NATIONAL CENTER FOR CASE STUDY TEACHING IN SCIENCE

NATIONAL CENTER FOR CASE STUDY TEACHING IN SCIENCE Giant Pandas, Hormones, and the Evolution of a Lazy Bear by Patricia J. Moore Division of Biological Sciences University of Georgia, Athens, GA

Introduction: • The giant panda (Ailuropoda melanoleuca) is a critically endangered species that lives in south-central China. • Although giant pandas are in the class Carnivora, the diet of the giant panda mostly consists of bamboo, although they occasionally eat other grasses, tubers, and fruits. • In the wild, the giant panda population is closely tied to bamboo abundance. • Working in small groups, propose an ultimate hypothesis for why giant panda might specialize on bamboo as a food source. 2

Panda bear diet: • Giant pandas have adapted to a diet made up almost exclusively of bamboo (a grass). • Bamboo has very low nutritional quality, so giant pandas need to eat a lot of bamboo every day. • Meat is easy to digest, while most of the energy in grass is stored in complex carbohydrates like cellulose that requires a specialized digestive system to break down efficiently. 3

• During evolution, the digestive tracts of carnivores and herbivores have diverged. © Open. Stax Biology 4

• Giant pandas are in the Ursidae clade and are descendants of a carnivore ancestor. Brown bear Polar bear American black bear Sun bear Spectacled bear Giant panda Raccoon Lesser panda Ursidae ancestral carnivore 5

Question: How do giant pandas survive on this poor diet? • Working in small groups, propose a proximate hypothesis for how giant panda specialize on bamboo as a food source. 6

Question: How do giant pandas survive on this poor diet? • Working in small groups, propose a proximate hypothesis for how giant panda specialize on bamboo as a food source. We will work through some proximate hypotheses examined in the paper: Nie, Y. , Speakman, J. R. , Wu, Q. , Zhang, C. , Hu, Y. , Xia, M. , Yan, L. , Hambly, C, Wang, L. , Wei, W. , Zhang, J. , & Wei, F. (2015). Science 349: 171 -174. 7

Hypothesis 1: Adaptation to a bamboo diet has been through morphological changes in the digestive tract CQ 1: Do you agree with that hypothesis? What type of digestive tract might you expect to see in a giant panda and why? A. I agree with the hypothesis. I expect to see an herbivore type of digestive tract, because herbivory requires a long, complex digestive tract. B. I disagree with the hypothesis. I expect to see a carnivore type of digestive tract, because the common ancestor of giant pandas was a carnivore and that places an evolutionary constraint on the digestive tract. 8

Hypothesis 1: Adaptation to a bamboo diet has been through morphological changes in the digestive tract On the scale below, showing intestinal length relative to body size, where would you expect to find pandas if Hypothesis 1 is correct? omnivore carnivore herbivore 9

Hypothesis 1: Adaptation to a bamboo diet has been through morphological changes in the digestive tract Data: Giant panda digestive tracts are very similar to other bear species, therefore adaptation to a bamboo diet has not been though changes in the digestive tract omnivore carnivore herbivore 10

Hypothesis 1 Adaptation to a bamboo diet has been through morphological changes in the digestive tract is not supported by the data. What do we know now? • Bamboo has low nutritional value. • Giant panda digestive tract is not specialized to extract maximum energy from plant material. • Can you propose an alternative hypothesis? Brainstorm with your neighbors; think about what you know about energy, metabolism, and homeostasis. 11

Organisms operate on an energy budget – they must balance energy in against energy used Some energy will be lost during digestion Energy available in food Energy available for giant panda to “spend” on metabolic functions 12

Hypothesis 1 proposed that giant pandas might increase their “energy budget” by reducing the amount of energy lost during the transformation of the energy stored in the bamboo to energy available for metabolism Less energy lost will result in more energy available Energy available in food Energy available for giant panda to “spend” on metabolic functions 13

But the data did not support Hypothesis 1 – giant pandas do not have a digestive tract that is more efficient at extracting energy from their food than other carnivores. What else can a giant panda do to maintain its energy budget? Some energy will be lost during digestion Energy available in food Energy available for giant panda to “spend” on metabolic 14 functions

