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Spider Behavior Laboratory
Based on the research of Dr. Todd Blackledge from The University
of Akron
1
BACKGROUND:
Summary:
Using the common house spider, Achaearanea tepidariorum, you will investigate the foraging ecology
of cobweb spiders. You will develop and test hypotheses about how the shapes of spider webs may be
related to the foraging success of these ubiquitous spiders.
Background:
All animals need to eat to survive and reproduce. However, the behaviors that animals use to
hunt or forage also cost time and energy, which could otherwise be invested in things like growth and
reproduction. Furthermore, foraging can expose animals to greater risk of capture by their own predators.
Animals therefore need to balance the benefits and the costs of foraging. Optimal foraging theory
suggests that natural selection will act on the behaviors of animals in ways that maximize the difference
between the benefits and costs of foraging behaviors. In other words, the optimal strategy for foraging
isn’t always the behavior that obtains the most calories but rather the behavior that obtains the largest
number of calories for a given amount of effort.
Spiders are an excellent model system to study foraging behaviors because their webs provide
easy ways to measure foraging effort (i.e. a larger web with more silk in it costs more to spin) and
biologists can design experiments to test how effective different webs are at capturing food. One
commonly studied type of spider web is the orb web. This web consists of a framework of dragline
silk, which looks a lot like the spokes on a bicycle wheel, and a spiral of sticky capture silk. The
dragline silk is a stiff and strong support while the capture silk is elastic like a rubber band and is coated
with sticky glue that adheres to insects. These webs are excellent at capturing flying insects and past
research has shown that hungry orb spiders spin larger webs than do sated (full) spiders. From an
optimal foraging viewpoint, this makes sense because starved spiders can gain a lot of fitness from
capturing a meal (otherwise they might starve to death) while sated spiders cannot gain that much fitness
by capturing an additional meal (because they are already full of food!). Thus, the additional cost for
starved spiders of spinning a bigger orb web is more than compensated for by the large benefit of
capturing food.
A. tepidariorum feeds on a variety of prey, including German cockroaches and scorpions (Archer 1947;
personal observation). While awaiting prey, spiders are usually positioned in the middle of their webs, but resting
individuals may be nearer a lateral or upper edge of the web, where the complex color pattern on the spiders'
bodies near the substrate may help camouflage them against some enemies. Frequently males may be seen
hanging in webs of adult and subadult females.
2
In this lab, we are going to study a different kind of spider, the common house spider
Achaearanea tepidariorum. The common house spider spins a cobweb and, as its name suggests, is
often found in the basements and garages of homes. Cobweb spinning spiders are closely related to orb
weaving spiders and they probably evolved from an orb weaving ancestor in the late Jurassic. But, their
webs look very different. Cobwebs completely lack a stretchy capture spiral and the supporting
framework of dragline silk isn’t arranged into stereotyped radial threads. Instead, the common house
spider’s cobweb consists of two regions – a retreat and sticky gumfooted threads. Both regions are
spun from the same type of dragline silk but the retreat portion of the cobweb is a three-dimensional
cloud of silk threads that surrounds the spider when it is resting while the sticky gumfooted threads are
only found in the bottom halves of webs. Sticky gumfooted threads are always single, long threads of
silk that run down from the retreats to the ground. They can be identified by the tiny droplets of glue that
coat the last centimeter of the threads where they attach to the ground. Cobwebs capture arthropod prey
that walk on the ground and sticky gumfooted threads play two important roles (it is not known if one is
more important than the other). First, the gluey “feet” of the threads adhere to prey, slowing them down
and sometimes even pulling small insects off of the ground. Second, since these spiders have 8 eyes but
poor eyesight and rarely leave their web unless they are disturbed, the sticky gumfooted threads transmit
the vibrations of insects up to the retreat where the spider senses the vibrations and uses them to locate
potential prey within the web. Both of these functions would clearly be improved by spiders that spun
webs containing lots of sticky gumfooted threads. However, spider silk is composed of protein and it
takes time for the spiders to spin these threads so that webs with many sticky gumfooted threads cost
more to construct. Thus, variation in the numbers of sticky gumfooted threads between different spiders’
webs should reflect differences in how individual spiders are “assessing” the energetic costs and benefits
of capturing prey.
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Retreat
Sticky
Gumfooted
Threads
Glue
4
Lab One Procedures:
1. You will be provided with one spider cage—each cage houses one spider. Remove the
cardboard foundation from each cage (carefully!) and place it within the shadow box that is
provided (the spider will not move from its web, don’t worry). You will need to position a
florescent light so that you are able to clearly see the structure of the web. You may need to
move the light around to better see different regions of the web.
2. At the base of each cage is a piece of black paper which is divided into nine (9) squares. Position
the light so that you can see the gumfooted threads (only those threads with glue attached at the
end) and examine the threads closely with the magnifying lens.
a. If it appears that there are fewer than 30 gumfooted threads, count the total number.
b. If it appears that there are more than 30 gumfooted threads, count the number of
gumfooted threads within three of the nine squares. The three that you will count will be a
random selection (remember that you do not want to impose any form of “researcher bias,”
so randomization is the best method to use to prevent this).
How to select random #’s:
1. Place #’s between one and nine into a hat and pull out one number at a time.
2. Use a random numbers table:
a. Go to www.random.org/integers
b. Values should be between one (1) and nine (9)
c. Click “Get Numbers”
3. You now need to determine the average number of gumfooted threads per square.
4. You may also want to record descriptive data about the web, and possibly take a picture of the
web architecture. Once you introduce prey into the web, the web architecture will be damaged,
so you will need to record as much data as possible before you continue.
