the mean female chela length is 3.85 cm (Figure 1). This table shows that on average, male crayfish have larger chela than female crayfish. Also, when deciding which sex has the longer cephalothorax, it is important to look at the mean for each sex, which shows that the average length for males is 5.304 cm, while the average length for females is 5.401 cm (Figure 2). These results indicate that males, while having larger chelae, do not have larger bodies, as they are typically the same size, or may even be slightly smaller than females, as our results show. Furthermore, to truly understand how male and females vary in body, it is important to notice the mean ratio of chelae-cephalothorax, which from our studies indicate that males have a mean of .857 cm while females have an average ratio of .713 cm (Figure 3). These results conclude that for males, the chela takes up a larger percentage of their total body weight/mass than for their female counterparts. Figure 1- Average male and female chela length (based on measurements of 5 male and 5 female crayfish) Figure 2- Average male and female cephalothorax length (based on measurements of 5 male and 5 female crayfish) Figure 3- Average male and female ratio of chela-cephalothorax length (based on measurements of 5 male and 5 female crayfish) Discussion: Upon completion of this experiment, the results indicate that for two out of our three tests, we are able to say that males, on average, have a larger chela length and a larger chela-cephalothorax length ratio. For chela length, our initial hypothesis stated that male cheliped size is less than or equal to female cheliped size, while our alternative hypothesis states that male cheliped size is greater than female cheliped size. With a test statistic of 5.09 and a t critical value of 1.66, the returned p-value for this individual test is .00, less than any alpha used when analyzing p-values. (Also the test statistic is way above the t critical value) With such a small p-value, we are able to reject the initial hypothesis with sufficient evidence to accept our alternative hypothesis, which states that male cheliped size is greater than female cheliped size. For cephalothorax length, however, the results indicated a different conclusion. While both hypotheses stayed the same (Ho: male cephalothorax length is less than or equal to female cephalothorax length; Ha: male cephalothorax length is greater than female cephalothorax length) the returned statistics are different. With a test statistic of -1.19 and a t critical value of 1.66, the returned p-value for this individual test is .12, greater than any alpha used when analyzing p-values. (Also the test statistic is not greater than the t critical value) In turn, we fail to reject our initial hypothesis, so we have insufficient evidence to claim that male cephalothorax length is larger than female cephalothorax length. Therefore, on average, male cephalothorax length is either less than or equal to female cephalothorax length. Finally, for the chela length to cephalothorax length ratio, our initial hypothesis stated that male ratio is less than or equal to the female ratio, while our alternative hypothesis stated that male ratio is greater than the female ratio. With a test statistic of 5.70 and a t critical value of 1.67, the returned p-value for this individual test is .00, less than any alpha used when analyzing p-values. (Also the test statistic is way above the t critical value) With such a small p-value, we are able to reject the null hypothesis; we have sufficient evidence to assume that males have a greater chela length to cephalothorax length ratio than do females. To bring it all together, we can conclude that on average, males have longer chela lengths than females, while shorter or equal cephalothorax lengths as females, and when comparing ratios, it’s important to note than on average, a male’s chela will take up a larger percentage of its body. For males, this is a crucial trait, as the chela is an important body part for crayfish on a day to day basis. As previously stated, a larger chela size means a better ability to prevent predatory invasions because predators will choose to attack a crayfish with a smaller chela (Garvey and Stein 1993). This leaves females in more danger than males. Also, a larger chela helps a crayfish hold down prey more adequately, allowing for capture and consumption (Garvey and Stein 1993). With a larger chela, crayfish are able to easier strike other crayfish and assert a position of dominance, which is also important for retaining shelter, acquiring food, and attracting mates (LePine et. al. 2012). Furthermore, chela size is considered first and foremost an important trait for reproduction (Stein 1975). With a bigger chela size, male crayfish can hold up female crayfish easier while mating, which is seen as a great advantage by female crayfish (Garvey and Stein 1993). One of the biggest ideas to take away from our experimentation is that the chela size could directly lead to the male’s dominance over females. As our studies show, male cephalothorax length, on average, is less than or equal to females. History shows, however, that males are typically dominant to females (Blank et. al. 1999). This could very well be due to chela size, which our research shows are bigger in the male crayfish. When comparing males to females, and chela size to cephalothorax size, our conclusions lead to noting how important chela size really is; a larger chela leads to shelter, food, mating, and even dominance over females, even when the cephalothorax size could be the same, or even smaller. 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