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Biology homework help. Evaluating Sexually Dimorphic Characteristics in Crayfish
There are 3 main objectives for this exercise:
1) Introduce students to sexually dimorphic traits that distinguish males from females
2) Give students more practice with study design and data analysis
3) Give the students more practice in scientific writing
Biologists define fitness as the number of copies of genes an that individual passes into future generations. Fitness can be thought of as a combination of the quantity and quality of the offspring produced.
So, if your offspring are unable to reproduce will you have a higher or lower fitness?
Natural Selection can act on any heritable trait that affects fitness. Obviously many physical characteristics are acted on by natural selection; in addition, many behavioral traits have very strong genetic components and are acted on by natural selection (bird songs).
Sexual Dimorphism is the occurrence of morphological differences between the genders that are not directly related to reproduction (not gonadal).  This often results from gender related differences in reproductive potential (i.e. non-random mating/mate selection).
What would determine female reproductive potential? What about males?
Because of the differences in reproductive potential, males and females typically evolve different reproductive strategies to enhance their fitness.
Females – Since the quantity of offspring produced is usually limited, they can enhance their fitness by investing in quality.  How do they do this?  Females are picky.  Since half of the genes in a female’s offspring will come from her mate, females invest in offspring quality by mating with high quality males.
Males  – Males can also invest in offspring quality by being picky about their mates, but a better approach  to enhancing fitness is to invest in quantity by mating with as many females as possible.  The chances are that some of the females will be high quality.
These differences frequently result in one or a combination of the following three types of sexual dimorphism:

  • Females are larger than males – This occurs when a female’s egg production is a function of her size – many invertebrates (spiders) and most fish.
  • Males are larger then females (or possess weaponry i.e. Antlers) – This occurs when males physically compete with each other for access to the females.
  • Males possess elaborate secondary sexual characteristics (peacock tail feathers) – this occurs when females actively choose their mates. The elaborate characteristics are thought to be an indicator of the males’ quality.  If he can feed himself, maintain his health and avoid predators while dragging that stupid tail around, he must have one mean set of genes.

Since sexual dimorphism in cheliped size is known to exist in some decapods (crabs and lobsters), we want to know if it also occurs in crayfish (Procambarus clarkii).  We suspect that it might because we know that male crayfish compete with each other for access to the females.
What are our hypotheses for this exercise?:
Chela size:
How many tails do these hypotheses have?
We will use the following methods to determine chela size and body size of about 60 male crayfish and about 60 female crayfish.  Form groups of 2-3 and measure the following on 5 crayfish of each gender. Record your data for both variables in a table and then enter it into the spreadsheet at the front.
First you must retrieve your crayfish from the bucket. Though we are evaluating sexual dimorphism, which again is non-gonadal differences between sexes, in order to accurately assess the sexual dimorphism, we must accurately sex the crayfish – which is done by identifying the gonads (see below).
Once you have identified 5 males and 5 females, we will begin taking measurements as described below.
Chela Size:  When measuring the length of the chela, you will be measuring only the chela. Your measure will start at the base of the claw and you will measure to the tip.
Cephalothorax Size:  This is our control measure for larger crayfish in general – by measuring the body size and the chela size we can control for the chance that the crayfish selected were larger than average. For this you will measure from the tip of the rostrum to the base of the tail.
After we have the measures of the chela and the cephalothorax, we will use them to generate a body size ratio.
Data Collection Procedures:
*Feel free to use the data table on the bottom of the page instead of the blanks at the top of the page.
-Do only 1 gender group at a time to avoid mixing up male & female crayfish.
-Work in pairs.
-Take 5 male crayfish OR 5 female crayfish.
Once you are done with them you will still keep them at your table to avoid multiple measures of the same crayfish.
-Get the length of the chela  _____________ (A)
-Get the length of the cephalothorax  _____________ (B)
-What is your ratio? ___________________(A/B)
Once you have your measurements for your crayfish, enter them into the class spreadsheet on the computer.
Analysis Procedures:
The hypotheses you have made can be tested using the Students t-test (as you did in 1111).
We are going to be using Excel to run our data analysis. You will not be doing the statistical testing by hand (this is because rounding and other human error can cause enormously variable results in data analysis)
You can compare the t-stat and t-crit to determine if you should reject or fail to reject your null hypothesis. If the p<0.05, you can reject the H0 because 95% confidence that the variance you see is not due to sampling error. However, if the p>0.05, you fail to reject the H0 because the observed differences are likely due to sampling error.
Conduct a t-test and analyze the data for chela length, cephalothorax length and the body size ratio using male and female as your comparison groups But how do I run a t-test in Excel?
Make bar graphs in Excel- one for each of the variables above for your paper. How do I do this?
Each graph should show means + S.D. for each variable using error bars. How do I add error bars and what do they represent?
Written Assignment:
Write up this lab exercise in scientific format using the instructions provided in Appendix II.  In your report make sure to cite relevant primary literature as appropriate.  The following articles have been placed on electronic reserve in the library to serve as a start in searching through the primary literature on this subject.  You will also find some (minimum 3) primary references on your own. You can use these articles as a guide to determine what information should go into each of the sections of the report.  Pay special attention to the way in which citations are made in the text of the articles and how the full citation is provided at the end.  Notice that authors seldom quote their references, instead they provide the pertinent information in their own words and then cite the reference (Smith and Jones 1999).  You will not use quotes in your paper since there are only very limited circumstances in which this is acceptable in scientific writing. Pay attention to the way data is presented in figures and tables and how table and figure captions are written.
Articles on reserve:
Bildstein, K.L., McDowell, S.G., and Brisbin L.  (1989).  Consequences of sexual
dimorphism in sand fiddler crabs, Uca pugilator: Differential vulnerability to avian
predation.  Anim. Behav. 37: 133-139.
Garvey, J.E. and Stein, R.A. (1993).  Evaluating how chela size influences the invasion
potential of an introduced crayfish (Orconectes rusticus).  Am. Mid. Nat. 29 (1): 172-
Stein, R.A.  (1976).  Sexual dimorphism in crayfish chelae: Functional significance
linked to reproductive activities.  Can. J. Zool. 54: 220-227.

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