Name _____________________________ Class _________________ Date _________________
That’s Amazing! Worksheet
Farmer Ants with Dr. Michael Heithaus
Teacher Notes and Answers BACKGROUND
Next to humans, leaf-cutter ants are one of the largest and most complex animal societies on Earth. Leaf-cutter ant populations can reach into the millions, and their colonies can cover an area the size of a tennis court. Known for their remarkable abilities to slice through almost any type of leaf matter, these charismatic critters can carry 20 times their weight in leaves. Surely, these leaves must be their food! But, surprisingly, they aren’t. These ants are farmers! They use the leaves as fertilizer for gardens of fungus that they eat. In the rain forest, fungi can succumb easily to infections, but the leaf-cutters use beneficial bacteria to manufacture incredible antibiotics to keep their fungal gardens safe. In this video, Mike Heithaus travels to Costa Rica, where he meets up with microbiologist Cameron Currie of the University of Wisconsin–Madison. They work together to uncover how these miniature pharmacists have managed to keep their crops free of disease and resistant to infection for millions of years. INVESTIGATION
1. Graph should show high scores for the Fungal Garden Tenders and lower scores for Foragers and Waste Tending Workers. 2. Accept all reasonable answers. Sample answer: Workers tending the garden would have higher levels of the antibiotic to help ensure that their food source remains uninfected. However, workers gathering leaves or getting rid of waste don’t need to carry as much bacteria, because carrying the bacteria is costly and the parasitic fungi only infect the fungus garden and not the ants themselves.
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Protists and Fungi Section 6: Ecology of Fungi
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Farmer Ants—Teacher Notes and Answers continued
3. Table 1. Bacteria load scores for ten ants from three types of leaf-cutter
worker ant Ant number
Waste tender
Garden tender
Forager
1
0
12
3
2
0
12
3
3
3
12
3
4
0
12
0
5
0
12
0
6
3
9
3
7
0
12
3
8
0
12
3
9
0
12
6
10
3
9
3
Mean
0.9
11.4
2.7
4.
5. If the predictions are as suggested above, the data do support the predictions. Note. An interesting pattern in the worker ants is that they change their jobs
and bacterial loads as they age. This change is probably due to the costs of housing the bacteria. When ants are young and strong, they tend the gardens and are covered in bacteria. As they age, they lose bacteria and become foragers. After that, they tend the waste piles.
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Protists and Fungi Section 6: Ecology of Fungi
Name _____________________________ Class _________________ Date _________________
Farmer Ants—Teacher Notes and Answers continued
6. Although not conclusive, there is some support for the hypothesis. The patterns in which types of ant workers carry bacteria are consistent with using bacterial products to treat the fungus. In other words, garden tenders, which would need the bacteria, have the bacteria, while those that would not need it do not have it. However, these data do not tell us that an antibiotic is produced or, if so, what it might be used for. Note. You may want to ask students if this provides enough data to answer
the question convincingly. Students should come up with the idea that more tests to determine what the bacterial products actually do are needed. (Do the bacteria produce antibiotics? Do the antibiotics inhibit the parasite?) 7. The set of plates with sterile water and fungi demonstrates the growth pattern of the fungi on the plate in the absence of bacteria. This is important for comparing the growth patterns of fungi in the presence of the bacteria. This also helps to determine whether there are other factors that may affect the zone of inhibition. 8. The sterile water + fungi should have the smallest zone of inhibition, followed by the general fungi. The specific fungi should have the largest zone of inhibition. The sterile water + fungi should show no inhibition because no bacteria are present to produce antibiotics that would prevent fungal growth. If the bacteria produce antibiotics that target specific fungi, we would expect the bacteria to have the greatest impact on the specific fungi; thus, the specific fungi should have the largest zone of inhibition. 9. Table 2. Size of the zone of inhibition for each treatment Size of the zone of inhibition (cm)
Plate number
Sterile water plus fungi General fungi
Specific fungi
1 2 3 4 5 6 7 8 9 10
0 0 0 0 0 0 0 0 0 0
0.45 0.65 0.55 0.65 0.45 0.55 0.6 0.55 0.7 0.35
2 1.5 2.5 2 1.5 2.5 2 2 2.5 2
Mean
0
0.55
2.05
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Protists and Fungi Section 6: Ecology of Fungi
Name _____________________________ Class _________________ Date _________________
Farmer Ants—Teacher Notes and Answers continued
10.
