} CAS: Teachers - Tree Frogs and Evaporation

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Connected Experience: Tree Frogs and Evaporation


In this laboratory activity, studentswill perform an investigation that illustrates how surface area,anda suite of external conditions,may affecttherate of evaporationfrom an object, using a model frog made from sponge.They will learn aboutwhy a frog’s skin needs to stay moist and play a game that illustrates how surface area can affect evaporation. Throughout theprogram, students will apply scientific process skillsby making a prediction, following experimental procedure, controlling variables, recording data, and analyzing graphs.


In this lab, students will:
  1. apply the scientific practices by making predictions, carrying out an experiment, recording data, analyzing graphs, and evaluating the utility of an experimental model to explore a real-world phenomenon;
  2. understand how surface area affects evaporation;
  3. discuss potential environmental factors affecting frog survival in the rainforest.


  • Teaching Visuals
  • thin sponges (you will need 1.5 3?x4? sponges per group)
  • Frog Templates
  • small plastic weighing dishes (2 per group)
  • scales, accurate to at least 0.01g (groups can share if necessary)
  • small beaker or cup with water (2 per group)
  • eye dropper (2 per group)
  • Data Sheet (1 per group)
  • Student Worksheet (1 per student)
  • Condition Cards (optional)
  • small dish (such as a 10 cm petri dish or  the bottom of a plastic cup) (optional)
  • flat tray or plate for each group at least a foot in diameter (optional)
  • 1-2 bags of dried black beans (optional)
  • chopsticks (optional)
  • rulers (optional)


  • evaporate: to change from liquid water to water vapor
  • bromeliad: a tropical plant that grows on the branches of other rainforest trees, sheltering tree frogs in the pockets of water that form between leaves
  • respiration: the process by which an animal exchanges gases with its environment, capturing oxygen and releasing carbon dioxide as waste
  • mucus: a viscous, slimy substance secreted by cells to protect surfaces and keep them moist
  • surface area:  the total area of the surface of a three-dimensional object or animal (e.g. the area of all sides of a frog: its belly, back, legs, head, etc)
  • surface-area-to-volume ratio: the amount of surface area per unit volume of an object.  For a given shape, the surface-area-to-volume ratio is inversely proportional to size.  For example, a two inch cube will have half the surface-area-to-volume ratio of a one-inch cube.



  1. Print out Data Sheets, 1 per team, and Student Worksheets, 1 per student.
  2. Cut out sponge frogs using the large and small frog templates.  Each group of 4 to 6 students will need one large and one small frog.  Use thin sponges (about 1 cm thick) and make sure the sponges have the same type of surface on all sides (no scrubbers).  You may choose to have students cut them out on their own.
  3. Fill beakers with about 2 inches of water and place droppers in beakers.
  4. Place scales where each team has access to one.


  1. If you have already come to the Academy on a field trip, have them think back to their experience in the Rainforest. If they haven’t been to the Academy, have them just imagine what it might be like.  Ask how they might feel in a rainforest?  What would they see and hear? When you visit, we recommend you have students do the Troubled Tree Frog Scavenger Hunt.
  2. Collect student ideas about frogs.  What do they know about them? Where do they live? What do they feel like? Make sure to emphasize that a frog’s skin is wet.
  3. You may also wish to play the Bean Grab Race (see Extensions) here to help them conceptualize surface area before they begin.  Alternatively, have students do the Dry My Laundry!
  4. Set the stage that they are biologists in a rainforest and they study tree frogs – 2 different species (show a visual of the two frogs). Ask them to point out the differences between the two species (size only).  Explain that there has been unusually dry air in the last few days and that they are worried that the frogs might get dehydrated.  Explain that they will be running an experiment to see which of the two frogs might be in danger.


