1. Ask students, “What affects whether you gain weight or lose weight?”

Record all responses on the board. On the basis of their experiences in the previous lessons, students should identify activity level and the amount of food eaten as important factors that determine weight gain or loss. Others factors may be less apparent to students, such as variations in BMR, gender, and heredity. You may need to lead students to these genetic factors, or you can add them yourself at appropriate times in the discussion.

Content Standard G:
Scientists formulate and test their explanations of nature using observation, experiments, and theoretical and mathematical models.

2. Ask students, “How can scientists determine whether these factors affect energy balance and weight change?”

From earlier lessons, students have a sense that physical activity and food intake alter energy balance. Make the point that in order to conclude that these things really do affect energy balance and weight change, scientists need evidence obtained through observation and experimentation. Their conclusions must be verified by repeated tests.

Students may suggest that scientists prescribe specific amounts of food and physical activity for people and measure the impact on their weight. Follow this suggestion with the questions in Steps 3 and 4.

3. Ask students, “What are the advantages of using humans for research on energy balance?”

A major advantage is that humans demonstrate how humans behave. You can give them instructions that you know are understood. They can communicate with you.

4. Follow this by asking, “Can you suggest any disadvantages of using humans for the experiment you suggested?”

A major disadvantage is that it would be difficult to control all of the variables in the experiment. It would be very difficult, if not impossible, to maintain large groups of humans in exactly the same environmental conditions. There are also ethical issues to consider, such as the possibility of negative health effects on experimental subjects. It would be impossible to rule out the impact of genetics on energy balance because large numbers of genetically identical humans do not exist. Finally, asking people to record their food intake and physical activities may cause them to change their usual behavior patterns.

Tip from the field test: To emphasize the last point, remind students about the physical activity diaries they completed in Lesson 1. Ask them whether they changed their activities because they knew they would be recording them in their diaries.

5. Ask, “How could you get around these disadvantages?”

If students do not suggest using animal models, point out that this is a common way to study phenomena that affect both humans and other animals. In particular, all mammals have a similar physiology, so in many cases, what is true for mice, rats, or goats is also true for humans.

One advantage of using animal models is that researchers can control the animal’s environment. Students may not realize that another advantage is that genetically identical animals can be produced and used. Using the analogy of identical twins may help students understand this. By carefully selecting and breeding mice for many generations, scientists have produced genetically identical strains of mice. All mice in such strains could be considered identical twins of each other. Male and female mice differ only by those genes involved in determining sex.

You may want to provide some examples of the many beneficial outcomes from experimentation involving laboratory animals. The benefits include the discovery of the causes and prevention of, and effective treatments for, many infectious diseases and the development and perfection of surgical techniques. For example, Louis Pasteur used rabbits and guinea pigs to identify the bacteria that cause anthrax and to develop a vaccine against this disease. Successful open-heart surgery and organ transplants are based on years of animal experimentation.

6. Ask, “What are disadvantages of using animals other than humans for research on energy balance?”

A major disadvantage is that the animals may not respond exactly the same as humans. An animal’s physiology may or may not be similar enough to a human’s to make comparisons. Predictions about human responses to particular conditions can be made based on the results of experiments with animal models. If the predictions are accurate, the appropriateness of the animal model is confirmed. At the end of this lesson, students make predictions about weight changes in humans based on the results of their experiments with mice. They then examine existing evidence about human weight change to decide whether mice are useful models for investigating energy balance in humans.

Teacher note: Ethical treatment is a concern for animal subjects as well as human subjects. However, try to avoid emotional issues by pointing out that scientists who use animals in their research must comply with stringent guidelines for humane treatment of the animals. For this discussion, focus on the limitations of using human subjects and the advantages and limitations of animal models.

1. Display a transparency made from Master 4.1, Award to Study Factors Affecting Energy Balance, and read it with the class.

The memo describes a research grant given to the “Energy Balance Institute.” Students play the role of research scientists who design and analyze experiments to answer the three research questions in the memo. The first research question is investigated in Activity 1, and the second and third research questions are investigated in Activity 2.

