What is causing frog deformities in this pond?

Course: ISB 202: Applications of Environmental Science and Organismal Biology

Level: lower-level undergraduate non-science majors

Class Size: 180

Source: Images from http://info.hartwick.edu/biology/def_frogs/

Annotation by:Debra Linton and Diane Ebert-May

Keywords: experimental design; global change


Goal: Engage students in a large (or small) class in the practice of science by developing hypotheses, designing methods to test the hypotheses, and evaluating data from research about deformities in frogs. As a result of this activity, students should demonstrate understanding of frog population responses to changes in abiotic variables in the environment and the potential causes for changes in both the frogs and the environment. The activity is based on research of the causes of frog deformities. Simplified data from actual research studies is used. No statistics are used to analyze the experimental data, however statistics could easily be incorporated into the activity with more detailed data on experimental design (e.g. sample sizes and replication). Images are from the Deformed Amphibian Research website at Hartwick College, which also includes a differently structured activity using the frog deformity question.


Students will be able to...

  1. Demonstrate their understanding of the process of science by generating hypotheses, designing ways to test questions, and evaluating the science done by others.
  2. Analyze and interpret a simulation about an ecological problem.

Instructional Design:

  1. Introduce the question - Since 1980, populations of hundreds of the world's estimated 5,100 amphibian species have been vanishing or declining in almost every part of the world, even in protected wildlife reserves and parks. In some locales, frog deformities (such as extra legs and missing legs), which usually lead to mortality, are occurring in unusually high numbers. Imagine you found this frog (above) in a pond on your property. On further investigation you discovered that over 30% of frogs in that pond have similar deformities. How will you determine what caused the deformities?
  2. Students
    • In your group, develop a testable hypothesis about the cause of frog deformities in this pond. Consider both abiotic and biotic variables. Write your hypothesis on your carbonless paper.

  3. Report out
    • Groups report out, generating a list of possible causes of the frog deformities.
    • Instructor focuses to three hypotheses: Scientists working on this question have narrowed down their investigations to three possible causes:
      1. increased UV-B radiation
      2. chemical pollution
      3. trematode parasites
  4. Students Work in Groups: (assign hypotheses to groups to ensure coverage)
    • Design an experiment to test the hypothesis assigned to you. Explain your experiment, in writing, on your carbonless paper, one per group.
  5. Report Out
    • Groups report out, generating three consensus experiments - one per hypothesis. Instructor may need to mold experiments to fit the data that will be provided.
  6. Students Work in Groups
    • Results are provided for your experiment. These data are based on compilation of results from several experiments, and represent actual trends in current research findings.
    • Graph the data in a way that will help you interpret it and present it to your peers.
    • Interpret the data.
    • Discuss and write your responses to the following question:
      1. Do the results support or not support your hypothesis? In your group, explain your answer by referring back to your hypothesis. Write your explanation on the group carbonless paper
  7. Class Discussion of Results
  8. Students Work In Groups
    • Does the conclusion from this research explain the increased frequency of frog deformities worldwide (and associated decline in amphibian populations)?
    • Examine the following experimental results. (Kiesecker, 2002)
    • Treatment
      % of frogs with deformities
      No Atrazine/No Trematodes
      No Atrazine/No Trematodes
      No Atrazine/No Trematodes
      No Atrazine/ Trematodes
      No Atrazine/ Trematodes
      No Atrazine/ Trematodes
      Atrazine/No Trematodes
      Atrazine/No Trematodes
      Atrazine/No Trematodes
      Atrazine/ Trematodes
      Atrazine/ Trematodes
      Atrazine/ Trematodes

    • Graph the data.
    • Interpret the results.
    • What conclusion about the increased frequency of frog deformities is supported by this data?


  1. In Class: Students write a testable hypothesis - must include dependent and independent variables.
  2. In Class: Students design an experiment - must have appropriate controls and replication.
  3. In Class: Students answer discussion questions.
  4. In Class: Students interpret experimental data.
  5. On Exam: Students generate a hypothesis and design an experiment based on a novel ecological problem.
  6. On Exam: Students interpret experimental data.