Hypothesis 2: Giant pandas have low energy demands relative to other bears CQ 2: Do you agree with that hypothesis? Why or why not? A. I agree with the hypothesis. A large mammal would not be able to survive on such a low quality diet if it had similar energy requirements as other mammals of the same size. B. I disagree with the hypothesis. Energy expenditure is based on body size – big animals have big energy requirements. 15

Hypothesis 2: Giant pandas have low energy demands relative to other bears CQ 3: What prediction arises from this hypothesis? A. Giant panda will have low energy expenditure. B. Giant panda will have low energy expenditure compared to other mammals of a similar size. C. Giant panda will have low energy expenditure compared to carnivores. D. Giant panda will have similar energy expenditure compared to herbivores. 16

Hypothesis 2: Giant pandas have low energy demands relative to other bears or mammals Test: measure the metabolic rate of pandas by measuring their daily energy expenditure (DEE) What is the purpose of this figure? 17

Hypothesis 2: Giant pandas have low energy demands relative to other bears or mammals Test: measure the metabolic rate of pandas by measuring their daily energy expenditure (DEE) CQ 4: The purpose of this figure is to demonstrate that: A. captive giant panda have lower energy requirements than wild giant panda. B. giant panda energy requirements are positively correlated with their body mass. C. giant panda energy requirements are low compared to other bears and mammals. D. giant panda energy requirements do not depend on body size. 18

Hypothesis 2: Giant pandas have low energy demands relative to other bears or mammals Test: measure the metabolic rate of pandas by measuring their daily energy expenditure (DEE) Giant panda 19

Hypothesis 2: Giant pandas have low energy demands relative to other bears or mammals Test: measure the metabolic rate of pandas by measuring their daily energy expenditure (DEE) CQ 5: The purpose of this figure is to demonstrate that: A. captive giant panda have lower energy requirements than wild giant panda. B. giant panda energy requirements are positively correlated with their body mass. C. giant panda energy requirements are low compared to other bears and mammals. D. giant panda energy requirements do not depend on body size. 20

Hypothesis 2: Giant pandas have low energy demands relative to other bears or mammals Test: measure the metabolic rate of pandas by measuring their daily energy expenditure (DEE) CQ 6: Does the data support this hypothesis? A. No, giant pandas are big animals and have high DEE. B. No, giant pandas are big animals, but their DEE is low compared to other mammals of the same size. C. Yes, giant pandas are big animals and have high DEE. D. Yes, giant pandas are big animals, but their DEE is low compared to other mammals of 21 the same size.

Hypothesis 2: Giant pandas have low energy demands relative to other bears or mammals Test: measure the metabolic rate of pandas by measuring their daily energy expenditure (DEE) Giant panda Sloth 22

Hypothesis 2, giant pandas have low energy demands relative to other bears or mammals, is supported by the data on daily energy expenditure. Is this the end of the story? What other questions might you want to ask about the giant panda bear adaptation to this unique ecological niche? 23

What do we know now? • Bamboo has low nutritional value • Giant panda digestive tract is not specialized to extract maximum energy from plant material • Giant pandas have low daily energy expenditures (don’t need to take many calories in, because don’t use many calories) To fully understand giant pandas, we might want to know how they maintain such a low energy expenditure. How do you think animals achieve low rates of metabolism? IDEAS? 24

How do animals achieve low rates of metabolism? IDEAS? Brainstorm with your neighbors to come up with some potential explanations for how some animals achieve low metabolic rates. Hypotheses about how animals might achieve low rates of metabolism could include: Ø Changes in behavior Ø Changes in morphology Ø Changes in physiology 25

Hypothesis 3 a: The low energy demands of giant pandas are due to changes in behavior Test: measure activity of giant pandas in captivity and the wild CQ 7: What prediction arises from this hypothesis? A. Giant panda will be less active than other bears, thus allowing them to use fewer calories per day. B. Giant panda will be more active than other bears because they need to find more food to make up for the lack of calories in bamboo. 26