2. Organize students into groups of three. Number each group sequentially beginning with 1. Explain that the groups are research teams at the Energy Balance Institute assigned to design and analyze experiments to answer the three research questions.
3. Distribute a copy of Master 4.3, Reference Manual for Scientific Research, to each team.

Explain that, just as in Lesson 3, the reference manual in this lesson contains helpful information. Point out several sections of the manual. The reference manual provides useful information about laboratory mice, including how to raise mice in the laboratory, and, even more important for students’ work in this lesson, information about writing hypotheses, designing experiments, analyzing data, interpreting results, and drawing conclusions.

4. Tell students that a research technician has completed an experiment to answer the first research question. Distribute a copy of the results, Master 4.4, Weights of Infant Mice from Strains A and B over Time, to each team.
5. Direct students to examine the tables and give them three to five minutes to write three observations about the data. List their observations on the board as they share them with the class.

Observations include the following: all mice have the same initial weight; they gain weight steadily for about six weeks and then maintain a constant weight; there is individual variation in the amount of weight gained; and overall, Strain B mice weigh more at eight weeks than Strain A mice.

6. Point out that the data collected in Experiment 1 focus on weight gain. Ask, “What does weight gain have to do with energy balance?”

Assessment:
This question also gives you the opportunity to assess informally what students have learned about energy balance from the previous lessons. They should be able to explain that in order to grow and gain weight, the mice have consumed more calories in their food than they are expending in their physical activities, or Energy_in = Energy_out + Energy_growth + Energy_stored.

7. Distribute a piece of graph paper (from classroom supplies or photocopies of Master 4.5, Graph Paper) to each student. Explain that researchers find it useful to graph experimental data to help them interpret their results.
8. Direct teams to prepare line graphs from the data, using “Week” on the x-axis and “Average Weight” on the y-axis.

The data for Strains A and B should be plotted on the same graph using different colors or different symbols for data from the different strains. Tell students to include a legend that identifies which line corresponds to which strain’s data. Allow about 20 minutes for students to complete their graphs.

9. While students complete their graphs, distribute one copy of Master 4.2, Memo to the Director on Research Question 1, to each student. Tell them to complete the memo when they have finished constructing their graphs. Explain that they will present their findings at the end of class.
10. Reconvene the class and ask three or four teams to show their graphs. Ask three or four other teams to give their answers to the questions on the memo. Ask the remaining teams to give their conclusion about energy balance.

Students should use the results of the experiment to justify their conclusions. Allow the other teams in the class to challenge the graphs, answers, and conclusions of their classmates, based on the results of the experiment.

Students should conclude that although the overall growth patterns of the two strains of mice are the same, genetics does affect the amount of weight gained.

Assessment:
Collect and review Master 4.2, Memo to the Director on Research Question 1, from each group.

Tip from the field test: Students may feel that 5 g is not much difference in final weight. If so, make the comparison to humans: 5 g in a 35-g mouse correlates to 15 pounds in a 100-pound human (5 g and 15 pounds are both about 15 percent of the total body weight). Thus, comparing mice weighing 30 g and 35 g is like comparing humans weighing 85 pounds and 100 pounds.

11. Collect Master 4.3, Reference Manual for Scientific Research. Remind students about Research Questions 2 and 3. Tell them they will have the opportunity to design an experiment and collect and analyze the data for one of those two questions in Activity 2.

You will redistribute copies of the reference manual when students begin Activity 2.

1. Organize students into their research teams from Activity 1. Display Master 4.10, Next Research Assignment. Read this memo with the class.

The memo, from the institute research director, assigns even-numbered teams to conduct an experiment to answer Research Question 2 (the effect of food availability on energy balance in juvenile mice) and odd-numbered teams to conduct an experiment to answer Research Question 3 (the effect of exercise on energy balance in adult mice).

2. Give each team a copy of Master 4.11, Experimental Design for Research Question 2, or Master 4.12, Experimental Design for Research Question 3, as appropriate. Redistribute a copy of Master 4.3, Reference Manual for Scientific Research, to each team.

Content Standard A:
Design and conduct a scientific investigation.