CONCLUSION
11. The evidence supports that leaf-cutter ants are using the antibiotics produced by bacteria as a drug to protect their fungal gardens. First, the bacteria are found in abundance only on worker ants tending the gardens, where the protection from the fungal parasite would be needed. Second, the bacteria are producing an antibiotic that inhibits fungal growth. The data from the inhibition experiment show that the effect is strongest for the “specific fungi,” i.e., the parasitic fungi that attack the ants’ gardens. Together, this evidence suggests that ants do use drugs.
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Protists and Fungi Section 6: Ecology of Fungi
Name _____________________________ Class _________________ Date _________________
That’s Amazing! Worksheet
Farmer Ants with Dr. Michael Heithaus
Problem Fungi play different roles in ecosystems. Many fungi are important decomposers. They break down organic material (including dead plants and animals) and return nutrients to the ecosystem. Some of these fungi are tasty—for example, people eat mushrooms and leaf-cutter ants consume the fungus that grows on the leaves they carry back to their nest. Other fungi are parasites. Some cause diseases in many animals, including people, and others even attack different species of fungi. One specific type of parasitic fungus attacks the fungus grown by leaf-cutter ants. This very specialized parasitic fungus doesn’t attack other fungi and doesn’t harm the ants. How do leaf-cutter ants keep their fungal farms free from the parasitic fungus? Cameron Currie suspects that leaf-cutter ants might be using antibiotics to do this. So, do bugs use drugs? INVESTIGATION Leaf-Cutter Ant Behavior
We will first investigate the behavior of different worker ants in leaf-cutter colonies. Leaf-cutter ant workers specialize in different jobs for the colony. Some workers are foragers that go into the forest, cut the leaves, and carry them back to the colony. Other workers tend the fungus gardens, others tend to the colony’s waste, and still another group of workers tends to the queen. Some leaf-cutter ants are covered in so many bacteria that it forms a thick white coat. That’s why Cameron calls them “sheep.” But not all leaf-cutter workers are covered in bacteria. If the ants are using drugs from the bacteria to treat their gardens, why wouldn’t all the ants be covered in it? Cameron’s work shows that carrying a heavy load of bacteria is costly! “Sheep” ants use 25 percent more energy, compared with ants that are not covered by bacteria. Because carrying the bacteria appears to have a high cost to the ant, we would expect the ants to gain some significant benefit from doing so Cameron and his colleagues have created a scoring system to indicate how much bacteria an ant is carrying (Figure 1).
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Protists and Fungi Section 6: Ecology of Fungi
Name _____________________________ Class _________________ Date _________________
Farmer Ants continued
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Protists and Fungi Section 6: Ecology of Fungi
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Farmer Ants continued
Before we get to the data, make a prediction. 1. Draw a bar graph that shows the average bacteria cover score you think the following groups of workers will have: Foragers, Fungal Garden Tenders, Waste Tending Workers. (Hint: Look at Figure 1.)
2. Explain why you made the predictions in your graph above. ____________________________________________________________ ____________________________________________________________ ____________________________________________________________ ____________________________________________________________ Now use the data in Table 1 to test your predictions. 3. Data Analysis Complete Table 1 on the next page by calculating the mean bacteria load scores for the three types of worker ants.