5. Place the cardboard frame back into the plastic box.
6. I will assign each group a prey specimen – either a cricket or a pillbug (woodlouse).
7. Weigh the prey.
8. Select a random number between one (1) and nine (9) – see “how to select random #’s.” The #
you choose will tell you onto which of the black squares you will place the prey.
9. Using forceps or your fingers, gently toss the prey into your chosen numbered square (remember,
the crickets are very quick and will try to run away).
10. Leave the prey in the plastic spider arena for no more than 60 seconds!
11. Record the amount of time until the prey is captured (spider approaches, wraps, and bites).
12. If after 60 seconds the spider has not advanced toward the prey, stop the trial, remove the
prey, choose another random number and reintroduce the prey.
Start the stopwatch at zero when you reintroduce and record time to prey capture.
5
It is very important that you do NOT allow a trial to extend beyond 60 seconds!
13. If the spider has not advanced toward the prey after 5 trials, you may discontinue the experiment.
(Leave the prey in the cage though).
14. Your recorded data should be displayed in a format similar to the table below:
Average # GF
Type of Prey
Prey Captured (Y or N)
Trial # Captured Time to Subdue Prey
Prey Weight
Spider #7
Spider #8
15. You will need to present your data table so your entire class has access to your raw data.
RECORD YOUR INFORMATION ON THE TEACHER’S COMPUTER.
16. Read chi-square section of “t-test, chi-square, and regression analysis.”
17. Got to the online classroom and look at the woodlice lab subfolder which has a copy of how to perform
the test, or look in the statistics folder in the online classroom.
18. Practice a chi-square test if you have time.
6
STATISTICAL DATA ANALYSIS – 15 POINTS
Statistically analyze the CLASS DATA (not just your group’s data).
Answer questions using STATISTICS to obtain a p-value!
a. Is there any relationship between type of prey and whether it was captured?
__________________________________________________________________________________
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b. Is there a difference in the average number of sticky gumfooted threads in webs in which
prey were captured vs. webs in which prey were not captured?
__________________________________________________________________________________
__________________________________________________________________________________
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c. Is there any relationship between prey weight and whether it was captured?
__________________________________________________________________________________
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7
An Example (you do not need to answer this question):
Is there any relationship between type of prey and time to capture?
1. What are the two variables you want to compare?
1. Type of Prey (independent variable)
2. Time to Capture (dependent variable)
2. Are your variables qualitative (or categorical) or quantitative (or numerical).
1. Type of Prey is qualitative (cricket or woodlice)
2. Time to Capture is quantitative
3. Now determine which statistical test to use based on the type of variables:
chi-square = compares two qualitative (or categorical) variables
t-test = compares one qualitative variable with one quantitative variable
(regression = compares two quantitative (or numerical) variables; I’m not teaching you this
but have supplied the information in the statistics folder in the online classroom)
-
We would use a two-sample t-test to answer our question above.
We would form our two groups (a t-test only compares two groups) from our categorical variable.
We can divide type of prey, our categorical variable, into two groups – cricket and woodlice
We would place the numerical data for time to capture beneath the appropriate group.
Crickets
15.2
30.5
Woodlice
3.5
8.2
Staple and Turn In:
1.
2.
3.
4.
Any descriptive data your group may have recorded
Statistical results (TWO total - one for each test)
Brief interpretation of your statistical results
Graph associated with your t-test (be sure to include error bars!)
8
EXPERIMENTAL DESIGN
Design an experiment in which you will alter the probability of prey capture.
Some possible experiments include:
A. manipulate architecture of web to alter probability of prey capture
B. alter type or size of prey to alter probability of prey capture
C. alter condition of spider or prey to alter probability of prey capture
Use the proposal worksheet below to detail the experiment you wish to conduct next lab.
If there are any materials that you will need, please state these as specifically as possible.
9
Proposal Worksheet
Be sure to follow the rubric provided in lab as you complete your proposal worksheet.____
1. State your hypothesis and the rationale for your hypothesis.
2. How do you plan to conduct your experiment (don’t forget to mention sample size)?
3. Provide a sketch of the Excel spreadsheet you will use to analyze your data.
4. What is the control group in your study? If no control group, explain why one is not needed.
5. Discuss how you will analyze your data (mention which statistical test and how you’ll interpret).
6. Sketch a graph to show how you will present your data.
10
Lab Two Procedures:
1.
2.
3.
4.
Complete your experiment and record your raw data.
Use a chi-square test or t-test to analyze your results.
Provide a typed summary and discuss all criteria included on the summary rubric.
Staple and turn in your raw data, graph (if applicable), statistical analysis, and summary.
11
Faculty Data Interpretation – 10 Points
Figure 1. Linear relationship between the total mass of silk and body condition of spiders.
Body condition measures how heavy spiders are relative to their size. Body condition is similar to the
human body mass index and provides an indication of how well-fed spiders are. A positive body
condition indicates a spider that is heavier than average for its body size (i.e. is well fed).
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Figure 2. Linear relationship between # of gumfooted threads and spider body condition.
Review the above data with group members and answer the following questions:
1. What is a line of best fit (if you have never taken a statistics course, you will have to research this)?
2. Based on figure 1 (page 10), which spider is more likely to spin a web with more silk?
A. A spider that just caught a large meal
B. A starved spider
3. Based on figure 2 (above), which spider is more likely to spin a web with more gumfooted threads?
A. A spider that just caught a large meal
B. A starved spider
4. How would the # of sticky gumfooted threads affect the likelihood that a spider will capture prey?
5. Come up with a logical explanation to explain the trends observed in Figures 1 and 2.
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