Pick your battles

This exercise is a part of Educator Guide: Fight Like an Animal / View Guide

Directions: After students have had a chance to review the article “Fight like an animal,” lead a classroom discussion based on the questions that follow. These questions are intended to illustrate some of the steps in planning, carrying out and analyzing scientific experiments.


Discussion questions:

1. What are variables in an experiment?

Variables are factors, traits or conditions that can change in the course of an experiment. Variables can be qualitative (descriptive, like appearance or odor) or quantitative (measurable, like mass or length). Variables can also be continuous (any possible value, like position along a ruler) or discrete (only certain allowed values, such as the number of animals in a cage).

2. What are independent, dependent and confounding variables in an experiment?

An independent variable is a variable that changes during a scientific experiment but that is not changed by the other variables that are being measured. It is controlled by the scientist. The independent variable is often plotted on the horizontal (x) axis of a graph. Note: The independent variable doesn’t always change.

A dependent variable is a variable that changes during a scientific experiment based on other factors in the experiment. A researcher does not control it. For example, a test score is a dependent variable. It depends on how much someone studied, how difficult the test is and even how much sleep a student got the night before. The dependent variable is often plotted on the vertical (y) axis of a graph.

Confounding variables are factors other than the independent variable that may also affect the dependent variable. A good experiment should have one dependent variable and one or maybe two independent variables with value ranges that are methodically tested in the experiment. Confounding variables should be minimized by the experimental design, and any remaining confounding factors should be accounted for when the results are analyzed.

Extension prompts:

3. In the “Beetle body size and horn length” graph in the article “Fight like an animal,” what are the independent, dependent and possible confounding variables? What relationships among the variables does the graph show?

The independent variables are thorax width and beetle sex. The dependent variable is horn length. Possible confounding variables that are not addressed might be beetle age, health, presence of male competitors, presence of females and temperature. The graph shows that horn length is very short and relatively constant for females regardless of thorax width. The graph also shows that for males, horn length increases approximately linearly with thorax width (or more precisely, approximately linearly with any increase in thorax width beyond about 1.5 centimeters or so.)


Discussion questions:

1. Look up an original study for an animal mentioned in “Fight like an animal” and read that study’s abstract (citations are listed at the bottom of the online version of the Science News story). What are the independent and dependent variables, and what is the hypothesis for the relationship between those variables?

Example student responses:

Exaggerated rostra as weapons and the competitive assessment strategy of male giraffe weevils.
Dependent variable: contest duration.
Independent variable: body length difference.
Hypothesis: Contests are shorter when the two competing males have very different sizes, since the larger one quickly wins.

Male-male lethal combat in the quasi-gregarious parasitoid Anastatus disparis (Hymenoptera: Eupelmideae).
Dependent variable: fighting intensity.
Independent variables: competitor density, female presence.
Hypothesis: Fighting intensity increases with increasing competitor density and/or increasing female presence.

2. If you were to do a science research project on animal weapons, what species would you choose to study?

Nematodes, fig wasps, anemones, Caribbean rock mantis shrimp, Asian rhinoceros beetles, etc.

3. What central question about that species’s weaponry would you investigate?

  • How is aggressive behavior affected by the density of animals within an environment or by other factors?
  • How are male-male competitive behaviors and female-female competitive behaviors for [species] similar or different? How common is competitive behavior, instead of cooperative or sexual behavior, between members of the opposite sex for [species]?
  • Are there multiple uses for [weapon] in [species]?

Extension prompts:

4. What is your hypothesis (the answer you expect to find)? What are your independent and dependent variables?

Higher density of animals (independent variable) promotes more aggressive behavior (dependent variable). One sex (independent variable) is more competitive (dependent variable) than the other, depending on the species. Some weapons (independent variable) are dual-use (dependent variable), useful for fighting rivals but also helpful for mating or acquiring food.

5. What experiments could you conduct to investigate that question and hypothesis?

Putting animal populations into environments with different densities and measuring aggressive behavior. Testing same-sex aggression for both genders under various conditions. Determining weapon use when rivals are present and when they are not.

6. What supplies and requirements would you need to conduct your experiments?

  • A considerable number of animals. An appropriate habitat. Suitable food. Student safety and animal welfare approvals if needed.
  • Handbooks for caring for and studying that species (and telling male from female).
  • One or multiple video cameras to record animal behavior.
  • Computer and software to store and analyze recordings and other data.
  • Rulers and balances for measuring animal size and mass.
  • Thermometer to measure habitat temperature; perhaps even equipment to maintain a constant habitat temperature.
  • Nontoxic paint or another way to mark animals to identify them.
  • Compartments or barriers to separate animals or to control the size of their habitats.
  • Method to sedate animals (anesthetic, cold, smoke).
  • Humane method to kill animals if necessary.
  • Tools to dissect animals.
  • Dissecting microscope.

7. What measurements would you need to make?

Weapon length or mass from a live or dead animal. Volume or useful surface area of animal habitat. Habitat temperature. Number of animals within a habitat. Number of aggressive actions over a given time as recorded on video. Mating success as recorded on video (or confirmed by genetic testing if resources are available). Weapon use under various conditions as recorded on video.

8. How could you minimize errors in your experiments?

Make sure all instruments (balance, thermometer) are properly calibrated and used. Conduct multiple trials with the same animal. Conduct multiple trials with other animals of the same species. Make measurements several times over the course of the experiment. Watch out for other variables that might be affecting the results (temperature, humidity, light, time of day, amount of food, noise, vibration and other animals in a nearby habitat).

9. How could you graph your results?

Plot (independent variable) density of animals per habitat volume or area on x-axis and (dependent variable) number of aggressive events per hour on y-axis. Plot number of behavioral events per hour over the course of each day. Use bar graphs to compare results for different animals. Use pie charts to quantify animals’ reproductive success by showing the number of offspring that each individual animal produced. Include error bars wherever appropriate.


Discussion questions:

1. What engineering projects could you do based on animal weapons?

  • How can [human weapon or tool] be improved based on the performance of [animal weapon]?
  • How can [human material or tool] be improved based on the performance of [animal armor]?
  • How much [force/pressure/etc.] can [animal weapon] produce?
  • How much [force/pressure/etc.] can [animal armor] withstand?
  • How can [animal] be genetically engineered to improve their weapon or give them new weapons?  

2. What supplies would you need to conduct your experiments?

  • Methods of applying consistent force (springs, rubber bands, motors, levers, etc.).
  • Methods of measuring force or pressure (depth of impression left in a deformable material, spring scale, piezoelectric force sensors, air or water pressure gauges, etc.).
  • Methods of evaluating weapon efficiency (measure length, mass, angles, shape, density, hardness, etc.).
  • Methods of evaluating armor efficiency (measure thickness, density, hardness, etc.).
  • Video camera and computer to record experimental trials and play back in slow motion if necessary.
  • Software to analyze genes involved in natural weapons and propose modified or new genes that could improve the weapons in an animal.

Extension prompts:

3. What practical applications might result from your project?

Improving the performance of human-designed weapons or tools. Improving the strength per weight, reducing the cost or improving how recyclable certain human materials are.