3. Instruct teams to complete their master by first writing a hypothesis for their research question and then selecting two groups of mice appropriate for testing their hypothesis.

Remind students that the reference manual provides advice for developing a hypothesis and selecting research subjects. Four groups of mice will be available to students, and they may select any two groups for their experiment.

Assessment:
Move among the groups and listen as students develop their hypotheses and plan their experiments. Check that hypotheses are appropriate for the research question and that experiments are appropriate for the hypotheses.

4. Give the research teams the appropriate data sets for the two groups of mice they selected.

Although students can select any pair of the four mouse groups available for their experiment, only two pairs are appropriate for answering each research question.

For Research Question 2 (juvenile mice), the correct pairs of mouse groups are Master 4.13, Weights of Juvenile Mice with Limited Food and Regular Exercise over Time, with Master 4.14, Weights of Juvenile Mice with Unlimited Food and Regular Exercise over Time, or Master 4.15, Weights of Juvenile Mice with Limited Food and No Exercise over Time, with Master 4.16, Weights of Juvenile Mice with Unlimited Food and No Exercise over Time. The second pair of mouse groups is probably more appropriate because no exercise is the standard condition for raising mice. However, the first pair of mouse groups also allows students to answer the research question. Either pair leads students to the same conclusion: juvenile mice with limited food gain less weight than juvenile mice with unlimited food.

Content Standard C:
Some traits of an organism are inherited and others result from interactions with the environment. Behavior is one kind of response an organism can make to an internal or environmental stimulus.

For Research Question 3 (adult mice), the correct pairs of mouse groups are Master 4.17, Weights of Adult Mice with No Exercise and Unlimited Food over Time, with Master 4.18, Weights of Adult Mice with Regular Exercise and Unlimited Food over Time, or Master 4.19, Weights of Adult Mice with No Exercise and Limited Food over Time, with Master 4.20, Weights of Adult Mice with Regular Exercise and Limited Food over Time. The first pair of mouse groups is probably more appropriate because unlimited food is the standard condition for raising mice. However, the second pair of mouse groups also allows students to answer the research question. Either pair leads students to the same conclusion: adult mice that exercise regularly lose weight, while adult mice that do not exercise regularly gain weight. However, the amounts of weight gain or loss vary considerably depending on food availability.

Using other pairs of mouse groups may answer a research question different from the one assigned to students, or students may not be able to determine which factor (exercise or food availability) has affected weight change. For example, if students compare weight gain in juvenile mice that had unlimited food and no exercise with weight gain in juvenile mice that had limited food and regular exercise, they cannot determine whether it was lack of food or regular exercise that limited weight gain in the second group.

5. Give each student one piece of graph paper and tell them to prepare line graphs of their data following the same procedure as for Activity 1.

Assessment:
You can collect the completed memos and review them to evaluate students’ understanding of data analysis and interpretation of the results.

6. Distribute copies of either Master 4.6, Memo to the Director on Research Question 2, or Master 4.7, Memo to the Director on Research Question 3, as appropriate, to each student while they prepare their graphs. Tell them to complete the memo by entering the results of their experiment.

Move among the teams during this time, answering questions and providing suggestions as necessary. Point out that the reference manual provides guidance for analyzing data and drawing conclusions from graphs.

7. When students have completed their graphs and memos, pair odd- and even-numbered teams and tell them to share their results. Encourage teams to challenge their partner team’s conclusions if they are not supported by the data.

Content Standard G:
Scientists formulate and test their explanations of nature using observation, experiments, and theoretical and mathematical models.

1. Reconvene the class and display Master 4.8, Summary of Research Findings. Ask students to summarize what they learned about energy balance by providing evidence-based answers to each of the three research questions.
2. Ask the class, “If what you learned about energy balance in mice is true for humans, what would you predict about weight change for human adults who consume their usual number of calories but increase daily exercise?”

On the basis of the results of Experiment 3 with mice, students should predict that adult humans would lose weight.