Part 1.  Experimental setup

  1. Pass out the Student Worksheet to each student.  Ask them to write down the question that is motivating the experiment (Will large or small frogs dry out faster?)
  2. Point out that they will not be using real frogs.  Instead, they will be using a model to represent a frog.  Have students brainstorm the similarities and differences between a real tree frog and a sponge frog.
  3. Introduce groups to their materials.  Students will work in groups of 4 to 6 students (then the group will split into large frog and a small frog pairs or threes.)   Each group should have two sponge frogs, one large and one small, or have students cut their own from the provided templates.
  4. Students will also have two weighing dishes where the sponge frogs will be kept.  Make sure the groups label one dish “large” for the large sponge frog and the other dish “small” for the small sponge frog.
  5. Have students point out the tool they will be using to collect data – a scale.  
  6. Practice using the scale by weighing the small frog dish. If many groups will be sharing a scale, make sure to demonstrate how to use it.  
  7. Next have students look at the data sheet.  Point out where the small frog dish weight should be recorded (in the “Small Frog” table, START column, “Dish” row).  Have each group weight their small and large dish.  The pair or three assigned to the different sized frog within each group should take responsibility for doing the work on only their assigned size.
  8. Tell students to place each frog in its appropriate dish (large frogs in “Large” and small frogs in “Small”).  From now on, the students should not touch the sponges, only the dishes. You can briefly discuss why (touching transfers water on to a finger or could squeeze the water out).
  9. Pass out the water beakers.  Model how to use droppers.  Tell each pair/three to drop EXACTLY 50 drops onto their assigned frog, spreading the drops evenly across the back surface only.  Then weigh the wet frog+dish, and add the weight to the data sheet in START column. Make sure the large and small frogs are being done at the same time.
  10. Students can then do the calculation of frog-only weight on data sheet.  Discuss the calculation on the data sheet, making sure everyone understands how to get the accurate weight of the frog only (by subtracting the dish weight from the frog+dish weight).  Use the analogy of weighing yourself at the doctor’s office and that you+clothes is not really your true weight.
  11. The frogs need to dry out for at least 30 minutes but not more than 50 minutes. While you wait, proceed to Part Two.

Part 2.  Real Frogs and Predictions

  1. Ask the class to share ideas about how frogs use water (swimming, laying eggs, drinking, finding food).
  2. If they haven’t already talked about it, ask if a tadpole needs water?  Remind them that a tadpole has gills.  Then, discuss what happens to a frog after it goes through metamorphosis. Do they still use gills to breath in the water?  (No, they use lungs and skin).  Briefly explain the difference between human and frog lungs and why they need to breath with their skin as well (see Teacher Background). Have students note what the lining of their lungs, nose, and mouth are covered in (mucus!) and that we need this moist layer to breath.  Since a frog also breathes through its skin, the skin must also stay covered in moist mucus.  
  3. If you haven’t already played the Bean Grabbing Race, this is a good time to have them play the game before making predictions.
  4. Redirect students to their sponge frogs.  How are the two different? Color, material, location, temperature?  The only difference is size.  How might this affect how they dry out? Refer back to the Bean Grabbing Race, if they have played it.
  5. Knowing that the surface area of their backs is the only difference, which size frog is likely to lose more water?  Ask students to discuss with their groups and write a prediction in the box on their worksheets.  Be sure to add “because” reasoning.
  6. If you still have time while the sponges are drying for 30 to 50 minutes, you can have students do the A Perfect Home Card Sort (see Extensions).

Part 3.  End Data Collection

  1. Just before you wrap up the experiment, ask students to predict if the wet frogs will have lost or gained weight?  Why? Also, ask them if they expect the dish to have lost or gained weight (No, plastic doesn’t evaporate or absorb water).  There may be some valid debate here, but tell them that any water in the dish will be counted as part of the frog still.
  2. After the sponge frogs have dried out for 30 to 50 minutes, ask the students to weigh their frogs+dish again and enter into END column on the data sheet.  Make sure that the large and small frogs are done at the same time.  
  3. Students should then calculate the true weight of their frog sponges by subtracting the dish weight as they did for the START column.  
  4. Lastly, explain that the important value they need is how much water each sponge lost.  How can we find this value?  Model how to subtract END weight from START weight.  Have them do this for the small and large frogs.

Part 4.  Data Sharing and Analyzing

  1. Before pooling the class data, ask each group to compare the water loss of their group’s two frogs.  To illustrate the difference, each student should make a bar graph of the group’s results on their worksheets, drawing their best approximation by hand.  Depending on teacher preference and grade level, they can create a scale for the y-axis on their own or use one decided as a group ahead of time.
  2. Pool the class data for analysis.  Each group should report the results of both their small frogs and large frogs and the data should be entered onto the board or into the class Excel spreadsheet projected onto a screen (or printed out and copied to each group).
  3. Ask the students to study the numbers in the table. What do they observe?  Do the numbers conform to expectations?  How do the large and small frogs’ values differ?
  4. If using a computer and projector convert the data to a bar graph in Excel, or have each group add their bars to a class-wide graph on the board. Explain that graphs are another way to represent data, and a visual display sometimes helps us see trends.
  5. Challenge each group to find its own data points on the class graph
  6. Which frogs lost the most water? By how much? Can a general conclusion be made about the relationship between surface area and evaporative water loss?  (Remember to guide the discussion toward the affects of surface area, not body size in general)  If you wish, this is a good place to do the Surface Area Calculation Challenge (see Extensions).  For your reference, if you used the frog templates printed at 100% the surface area for the large frog is about 40 cm2 and the small frog is about 20 cm2.
  7. Are the data from any group considerably different from the others? What might explain this?  Did everyone do the same procedure? Anyone touch sponges? Blow on them? Extra drops? Spread out drops? Etc.  Remind them of conditions that should have remained constant but that might have affected the rate of evaporation (all samples taken at same time, heat, humidity, light, original water amount, i.e. with the same environmental conditions).  You can have them brainstorm ideas on their worksheet.
  8. If students are comfortable with calculating averages, you might wish to them take the average of the data to come up with an overall conclusion about the class data.
  9. Have students compose conclusion about what the results show.  You can have them consider explanations for why they saw what they did.  There is space on the worksheet for this.
  10. Finally, have students reflect on ways to change the methods/procedures of the experiment to eliminate some of the variation between frogs of the same size.