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Protists and Fungi Section 6: Ecology of Fungi
Name _____________________________ Class _________________ Date _________________
Farmer Ants continued Table 1. Bacteria load scores for ten ants from three types of leaf-cutter
worker ant Ant number
Waste tender
Garden tender
Forager
1
0
12
3
2
0
12
3
3
3
12
3
4
0
12
0
5
0
12
0
6
3
9
3
7
0
12
3
8
0
12
3
9 10
0 3
12 9
6 3
Mean
4. Data Analysis Draw a bar graph of the mean bacteria load scores for the three types of worker ant.
5. Explain whether these data support the predictions you made. ____________________________________________________________ ____________________________________________________________ ____________________________________________________________ ____________________________________________________________ © Houghton Mifflin Harcourt Publishing Company
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Protists and Fungi Section 6: Ecology of Fungi
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Farmer Ants continued
6. Do the results of this portion of the study support the hypothesis that bugs use drugs? Explain your answer. ____________________________________________________________ ____________________________________________________________ ____________________________________________________________ ____________________________________________________________ Effects of Antibiotics on Fungus
Now that we know which worker ant types are carrying the bacteria, we need to find out if the bacteria are producing products that are capable of killing the parasitic fungi that attacks the ants’ gardens. To find out whether the bacteria are producing an antibiotic that kills a particular fungus, you can do an inhibition experiment. In these experiments, the bacteria are allowed to grow on a plate with nutrients. Doing so allows the bacteria to produce and secrete any antibiotics that they might make. Next, the fungus is spread across the plate. If the bacteria produce antibiotics that kill the fungus on the plate, then there will be a ring around the colony where no fungus grows, which is called a “zone of inhibition,” as shown in Figure 2.
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Protists and Fungi Section 6: Ecology of Fungi
Name _____________________________ Class _________________ Date _________________
Farmer Ants continued
Cameron conducted an experiment in which he grew bacteria on 20 plates. After the bacteria had enough time to grow and secrete any antibiotics that they might produce into the media, he introduced one of two types of fungi onto the plates. Ten plates received “General Fungi.” This included species that might be found near ant nests that could infest the colony, but do not specifically attack the fungus that the ants grow. The other ten plates received the “Specific Fungi,” which attacks only the fungus growing in the ants’ gardens. In another set of ten plates, Cameron introduced sterile water instead of bacteria, and then introduced fungi. 7. What is the purpose of the set of plates with sterile water? Why is this important for the experiment? ____________________________________________________________ ____________________________________________________________ ____________________________________________________________ ____________________________________________________________ 8. If the results of the experiment support the hypothesis that the bacteria produce antibiotics that target specific fungi, predict the relative size of the zone of inhibition for each of the three treatments. Which treatment would you expect to show the smallest zone of inhibition? Which treatment would you expect to show the largest zone of inhibition? Why? ____________________________________________________________ ____________________________________________________________ ____________________________________________________________ ____________________________________________________________ ____________________________________________________________ ____________________________________________________________ 9. Data Analysis Complete Table 2 (on the next page) by calculating the mean size of the zone of inhibition for each treatment.
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Protists and Fungi Section 6: Ecology of Fungi
Name _____________________________ Class _________________ Date _________________
Farmer Ants continued Table 2. Size of the zone of inhibition for each treatment Size of the zone of inhibition (cm)
Plate number
Sterile water plus fungi General fungi
Specific fungi
1 2 3 4 5 6 7 8 9 10
0 0 0 0 0 0 0 0 0 0
2 1.5 2.5 2 1.5 2.5 2 2 2.5 2
0.45 0.65 0.55 0.65 0.45 0.55 0.6 0.55 0.7 0.35
Mean
10. Data Analysis Draw a bar graph that shows the mean size of the zone of inhibition for each treatment.
CONCLUSION
11. Based on the data you have analyzed, do you think leaf-cutter ants use drugs to fend off fungal attacks? Use all lines of evidence to support your answer. ____________________________________________________________ ____________________________________________________________ ____________________________________________________________ ____________________________________________________________ © Houghton Mifflin Harcourt Publishing Company
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Protists and Fungi Section 6: Ecology of Fungi