Exam Question: This spring, a group of MSU students went to the Baker Woodlot and saw many frogs in the ponds located there. The students looked closer and noticed that many of the frogs were deformed. After consulting with the available scientific literature, they designed a laboratory experiment to test the effects and interactions of three different factors on frog development. Students collected data for the following three factors: (1) ultraviolet radiation (UV), (2) herbicide Atrazine, (3) parasitic trematodes. The data are summarized below:

% deformity
UV Only
Atrazine Only
Trematodes Only
UV + Atrazine
Atrazine + Trematodes


Do the data support the following hypotheses? Indicate "yes" (supports) or "no" (does not support) for each hypothesis.

_____Hypothesis 1: UV radiation increases the frequency of frog deformities.

_____Hypothesis 2: Atrazine increases the susceptibility of frogs to deformities caused by UV radiation.

_____Hypothesis 3: Atrazine increases the susceptibility of frogs to deformities caused by trematode infection.


Exam Question: Based on Kiesecker JM, Blaustein AR, Belden LK. 2001. Complex causes of amphibian population declines. Nature 410: 681-684.

Saprolegnia ferax is a fungus that infects frog eggs in ponds in the Pacific Northwest. In a “normal” year S. ferax causes mortality of approximately 20% of frog eggs. In some years, however, mortality reaches 60%. Researchers noted that this increased mortality occurs in years following an El Niño event, when precipitation falls far below normal levels. When precipitation is low, pond levels drop and frog egg-laying sites are closer to the surface of the water. UV radiation is attenuated (decreases) as it moves through water. Therefore eggs that are layed closer to the surface received more UV-B radiation. The graph below contains the data resulting from an experiment in which frog eggs were raised (in ponds that contained natural levels of S. ferax) at different depths, either in the presence or absence of UV-B radiation. Based on all of the data presented and the information above:

  1. Draw a model that represents the steps involved in this relationship and,
  2. Write an explanation of why frog mortality due to S. ferax is increased in years following El Niño events. Include in your explanation a discussion of synergism and how it applies here.


Fig. 3 Effects of water depth and exposure to UV-B radiation on average hatching success (not infected by S. ferax) for the frog eggs. Open bars represent the treatment NOT receiving UV-B radiation. The shaded bars represent the treatment that DID receive UV-B radiation.


Primary Sources:

Ankley GT, Diamond SA, Tietge JE, Holcombe GW, Jensen KM, DeFoe DL, Peterson R. 2002. Assessment of risk of solar ultraviolet radiation to amphibians. I. Dose-dependent induction of hindlimb malformations in the Northern Leopard Frog (Rana pipiens). Environmental Science and Technology 36(13): 2853-2858.

Ankley GT, Tietge JE, Holcombe GW, DeFoe DL, Diamond SA, Jensen KM, Degitz SJ. 2000. Effects of laboratory ultraviolet radiation and natural sunlight on survival and development of Rana pipiens. Canadian Journal of Zoology 78(6): 1092-1100.

Bridges CM. 2000. Long-term effects of pesticide exposure at various life stages of the Southern Leopard Frog (Rana sphenocephala). Archives of Environmental Contamination and Toxicology 39: 91-96.

Johnson PTJ, Lunde KB, Ritchie EG, Launer AE. 1999. The effect of trematode infection on amphibian limb development and survivorship. Science 284(5414): 802-804.

Kiesecker JM. 2002. Synergism between trematode infection and pesticide exposure: A link to amphibian limb deformities in nature? Proceedings of the National Academy of Sciences 99(15): 9900-9904.

Kiesecker JM, Blaustein AR, Belden LK. 2001. Complex causes of amphibian population declines. Nature 410(6829): 681-684.

Middleton EM, Herman JR, Celarier EA, Wilkinson JW, Carey C, Rusin RJ. Evaluating ultraviolet radiation exposure with satellite data at sites of amphibian declines in Central and South America. Conservation Biology 15(4): 914-929.


Secondary Sources:

Blaustein AR, Johnson PTJ. 2003. The complexity of deformed amphibians. Frontiers in Ecology and the Environment 1(2): 87-94.

Kiesecker JM, Belden LK, Shea K, Rubbo MJ. 2004. Amphibian decline and emerging disease. American Scientist 92(2): 138-147.