Part 5.   Conclusions and real-world implications (and complications!)

  1. Ask students to share their conclusions from the experiment.  Remind them that this experiment is really just about surface area only, not overall size or volume.
  2. To illustrate the impact of surface area only on evaporation, use the example of the two different sized swimming pools and how the large shallow one with lots of surface exposed to the drying effects of air, sun, and wind would evaporate faster than the deep narrow pool.  If you did the Bean Grab Race, refer back to that activity as well.
  3. Then relate the pool or Bean Grab Race example to tree frogs, ask the students to imagine two frogs of equal volume (same amount of water inside their bodies) in the same environmental conditions: one frog is extremely flat and wide and the other frog is shaped more like a ball.  If they were able to measure the surface area of the two tree frogs, which one would have the greater area? (The flattened frog.)  And which one would have the greatest water loss in the same amount of time? (The flattened frog.)
  4. Discuss the differences between the experimental sponge frogs and real frogs.  They can look back to what they wrote at the start of the activity and have them to add more if they want.  You might ask them:
    • Are real-life tree frogs perfectly flat or perfectly round or all the same volume? (No)  
    • Can frogs differ in their volume (i.e. how much water they can store) as well as their surface area? (Yes)
    • How much do they differ in their general shape or body plan? (Not much, all tree frogs are similarly shaped but can vary in their size.)  
    • Do smaller frogs have less volume or more volume of water inside them relative to their surface area than larger frogs? (Less)  
  5. Have students consider if they can answer their original question (Will large or small frogs dry out faster?) with the results of their sponge experiment.  
  6. If they need help getting there, emphasize that in their experiment the volume of water was the same for both sizes, but that this is not true in real frogs.  If a small and large real frog were caught in a hot, dry wind, the small frog would dehydrate faster (unlike in the sponge experiment).  This is because a smaller frog has a smaller amount of water stored in its body relative to its surface area of skin exposed to the drying air.  
  7. You may choose to further this discussion since this real-world complication demonstrates an important concept in science: that most experiments can test only one thing at a time and can’t take into account all the complexity of the real world (i.e. frogs are not perfectly flat and have many more differences between them than just surface area).  Surface area is positively correlated with evaporation rate; as surface area increases, evaporation rate increase. But in the natural world an animal’s rate of water loss is equally dependent on its surface-area-to-volume ratio.  A mouse is much more at risk of dehydration than an elephant!

Part 6.   Final Wrap-up

  1. Because tree frogs breathe through their skin in addition to their lungs, they need to stay moist and always maintain a layer of mucous on their bodies. How do tree frogs in the wild decrease their risk of drying out? With their behavior and habitat choice.  They avoid direct sunlight, they hide in the shade of leaves, and they try to stay close to water or in a humid environment at all times (e.g. bromeliads).  
  2. If you haven’t yet done A Perfect Home Card Sort (see Extensions), you can have students do the activity to learn more about what makes a good tree frog home.


Frog Observations at the Academy

  1. If you will be visiting the Academy before doing this activity, we recommend you have students do the Troubled Tree Frog Scavenger Hunt, which will help them observe a variety of frogs in our Rainforest exhibit.  
  2. Alternately, you could use this hunt between Parts 4 and 5 as a way to make better comparisons between real frogs and the sponge models.