Hypothesis 3 a: The low energy demands of giant pandas are due to changes in behavior Test: measure activity of giant pandas in captivity and the wild Mean time spent active by grizzly bears • What are some interpretations of this data? What other questions might you want to ask? 27

Hypothesis 3 a: The low energy demands of giant pandas are due to changes in behavior Test: measure activity of giant pandas in captivity and the wild Mean time spent active by grizzly bears CQ 8: Does this data support the hypothesis? A. Yes B. No 28

Hypothesis 3 a, the low energy demands of giant pandas are due to changes in behavior, is supported by the data on their behavior in both the wild and in captivity. The relationship between “sluggish” behavior and low metabolic rate is a correlation. What does that mean? • Do low levels of activity result in low metabolic rates? • Or do low metabolic rates result in low levels of activity? • What other explanations should we test for the low energy demands of giant pandas? 29

Hypothesis 3 b: The low energy demands of giant pandas are due to changes in morphology Test: compare the morphology of giant pandas to other mammals of similar size What do we know about morphology and metabolic rate? • Animals can achieve low metabolic rated by relaxing homeothermy • Giant panda internal body temperature indicate that they do not engage in either daily torpor or hibernation • Do giant pandas have better insulation to reduce body heat loss to the environment? 30

Hypothesis 3 b: The low energy demands of giant pandas are due to changes in morphology Test: compare the surface temperature of giant pandas to other mammals of similar size • What do you predict if the pelage of giant panda provides superior insulation? Ambient air temperature ≈ 4° C (blue boxes) 10 ° C (brown boxes) 31

Hypothesis 3 b: The low energy demands of giant pandas are due to changes in morphology Test: compare the morphology of giant pandas to other mammals of similar size What do we know about morphology and metabolic rate? • The dominant component of the daily energy budget is resting metabolic rate, which is the sum of the metabolic rates of all body components. • Brain, liver, kidney, and heart contribute disproportionately to the total. • Do giant pandas have smaller brains, kidneys, and hearts than other mammals relative to body size? 32

Hypothesis 3 b: The low energy demands of giant pandas are due to changes in morphology Test: compare brains, kidneys and hearts of giant pandas to other mammals of similar body size Data: giant pandas do have smaller brains, livers, and kidneys than expected based on body size. 33

Hypothesis 3 b: The low energy demands of giant pandas are due to changes in morphology Test: compare brains, kidneys and hearts of giant pandas to other mammals of similar body size CQ 9: To confirm their findings, scientists often address a hypothesis from multiple angles. The hypothesis is strengthened when the data from different approaches agree. In this case, the data on the pelange and on organ size: A. agree, providing confirmation of the hypothesis. B. agree, but are too different in approach to confirm the hypothesis. C. don’t agree, thus allowing us to reject the hypothesis. D. don’t agree, thus requiring us to undertake further studies. 34

Hypothesis 3 b, the low energy demands of giant pandas are due to changes in morphology, is supported by the data. Like the data on behavior, the relationship between organ morphology and metabolic rate is a correlation. • Can we start to get closer to a physiological explanation by looking at the endocrine control of metabolic rate? 35

Hypothesis 3 c: Changes in hormone levels lead to changes in energy expenditure Test: What hormones should we look at? CQ 10: What hormones should we measure? A. Gonadotropin releasing hormone B. Posterior pituitary hormone C. Thyroid hormones D. Adrenocortex stimulating hormone 36

Hypothesis 3 c: Changes in hormone levels lead to changes in energy expenditure Test: What hormones should we look at? CQ 11: What hormones should we measure? A. Thyrotropin releasing hormone B. Thyroid hormones C. Thyroid stimulating hormone 37

Hypothesis 3 c: Changes in hormone levels lead to changes in energy expenditure Test: What hormones should we look at? Test: resting metabolic rate is influenced by thyroid hormones T 4 and T 3. Thyroid hormone binding to their receptors stimulate multiple metabolic activities, including lipid and carbohydrate metabolism, leading to an increase in basal metabolic rate. Measure the levels of these hormones in giant pandas. 38

