Roman II:
Planning Studies
(Experimental design)
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Clam Dancing!?!?!?
The life of a typical garden variety clam would seem to be pretty uneventful. In general,
people who study marine invertebrate biomechanics (e.g. clam locomotion) are of the
opinion that ocean wave motion erodes the sand around a clam and then propels it toward
the beach. However, this simple theory may not hold for northern coquina clams (Donax
fossor) in North Carolina. These creatures may attempt to stay near the water’s edge by,
in effect, “surfing.” These clams have a muscular tongue-like foot and it may be they
use it to propel themselves toward the beach – in effect, surfing – to where the waves
break and stir up the plant and animal material that is the clam’s food.
As one can imagine the surf propels both live and dead clams toward the beach, and an
experiment is envisioned using the “life status” of the clams as an explanatory variable.
The general idea is that if clams have a developed locomotive capability and ride the
waves toward shore, the live clams should get further toward
shore than the dead clams. The experimental plan is to place the
clams in the sand at different locations near the surf and observe
their progress toward shore.
The experimental location exists for roughly 100 meters along
the beach, and about 20 meters toward shore from the ocean at
low tide. The elevation of the shore decreases slowly at a fairly
constant rate over the 20 meters. Over the 100 meters the beach
ranges from large plant density in the North to a more rocky
terrain in the South. Tides come in and go out twice a day, once
during daylight hours and once during nighttime hours.
Your task is to design an experiment to address this question: do
the coquina clams propel themselves toward the food source?
Be particularly clear about how you assign the clams their initial
positions and the times of positioning in the sand. Since the
clams may not move very far during the experimental period,
you will need to be precise in your description of the
measurement process. You may assume that identifying the
clams with a life status and number, e.g. “L-#” and “D-#”, will
not attract any predators, and that the experiment is of short
enough duration that the likelihood of experimental mortality is
vanishingly small.
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Live clam
Dead clam
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Wise Old Owl vs. Bambi Mouse
Rodents in desert communities in the eastern part of Southwestern North America have
two fundamental problems: (a) finding food and (b) avoiding becoming food. It may be
that these two problems are related; where rodents forage may depend on their
“strategies” for avoiding predation. Small rodents such as the deer mouse (Peromyscus
maniculatus) generally seem to spend more of their search efforts in spaces under bushes
and other desert cover rather than
foraging in open spaces where seeds
would be easier to locate visually.
Ecologists have two theories about why
deer mice look for food in different
areas.
The first theory is that the deer mouse is
able to climb and maneuver in bushes,
and searching for seeds there is less
tiresome than scampering across open
spaces. The second theory is that their
foraging strategy is the result of
attention to predation risk. This
The prey: a deer mouse
hypothesis suggests that a deer mouse in
the open is a proverbial “sitting duck”
and therefore would choose to forage in dense vegetation to avoid predation.
An investigator is designing an experiment to
evaluate these two theories. A fenced-in
rectangular pen approximately 10 acres in size has
been constructed in the Nevada desert. The plan is
to release deer mice individually into the pen and
record the amount of time spent foraging in the
open. The pen will contain a rectangular grid with
small boxes of seed placed at intervals. These
boxes will be placed in the middle of each open
space and under each brushy space. That way, an
open space is always presented close to a brushy
space, and a brushy space is always presented close
to an open space. This would suggest to the deer
mouse that all foraging options are open at all
times. Each deer mouse will forage in the pen for
12 hours, 6:00pm to 6:00am, and the amount of
time spent foraging in the open (as opposed to
foraging in the brushy parts of the pen) will be
The predator: A Long-eared owl
measured.