Bean Grabbing Race

  1. Divide class into groups of 6 (no fewer than 5), and give each group a container of enough beans to fill up the small dish with a few layers.  Hand out the one small dish and larger tray/plate, one of each per group.
  2. Ask the students in each group to cluster around the larger tray and pick one student to spread the beans out on the large tray/plate.
  3. For students in grades 6 and above, this activity works best using chopsticks.  For grades 4 and 5, students should use pincher fingers and a straight crane arm (no bending elbows!).  Have students practice using the chopsticks or “pincher cranes” to pick up beans and remind them to take only one bean at a time.
  4. After practicing, go over the rules (one bean at a time, they must take beans from the tray, not any dropped ones). When they are ready to go, give them 20 seconds to pick up as many beans as possible.  Each student should take note of the number of beans he/she grabbed.
  5. Have them repeat the “bean grab” with the small dish for 20 seconds, reminding them they can only take one bean at a time and only from the surface of the dish (not any spilled beans).
  6. Have the students compare how many beans they picked up with the larger vs smaller container.
  7. Discussion: From which container did they get the most beans?  From which was it easier to physically grab the beans? (probably the shallow, larger tray)  Why? (because the beans are spread out on the tray and more “exposed”).  If the students with their chopsticks or fingers represent the “air” and beans represent the water how does this experiment relate to their sponge frog experiment?  How does the number of beans grabbed change between the two containers?

A Perfect Home Card Sort

  1. Hand out one set of “Conditions” cards to each group
  2. Ask students to work together to separate them into two piles:  The “good” environment for frogs and the “bad” environment for frogs.  
  3. Discuss together which conditions would provide the perfect home.  You could also have students come up with their own threats/good environments.
  4. Use the photos of bromeliad plants as an example of a good home/habitat for frogs high up in the canopy.  Bromeliads provide shelter, water, and protection from predators.

Surface Area Calculation Challenge

  1. If you will be having them calculate the surface area of their own group’s sponge frogs, then make sure they trace the outline before adding water to them.  Alternately, use the provided copy of the template frogs on the Student Worksheet.
  2. Use the worksheet to have students calculate the surface area of the two sizes of frogs.  
  3. Once students have the surface area calculated, you can have them graph the amount of water evaporated vs. the total surface area on one large class graph.

California Content Standards

Grade 4

Investigation and Experimentation

  • 6b. Measure and estimate the weight, length, or volume of objects.
  • 6c. Formulate and justify predictions based on cause-and-effect relationships.
  • 6d. Conduct multiple trials to test a prediction and draw conclusions about the relationships between predictions and results.
  • 6e. Construct and interpret graphs from measurements.

CCSS for Mathematics

  • 4.NBT.2 Read and write multi-digit whole numbers using base-ten numerals, number names, and expanded form. Compare two multi-digit numbers based on meanings of the digits in each place, using >, =, and < symbols to record the results of comparisons.
  • 4.MD.2 Apply the area and perimeter formulas for rectangles in real world and mathematical problems.
  • 4.MD.4 Make a line plot to display a data set of measurements in fractions of a unit (1/2, 1/4, 1/8). Solve problems involving addition and subtraction of fractions by using information presented in line plots.

Grade 5

Life Sciences

  • 2a. Students know many multicellular organisms have specialized structures to support the transport of materials.

Earth Sciences

  • 3b. Students know when liquid water evaporates, it turns into water vapor in the air and can reappear as a liquid when cooled or as a solid if cooled below the freezing point of water.

Investigation and Experimentation

  • 6e. Identify a single independent variable in a scientific investigation and explain how this variable can be used to collect information to answer a question about the results of the experiment.
  • 6f. Select appropriate tools (e.g., thermometers, meter sticks, balances, and graduated cylinders) and make quantitative observations.
  • 6g. Record data by using appropriate graphic representations (including charts, graphs, and labeled diagrams) and make inferences based on those data.
  • 6h. Draw conclusions from scientific evidence and indicate whether further information is needed to support a specific conclusion.

Grade 6

Investigation and Experimentation

  • 7b. Select and use appropriate tools and technology (including calculators, computers, balances, spring scales, microscopes, and binoculars) to perform tests, collect data, and display data.
  • 7c. Construct appropriate graphs from data and develop qualitative statements about the relationships between variables.
  • 7e. Recognize whether evidence is consistent with a proposed explanation.

CCSS for Mathematics

  • 6.G.1. Find the area of right triangles, other triangles, special quadrilaterals, and polygons by composing into rectangles or decomposing into triangles and other shapes; apply these techniques in the context of solving real-world and mathematical problems.
  • 6.SP.3. Recognize that a measure of center for a numerical data set summarizes all of its values with a single number, while a measure of variation describes how its values vary with a single number.
  • 6.SP.4. Display numerical data in plots on a number line, including dot plots, histograms, and box plots.
  • 6.SP.5. Summarize numerical data sets in relation to their context.