Hypothesis 3 c: Changes in hormone levels lead to changes in energy expenditure. Test: resting metabolic rate is influenced by thyroid hormones T 4 and T 3. Measure the levels of these hormones in giant pandas. eutherian mammals Giant pandas eutherian mammals CQ 12: Does this data support the hypothesis? A. Yes B. No Giant pandas 39

Hypothesis 3 c: Changes in hormone levels lead to changes in energy expenditure. Test: resting metabolic rate is influenced by thyroid hormones T 4 and T 3. Measure the levels of these hormones in giant pandas. Data: • Levels of T 4 and T 3 are lower in giant pandas relative to other eutherian mammals relative to body size. • Levels of T 4 and T 3 are lower even than those found in hibernating bears. 40

Hypothesis 3 c, changes in hormone levels lead to changes in energy expenditure, is supported by the data. 41

• Ultimate explanation – giant pandas are able to exploit a unique ecological niche and avoid competition with other mammals of the same size. • Proximate explanation – adaptations in behavior, morphology and endocrinology reduce the required daily energy expenditure so that they are able to use a resource of low nutritional quality. In particular, the endocrine system of giant pandas leads to reduced metabolic rates. 42

CQ 13: Imagine similar data was collected on a newly discovered species of bear that also has a very sluggish lifestyle and a low metabolic rate. However, the levels of T 3 and T 4 are similar to other animals with similar body weights. Thinking about what you have learned about how hormones work, which of the following is the best explanation? A. The levels of T 3 and T 4 are not necessary for cell metabolism. B. Cell metabolism may vary in this species due to a decrease in thyroid stimulating hormone. C. The energy requirements of this new bear species must be high, despite the low metabolic rate. D. The response to T 3 and T 4 might be reduced due to reduced numbers or reduced activity of the receptors for these hormones. 43

Consolidate your learning: Imagine similar data was collected on a newly discovered species of bear that also has a very sluggish lifestyle and a low metabolic rate. However, the levels of T 3 and T 4 are similar to other animals with similar body weights. Thinking about what you have learned about how hormones work and how they are regulated, how might you explain this? 44

CQ 14: Genomic analysis in the giant panda show that there is a mutation in a gene required for the synthesis of T 3 and T 4. Thinking about how feedback control works, what might you predict for the levels of TSH in giant pandas? A. The reduction in T 3 and T 4 leads to a decrease in TSH. B. The reduction in T 3 and T 4 is caused by a decrease in TSH. C. The reduction in T 3 and T 4 leads to an increase in TSH. D. The reduction in T 3 and T 4 is caused by an increase in TSH. 45

Consolidate your learning: Genomic analysis in the giant panda show that there is a mutation in a gene required for the synthesis of T 3 and T 4. Thinking about how feedback control works, what might you predict for the levels of TSH in giant pandas? 46

CQ 15: American pika are small mammals that are related to rabbits and usually live above the tree line in the mountains of western North America. Pika are herbivores that maintain a high basal metabolic rate and high body temperature as adaptations to living at high elevations in the mountains where temperatures are cold. If you were to measure the levels of T 3 and T 4 hormones in pika, what would you predict? A. Pika are herbivores, therefore their adaptation to their diet must be through a specialized digestive tract, therefore I would predict that the levels of T 3 and T 4 hormones would be similar to other rabbit relatives. B. Pika live above tree line and must hibernate, therefore I would predict that the levels of T 3 and T 4 hormones would be similar to other rabbit relatives during the summer months. C. Pika have a high basal metabolic rate to maintain a high body temperature, therefore I would predict that the levels of T 3 and T 4 hormones would be higher than expected for a mammal their size. D. Pika have a high basal metabolic rate to maintain a high body temperature, therefore I would predict that the levels of T 3 and T 4 hormones would be lower than expected for a 47 mammal their size.