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A natural nocturnal predator of rodents in the Nevada desert is the long-eared owl (Asio
otus). They are buoyant fliers, gliding noiselessly even when flapping their wings. They
maneuver well and can fly through fairly dense brush, low to the ground, listening for
prey. Experimental evidence has previously shown that the long-eared owl is an
effective predator, especially in the light of the moon. This is thought to be because (a)
their prey is easier to see, and (b) the prey cast shadows, which increase the likelihood of
being spotted from the air. A sound system has been created with speakers placed in the
corners of the pen, creating treatments of “owl-present” and “owl-absent.” In the “owlpresent” treatment the owl’s mating calls will be sounded at random times during the
evening, suggesting the presence of owls to the deer mice. The set up of the pen is
diagrammed below, showing the speakers as owl icons. The locations of the brushy areas
are shown as little plant icons.
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The investigators expect that if the first theory – brush preference – is true, the deer mice
should spend about the same time in the open with the owls present as when the owls are
absent. On the other hand, if the second theory – predation risk reduction – is true, there
should be a difference in the amount of time spent in the open over the 12-hour period
depending on whether owls are thought to be present or not. Because each trial is run
only at night for a 12-hour period, only one trial can be run per 24-hour cycle. The
experiment will consist of a sequence of 60 trials, and there is some concern that over the
length of the experiment (60 days) the moon will provide different illumination over the
pen. It is felt that the deer mice, knowing from experience that the owls are better
predators during moonlight, might forage differently over different phases of the moon.
Because of the large number of potentially confounding variables, the investigators are
not sure how to assign the treatments over the course of the experiment, and have asked
you for assistance. You may order any needed random number generator from the
Charles River Experimental Design Obligato (CREDO) store at a nominal cost.
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Golly, Toto, these don’t look like corn plants!
Canary Island palms (Phoenix canariensis) may live for as long as 100 years. These
massive trees grow up to 60' tall, and support a huge crown of over 50 leaves that can
reach 18' in length. They may be planted in warm areas in the western U.S. in places
such as Arizona, California and Nevada. As an example, Palm Springs, CA, has almost
3,000 palm trees, maintained by the city, many of which are planted along streets to
enhance the natural scenic beauty.
With proper care, fertilization and water, Canary Island palms are very hardy – just the
sort of tree you would want to live in if you were a bud rot fungus (Phytophthora
palmivora). Bud rot fungus is spread from tree to tree by contaminated garden tools, and
unfortunately may actually kill the palm tree. This, of course, is not appreciated by those
who enjoy the Canary Island palm’s beauty.
Bud rot fungus can be controlled by two
methods, chemical and non-chemical.
Chemical controls such as mancozeb or basic
copper sulfate have been shown to be effective
in battling bud rot fungus, but use of chemicals
is not advisable in windy open areas. Another
strategy for controlling bud rot fungus is the
“virgin soil” technique. With this technique,
soil in which palms have never grown is placed
around the roots of the palm trees, protecting
the roots from the fungus. This method has the
advantages of being nonhazardous.
The City of Palm Slinky (sister city of Palm
Springs) is considering planting Canary Palms
along its main street, but has not decided
whether to use the chemical or non-chemical
treatment. They would like you to design an
experiment to see which method is more
effective at preventing bud rot fungus. Their main street – where the City fathers and
mothers wish to plant the palms – runs for about 5 miles through a variety of
neighborhoods, both business and residential, and has varying amounts of traffic at
different locations. They are planning on having trees on both sides of the road.
Your task is to design an experiment to compare the two methods of bud rot fungus
control. The City fathers and mothers will fund 60 trees for experimental purposes,
which of course can be ordered from the Charles River Experimental Design Obligato
(CREDO) store at a nominal cost.
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Bambi vs. Corn
The typical garden-variety deer
(Odocoileus spp.) can be a serious
problem even beyond the garden.
Deer are thought to cause more
damage to agricultural products than
any other species of wildlife. In
1993 over $30 million worth of corn
was lost to deer just in the 10 largest
corn-producing states. In late June
and early July corn reaches the
silking–tasseling stage of growth.
Cornfields are at particular risk
during the silking/tasseling stage
because (a) the corn is very tasty,
and (b) damage during this period seriously reduces yield.
It might be possible for farmers to minimize the amount of damage to crops by using
visual and/or acoustic frightening devices during the silking/tasseling stage of corn
growth. These devices range from (a) less than distressing to (b) terror inducing.