Assessment:
This is an opportunity to assess understanding of the experimental results. Students should be able to explain that among infant mice, weight gain was affected by genetics (Experiment 1); among juvenile mice, greater food consumption resulted in increased weight gain, regardless of exercise (Experiment 2); and among adult mice, no exercise resulted in weight gain while regular exercise resulted in weight loss, although the amounts of gain or loss were affected by food availability (Experiment 3).

3. Display Master 4.9, Impact of Exercise on Weight of Adult Males. Explain that the graph compares two groups of overweight adult males. Both groups consumed their usual number of calories for 12 weeks, but the exercise group added daily exercise.
4. Then ask, “Is your prediction confirmed by the evidence in this graph?”

Students should conclude that the information supports their prediction about energy balance in humans.

5. Ask, “On the basis of these results, can experiments with mice be used to test hypotheses about energy balance in humans?”

Students should conclude that mice are a good animal model for investigating energy balance in humans.

6. Set the stage for Lesson 5 by asking students, “Why does it matter whether adults gain weight?” Entertain several responses, then tell students that they will consider health concerns related to weight gain in Lesson 5.

Possible responses include that weight gain in humans matters for appearance, for good health, or for economic reasons. Accept all appropriate responses, but use this as an opportunity to emphasize that healthcare professionals are concerned about excessive weight gain in adults because of its impact on health.

1. What is the advantage of plotting the average weights each week rather than all the individual weights?

Students will probably recognize that plotting all the data results in a cluttered graph that is difficult to interpret. It would also take them much longer to draw such a graph. Plotting the average data makes interpretation easier. Students may not realize that using the average also helps minimize distortions in the results due to individual variation. Point out the individual variation in the data and ask the question (included on Master 4.2), Why do you suppose the technician collected data from 10 mice of each strain rather than just 1 or 2 mice?

2. Why is a line graph appropriate for the data on weight change?

Line graphs are used for measured data that relate trends in the measured characteristic, especially trends across time. The data collected in the experiments in this lesson are continuous; that is, the mice gained or lost weight continuously over the experimental time period. For example, you could accurately determine the unmeasured average weight of the infant mice at 2½ weeks by finding the y-coordinate at the midpoint of the line that connects their measured average weights at 1 week and 2 weeks. In other words, the points on the line in between the two measured intervals are logically meaningful.

3. Why is a bar graph appropriate for the data from Lesson 1, the amount of time spent at different physical activity levels?

A bar graph is used for data that are placed into qualitative categories. In Lesson 1, the five physical activity levels are qualitative categories. It is not appropriate to use a line to connect the amount of time spent in activities at the different levels because these levels are not expected to depict a continuing trend. For example, if an individual spent eight hours resting, you would not necessarily expect a trend of decreasing or increasing amounts of time spent at each higher level of activity. On the other hand, for the current Lesson, plotting the data as a bar graph would imply that there is not necessarily a correlation between the measured weights at each weekly interval.

4. What additional research questions could you answer using the groups of animals available to you?

Students could answer the research question, Does exercise affect weight gain in juvenile mice? by comparing juvenile mice with unlimited food and regular exercise with juvenile mice with unlimited food and no exercise or by comparing juvenile mice with limited food and regular exercise with juvenile mice with limited food and no exercise. The first pair is probably most appropriate because unlimited food is the standard condition for raising mice.

Similarly they could answer the research question, Does the amount of food available affect weight gain or loss in adult mice? by comparing adult mice with no exercise and unlimited food with adult mice with no exercise and limited food or by comparing adult mice with regular exercise and unlimited food with adult mice with regular exercise and limited food. The first pair is probably most appropriate because no exercise is the standard condition for raising mice.

5. Can you compare the effect of age on weight change in mice using the results from your experiments?

Students cannot do this directly with the Web version of the activity (the program does not allow students to pair a group of juvenile mice with a group of adult mice). However, they can compare final graphs for juvenile and adult mice that experienced the same conditions (for example, unlimited food and regular exercise). Point out that the adult experiments were conducted across five months while the juvenile experiments were conducted across five weeks. Students could compare weight changes after one month by comparing the adult averages at Month 1 to the juvenile averages at Week 4.