Consolidate your learning: American pika are small mammals that are related to rabbits and usually live above the tree line in the mountains of western North America. Pika are herbivores that maintain a high basal metabolic rate and high body temperature as adaptations to living at high elevations in the mountains where temperatures are cold. If you were to measure the levels of T 3 and T 4 hormones in pika, what would you predict? 48

Image Credits Slide 1: Licensed photo © Richie Lomba | Dreamstime, ID 3943835. Slide 2: Modified image of giant panda face, Shiela Lau, https: //commons. wikimedia. org/w/index. php? curid=2266960, PD. Slide 3: Modified image of giant pandas eating bamboo, Chi King, https: //commons. wikimedia. org/w/index. php? curid=10353046, CC BY 2. 0. Slide 4: Figure of herbivore and carnivore digestive tracts, Openstax Biology+ Chapter 34. http: //cnx. org/contents/GFy_h 8 cu@10. 53: Oestf 0 YE@6/Digestive-Systems, CC BY 4. 0. Slide 5: Modified image of a brown bear, Simm, https: //commons. wikimedia. org/w/index. php? curid=2533126, PD. Slide 5: Modified image of polar bear, US Fish and Wildlife Service, https: //www. flickr. com/photos/usfwshq/5124077764, CC BY 2. 0. Slide 5: Modified image of an American black bear, Jitze Couperus, https: //www. flickr. com/photos/jitze 1942/1750394833, CC BY 2. 0. Slides 5, 38 and 39: Modified image of a sun bear, Andrew Napler, https: //www. flickr. com/photos/28285551@N 03/4235578092, CC BY 2. 0. Slide 5: Modified image of giant panda, Jeff Kubina, https: //commons. wikimedia. org/w/index. php? curid=265560, PD. Slide 5: Modified image of red panda, Mathias Appel, https: //commons. wikimedia. org/wiki/File: Red_Panda_(24680619391). jpg, PD. Slide 5: Modified image of raccoon, Harlequeen, https: //commons. wikimedia. org/w/index. php? curid=5110451, CC BY 2. 0. Slides 6, 12 -14: Modified image of giant panda, original by Chen Wu, http: //www. flickr. com/photos/30073301@N 00/3171164610, CC BY 2. 0. Slide 12: Image of bamboo, Yo. Price. Ville, http: //gallery. yopriceville. com/Free-Clipart-Pictures/Decorative-Elements. PNG/Bamboo_PNG_Image#. V 03 Ov. Y 6 h. Ru 4, Free clipart collection Slide 12: Image of a Chinese coin, Marhawkman, https: //www. flickr. com/photos/11951562@N 08/albums/72157606482093440 , CC BY 2. 0. Slides 17 -18: Figure 1 Panel A, Nie et al. 2015. Exceptionally low daily energy expenditure in the bamboo-eating giant panda. Science 349: 171174. Reprinted with permission from AAAS. Slides 19 -22: Fig. 1 Panel D, Nie et al. 2015. Exceptionally low daily energy expenditure in the bamboo-eating giant panda. Science 349: 171174. Reprinted with permission from AAAS. Slides 27 & 28: Figure 3 Panels A & B, Nie et al. 2015. Exceptionally low daily energy expenditure in the bamboo-eating giant panda. Science 349: 171 -174. Reprinted with permission from AAAS. Slide 31: Figure 2, Nie et al. 2015. Exceptionally low daily energy expenditure in the bamboo-eating giant panda. Science 349: 171 -174. Reprinted with permission from AAAS. Slide 33: Table S 3, Nie et al. 2015. Exceptionally low daily energy expenditure in the bamboo-eating giant panda. Science 349: 171 -174. Reprinted with permission from AAAS. Slide 39: Figure 3 Panels C & D, Nie et al. 2015. Exceptionally low daily energy expenditure in the bamboo-eating giant panda. Science 349: 171 -174. Reprinted with permission from AAAS. Slide 43 -44: modified image of panda bear sleeping at National Zoo, Sandy Ussia, used with permission. Slides 45 -46: diagram of HYP-AP-THY axis, Patricia J. Moore, case author. Slides 47 -48: modified image of pika, Sevenstar, https: //commons. wikimedia. org/w/index. php? curid=935374, PD. 49