(a)
(b)
One frightening device under consideration is a bio-acoustic device that would mimic
animal communication signals such as alarm calls. (An alarm call warns others of
possible danger.) These devices typically consist of an infrared detection system that
activates an audio component that transmits recorded alarm calls. Researchers have
studied the effects of such devices on birds, but little is known of their effect on
mammals. If effective with deer, bio-acoustic alternatives would meet with public
approval and in addition could be used in both rural and urban settings.
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Deer are most active at dusk and dawn, and are frequent feeders at night. They tend
to stay close to wooded areas for safety; farms in Iowa are frequently interspersed with
wooded areas.
You are to design an experiment to collect data that would allow you to test the
hypothesis that sound devices can reduce deer predation on corn. For purposes of this
experiment every right-thinking Iowa farmer has volunteered his acreage for use as test
plots; you have complete freedom to choose cornfields for treatment and control plots.
Not only that, every right-thinking Iowa parent has volunteered their teenagers’ boom
boxes (in hopes that you are conducting a multi-year experiment.) These boom boxes
came in various brands, with the Aiwa CD player (model CDC-X217) being the most
popular. Infra-red animal-activated alarm and distress call systems have been purchased
from the Charles River Experimental Design Obligato (CREDO) and the corn yield will
be measured by yield monitors linked to Global Positioning Systems on harvesting
equipment.
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Fly vs. Rainbow
The typical intrepid trout fisher-person uses artificial
floating lures called “dry flies.” The general idea is that
these flies, artfully made with wool, fur, and feathers to
mimic adult prey, are cast onto the waters and confuse
the typical Rainbow Trout (Oncorhynchus mykiss) just
enough to chomp soundly on the fly.
The optimal design of dry flies is a matter of
controversy . The consensus is that flies must be
properly presented to the Rainbow before it can be
Blue Winged Olive
tempted to strike. However, some skeptics claim that
the trout is less discriminating than the fisherperson and may not actually be able to tell
one dry fly from another. In order to address this important social issue, two commercial
dry flies (shown at right) have been selected as distinct treatments.
When fishing with a dry fly, the
process is to use a big stick (called a
pole) with a small rope (called a line)
that has a dry fly affixed at one end.
The dry fly is thrown (by a motion
called a “cast”) to a certain position
in the water. This throwing motion is
replicated until a trout chomps
(known as a “strike”) on the dry fly.
Rainbow trout are hatched in streams
and remain there until they reach 6” –
9” in length, at which time they will
travel to lakes or oceans to bulk up before returning to
their stream or river to spawn. They tend to prefer
clear, flowing waters with turbulence (for
oxygenation) and are found in streams with gravel,
rock, and sandy bottoms. Three relatively
inexperienced Wisconsin fishermen – who by purest
chance happen to be the researchers -- have selected a
2-mile stretch of a Wisconsin river suspected to be
fully inhabited with Rainbow trout for purposes of this
experiment. The river bottom varies in turbulence,
depth, texture (sand vs. clay) , and speed over the
Green Drake Wulff
course of the 2 miles.
Your task is to design an experiment to address the question of whether fish are more
likely to bite for one type of fly than the other. You should use blocking to control at
least one confounding variable.
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A different kind of confidence interval?
When estimating the sizes of bird populations wildlife biologists wear brightly colored
clothes during hunting season as a safety precaution. This strategy may have interesting
consequences for the estimation of the bird populations for different species. It is
possible that some species of birds may be more easily frightened into motion -- and
therefore are more visible -- because of the brightly colored vests. What is known as the
“species-confidence” hypothesis states that birds prefer colors similar to their own colors,
and will avoid individuals with different coloring. If this hypothesis is supported,
wildlife biologists wearing orange vests might overestimate the number of -- say -American robins (Turdus migratorius) because they see more of them in their sample, but
underestimate the number of -- say -- Carolina Chickadees (Parus carolinensis) because
they fly away sooner, undetected, when they see the orange vests.
