A Case Study of Hypothesis-Based Science



A Case Study of Hypothesis-Based Science

From Observations to Question and Hypothesis . Many poisonous animals are brightly colored, with distinctive patterns in some species. This appearance is called warning coloration because it marks the animal as dangerous to potential predators. But there are also mimics. These imposters look like a poisonous species but are really harmless to predators. For example, a non-stinging insect called the flower fly is very similar in appearance to a stinging honeybee. The question that follows from these observations is: What is the function of such mimicry?

In 2001, a team of biologists designed a simple but clever set of experiments to test a hypothesis that was first suggested over a century earlier. Here's the hypothesis: Mimics (such as the flower fly) benefit because predators confuse them with the actual harmful species. Researchers David and Karin Pfennig, along with one of their college students, tested this hypothesis by studying mimicry in snakes that live in North and South Carolina. A poisonous snake called the eastern coral snake is marked by rings of red, yellow, and black. Predators rarely attack these snakes. A nonpoisonous snake named the scarlet kingsnake mimics the ringed coloration of the coral snake.

Testing a Prediction of the Hypothesis What is the explanation for this case of look-alike snakes? According to the mimicry hypothesis, the coral-snakelike appearance of kingsnakes repels predators. The hypothesis predicts that predators will attack snakes with the bright rings of red, yellow, and black less frequently than they will attack snakes lacking such warning coloration. To test this prediction, the researchers made hundreds of artificial kingsnakes out of wire and a claylike substance called plasticine. There were two types of artificial snakes: those with the red, yellow, and black ring pattern of coral snakes; and snakes with plain brown coloration.

The researchers placed equal numbers of the two types of artificial snakes in various sites throughout North and South Carolina. After four weeks, the team retrieved the artificial snakes and counted how many had been attacked by looking for bite or claw marks. The most common predators were foxes, coyotes, and raccoons, but black bears also attacked some of the artificial snakes.

Designing a Controlled Experiment Why did the experiment include artificial snakes that were plain brown along with the ringed snakes? A quick answer is that the contrast in coloration was necessary to see if predators attack snakes based on their color. If all the snakes were the same, the number of attacks would indicate nothing at all about the effect of the colored rings. This point illustrates an important requirement for designing experiments that test hypotheses. If you want to test the effect of one condition, you need to provide a contrasting condition as well. A condition that can differ within the experiment is called a variable. In the artificial snakes, the variable is the presence versus the absence of the colored rings. Most often, experiments test the effect of a difference in just one variable. An experiment that tests the effect of a single variable is called a controlled experiment.

By conducting a controlled experiment, scientists try to eliminate (control) other variables that could affect the outcome. This is not usually a simple task. For example, variables such as temperature or other weather conditions could influence the activities of predators in the snake experiment. In an ideal setting—such as a laboratory—scientists can keep temperature and other environmental conditions as constant as possible. But such control is usually impossible in a field experiment. And even in a laboratory, total regulation of all but one variable is often not practical.

Eliminating Unwanted Variables What is the solution to the problem of unwanted variables? Researchers divide the subjects (the artificial snakes, for example) into two groups: a control group and an experimental group. Since the snake experiment was designed to test the effect of the colored rings, the artificial snakes with the colored rings were the experimental group. The brown snakes served as a control group by showing what happens in the absence of colored rings. Everything else about the two snake groups was the same. For example, both ringed snakes and brown snakes were made of the same materials. Both kinds of snakes were placed at random in the same locations. Conditions such as light, temperature, and appetite of the predators varied, but both kinds of snakes were subject to the same variations. In this way, the brown snakes controlled, or cancelled out, the effects of the unwanted variables, leaving colored rings as the only consistent difference between the two groups of snakes. Then any difference in the number of attacks on the ringed snakes compared to the brown snakes could only have been due to the difference in coloration.

"If . . . , then . . ." Reasoning is a line of reasoning that assumes that “if” one thing is a cause, then a second thing will occur. Using the flowchart in Figure 2-15, you can follow the steps taken by the Pfennig team. Observations of snake coloration and attacks by predators led to a question. The Pfennigs posed a hypothesis that seemed reasonable based on other scientists' past research on different animal species. The Pfennigs then designed and performed a controlled experiment to test their hypothesis. The next step was analyzing the data.

Organizing Data and Interpreting Results Scientific inquiry is far from over once the data have been collected. Often, the results of an experiment only begin to make sense after much analysis of the data. For quantitative data, note again that it is often helpful to put the data in the form of a table or graph. These efforts may reveal patterns that were not obvious when the "raw" data were first collected.

The bar graph in Figure 2-16 summarizes the results of the artificial snake experiment. The graph also reinforces the purpose of using two groups of snakes. Of all attacks on artificial snakes, about 84 percent of attacks were on the plain brown snakes compared to about 16 percent for the snakes with colored rings. These data fit the prediction based on the mimicry hypothesis. The experiment supports the hypothesis that the kingsnakes' mimicry of coral-snake coloration helps protect against predators.

The research on look-alike snakes provides an example of how scientists use hypothesis-based science to test their explanation of natural phenonena. Notice again how hypothesis-based science works along with discovery science. Questions about nature usually arise from the observations of discovery science. Hypothesis-based science is a process for testing the possible answers to such questions.

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|Figure 2-15 |

|This flowchart summarizes the inquiry process followed |

|during the snake mimicry research. |

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Name______________________________________

1. Why are some animals brightly colored?

2. Why is it hypothesized that mimicry in nature is a benefit?

3. Describe the 3 kinds of snakes the researchers created

4. How did the brown snakes serve as a control?

5. What is the experimental variable being tested in this controlled experiment?

6. What is the researchers’ hypothesis?

7. What was the researcher’s evidence of attacks?

8a. What are some of the variables that cannot be controlled in this field experiment?

8b. Why is the effect of these variable minimized here?

9. Is the hypothesis supported by the data – explain.

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[pic]

Scarlet Kingsnake

[pic]

Eastern Coral Snake

[pic]

Fig. 2.16

Results of mimicry experiments using artificial snakes show a dramatic difference in the frequency of attacks on plain brown snakes compared to the snakes with colored rings.

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