Grade 7

Life Sciences

  • 5b. Students know organ systems function because of the contributions of individual organs, tissues, and cells. The failure of any part can affect the entire system.

Investigation and Experimentation

  • 7a. Select and use appropriate tools and technology (including calculators, computers, balances, spring scales, microscopes, and binoculars) to perform tests, collect data, and display data.
  • 7c.Communicate the logical connection among hypotheses, science concepts, tests conducted, data collected, and conclusions drawn from the specific evidence.

CCSS for Mathematics

  • 7.G.6. Solve real-world and mathematical problems involving area, volume and surface area of two- and three-dimensional objects composed of triangles, quadrilaterals, polygons, cubes, and right prisms.
  • 7.SP.1. Understand that statistics can be used to gain information about a population by examining a sample of the population; generalizations about a population from a sample are valid only if the sample is representative of that population. Understand that random sampling tends to produce representative samples and support valid inferences.
  • 7.SP.3. Informally assess the degree of visual overlap of two numerical data distributions with similar variabilities, measuring the difference between the centers by expressing it as a multiple of a measure of variability.
  • 7.SP.4. Use measures of center and measures of variability for numerical data from random samples to draw informal comparative inferences about the two populations.

Grade 8

Investigation and Experimentation

  • 9b. Evaluate the accuracy and reproducibility of data.
  • 9c. Distinguish between variable and controlled parameters in a test.
  • 9e. Construct appropriate graphs from data and develop quantitative statements about the relationships between variables.



This experiment explores how surface area affects evaporation rate.  Surface area and evaporation rate are positively correlated: as surface area increases evaporation rate increases.  For example, if two swimming pools have the same volume of water, and one is large and shallow and the other is small and deep; the large, shallow one will lose more water in the same amount of time because it has more surface exposed to the air.  Here, the same principle is explored using two sponge “tree frogs:” one with large surface area and one with smaller surface area, both containing the same amount of water.  When all other variables are held constant, the larger sponges will show more water loss than the small ones in the same amount of time.  
Tree frogs are the model “organism” in this lab because water loss and dehydration are real risks that tree frogs face in the natural world.  Many of the potential threats faced by tree frogs arise from their characteristic skin: a thin, scale-less, moist layer that easily exchanges water and gases with the environment.  This permeable skin is sometimes an advantage.  For example, many frogs have a ‘seat patch’ on their rear end, a special section of tissue that absorbs water particularly well.  So instead of drinking through its mouth, a frog can stick its hind end in a pool of water to quench its thirst!

Frog skin also performs an important role in frog respiration.  Because the lungs of an adult frog are hollow sacs, surface area is minimal and the exchange of gases is less efficient than in mammals.  Frogs don’t inhale and exhale regularly – like humans, dogs, or cats – but rather take in occasional large gulps of air.  The majority of a frog’s gas exchange is instead performed through its moist skin.  Oxygen from the air dissolves in a layer of mucus covering the body, diffuses through the skin into the bloodstream, and travels to the frog’s cells.  In summary, a frog both drinks and breathes through its skin!

The large surface area of the skin is thus helpful for providing the body with plenty of water and oxygen.  However, this skin can become a handicap when the environment is not ideal.  Variables such as low humidity, heat, and high wind increase the rate of evaporation, leaving a frog vulnerable to dehydration.  And without water covering its body, a frog will have a tough time capturing enough oxygen for its cells.  Other environmental threats include acid rain, water pollution, natural toxins, and fungus, which can injure a frog’s sensitive skin, or easily pass into the body.  

Although this experiment focuses on how surface area affects evaporation rate, in the case of real animals, there is another important biological concept to keep in mind.  A real frogs’ rate of water loss is actually more impacted by their surface-area-to-volume ratio rather than just their surface area.  Surface-area-to-volume ratio decreases as body size increases and the larger the surface-area-to-volume ratio, the faster water evaporates from an animal. This is because the area through which water can be lost in a small animal is increased relative to the volume of water that can be stored inside its body.  

Given that large frogs have a smaller surface-area-to-volume ratio than small frogs, large frogs will actually dry out more slowly than small frogs if all other things are equal (e.g. habitat, temperature, behavior).  The results of the sponge experiment looking at surface area only do not represent the whole picture in the natural world.  If all tree frogs were perfectly flat, then the experiment would reflect biological reality because their surface area would be the only factor affecting water loss.  But because tree frogs are not flat, their body plan and size demonstrates an adaptive trade-off between the need to have enough surface area of skin for respiration but not too much that they would lose water too quickly and risk dehydration.  Tree frogs minimize the threat of dehydration through their behaviors and habitat choice, which can also be discussed in class as part of this lesson.


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