Robin
Carolina Chickadee
The species confidence hypothesis could be tested by conducting an experiment.
Researchers could approach birds either wearing a brightly colored orange hunter vest or
the standard camouflage hunter vest, as determined by a coin flip. The distance from the
bird when it flushes and flies away (the “approach distance”) would be measured for each
bird. Two species, the American robin and the Carolina Chickadee have been chosen as
experimental units for the study. You have been given the task of designing an
experiment to collect data that will allow you to test the species confidence hypothesis.
Go for it!
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Snood Power!
The wild turkey, Meleagris gallopavo, is an example of a "sexually dimorphic species
exhibiting an array of extravagant behavioral and morphological [characteristics] that
serve no obvious function other than to attract mates." One of these characteristics is a
distensible process at the base of the upper mandible known as a “snood.” From the
female perspective better father turkeys will mean that the next generation of turkeys will
have a greater chance of survival; a big snood, it is suggested, is taken by the female to
indicate "good genes." Thus, from a biological standpoint it makes sense to select mates
from males with bigger snood. (Snoods?)
It is possible that the snood length
might also be regarded as a measure of
tough turkey maleness by male turkeys.
From the male turkey perspective, it
might not be a good idea to ruffle the
feathers of a big snooded male because
he might ruffle your feathers in
response.
Your task is to design an experiment to
address this question: does the male
turkey regard other males’ snood length
as a measure of don't-tread-on-me
capability? An experiment is
envisioned as follows. A male turkey
decoy is to be placed in a small arena
with a pile of birdseed nearby. (Using
a decoy will give direct control over
some possible confounding variables.
For example, a live turkey might provide a different aggressive display for different
situations, or be at different levels of hunger during the experiment.) A hungry live male
turkey would then be placed in the arena. It is reasoned that if male turkeys regard the
snood as indicative of a powerful turkey they will be less fearful of competing for food
against a short snooded turkey. (And, as anyone who has worked with turkeys can verify,
a turkey is NOT of sufficient awareness to be aware of his own snood size.)
You have budgeted for 60 male turkey subjects for your experiment, and can order the
animals to conform to your size specifications. For example, a 5 lb turkey would be
small; a 20 lb turkey would be “large.”
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References
These experimental designs have been adapted from reports of actual experiments
performed by some very creative people! They, of course, bear absolutely no
responsibility for these adaptations.
Britton, B. J. (1998) Does the fly matter? The CRACKPOT study in evidence based
trout fishing. British Medical Journal, 317:1678-80.
Buchholz, R. (1995) Female choice, parasite load and male ornamentation in wild
turkeys. Animal Behavior, 50, pp 929-943.
Buchholz, R., et al. (2004) Investigating the turkey’s ‘snood’ as a morphological marker
of heritable disease resistance. Journal of Animal Breeding and Genetics, 121(3), 176185.
Buchholz, R. (2004) Effects of parasitic infection on mate sampling by female wild
turkeys (Meleagris gallopavo): Should infected females be more or less choosy?
Behavioral Ecology 15 (4): 687-694.
Ellers, O. (1995) Behavioral control of swash riding in the clam Donax variabilis. The
Biological Bulletin 189(2), 120-127.
Gilsdorf, J. M., et al. (2004) Evaluation of a deer-activated bio-acoustic frightening
device for reducing deer damage in cornfields.
http://digitalcommons.unl.edu/icwdmother/14
Gutzwiller, K. J., & Marcum, H. A. (1997) Bird reactions to observer clothing color:
implications for distance-sampling techniques. Journal of Wildlife Management, 61(3):
935-947.
Kotler, B. P. (1984) Risk of predation and the structure of desert rodent communities.
Ecology 65(3), 689-701.
Kotler, B. P. (1991) Factors affecting gerbil foraging behavior and rates of owl
predation. Ecology 72(6), 2249-2260.
Turner, H J., Jr., & Belding, D. L. (1957) The tidal migrations of Donax variabilis Say.
Limnology and Oceanography, 2(2), 120-124.
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