Download Procambarus clarkii

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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