Metabolic Marvels of Bear Hibernation- Part 2

bear silhouette

Welcome and thanks for reading. This post is the second that discusses the tantalizing chemistry related to the metabolic marvel of bear hibernation. Click to read the first post. This second post focuses on the nutritional (and hence food energy) landscape bears must navigate to prepare for the long months of winter inactivity and caloric deprivation. Specifically, this post includes discussion about the following general chemistry topics/concepts- thermochemistry, unit conversions, and interpretation of numerical data.

Included are two in-class or takehome exercises/activities (identified as #1 and #2) that perhaps may prove beneficial to students. You are free to use, modify, and reproduce the two exercises/activities. Please though practice conspicuous attribution. And if you do modify one or both, please share your new ideas with the ChemEd X community so as to promote and ensure a continuous free-flow of ideas. Thanks as always...And first, a brief generic commentary on bear dietary habits.

When not hunkered down hibernating (4 to 7 months of the year depending on where bears live), bears are constantly on the move, oftentimes covering great distances over diverse food habitats as they roam the feast-or-famine, boom-or-bust nutritional landscape where they live. Bears roam because it's a biological and chemical urgency. They search for anything edible, preferably highly digestible energy-rich foods (a large nutritional energy/mass of food consumed ratio). A bear's roaming is not an aimless, energy sucking hit-or-miss endeavor as metabolic energy is too valuable to waste in a bear's live-or-die world. Rather bears search for food with precision and purpose, honed over millions of years from adaptive evolutionary pressure. A highly flexible, adaptable, and opportunistic omnivore, bears do not have picky culinary palates. They eat whatever meat, leafy vegetation, berries, and/or insects are available (Mowat and Heard, 2006; Fortin et al, 2013; Gunther et al, 2014; McLellan, 2011; Schwartz et al, 2014). Basically, if something is remotely edible, then a bear will eat it. If interested, at the end of this post below my sign off and above the Reference section is a figure showing the seasonal availability and annual variability from mid-March to late October of foods for a bear to eat in the Greater Yellowstone Ecosystem (GYE). 

Come late August/early September or thereabouts, bears enter a manic phase of binge eating and gorging known as hyperphagia. During this time, bears can consume upwards to 20,000kilocalories plus of digestible food content daily, a figure roughly four times greater than what is typically required during pre-hyperphagia (Nelson, 1980; Nelson et al 1983)! What's the end game for such food-eating hedonism? To put on as much fat as possible before going up to 120 to 150+ consecutive days without eating during hibernation.

So how chunky can a bear get? Since 2014 the public has had the chance to vote during (FBW) for the fattest bear of the (KNPP) in Alaska. FBW is the ursine equivalent of the NCAA basketball March Madness bracket. In October each year the public selects via online voting the fattest KNPP bear in a head-to-head photo competition. The 2019 FBW winner is 435 Holly. In a span of about two months, Holly, and all the other KNPP brown bears, has packed on the pounds in preparation for a foodless winter slumber. Come late April/early May 2020 when Holly awakes from hibernation, she will be even skinnier than the mid-summer July photo below.

Because of their habitat KNPP brown bears (Ursus arctos) are predominately carnivorous, feasting for most of the summer/early autumn on a protein- and fat-rich meat diet of migrating sockeye salmon and coastal seafood (clams, mussels, and dead whales and other oceanic animals that wash onto the beach). By contrast, non-coastal brown bears- aka grizzly bears (Ursus arctos horribilis), like those that live in the Alaskan and Canadian interior and further south in North America in Yellowstone (WY), Grand Teton (WY), and Glacier (MT) National Parks, and in the  (WA, ID, and BC, Canada) and (MT) Recovery Ecosystems- are omnivorous, with a greater proportion of their diet being more plant than meat-based. Upwards to 95% of the grizzly population in Glacier NP is plant matter based whereas it is around 50% for grizzlies farther south in Yellowstone National Park. Sadly, the sparse reproducing populations, low female reproduction rates, and low genetic diversity of grizzlies in the rugged Selkirk and Cabinet-Yaak ecosystems mean that these grizzlies are approaching functional biological extinction.

So how can this knowledge about the bear and natural worlds be used to teach some chemistry? Below are two class or takehome activities that relate bear diet to thermochemical information. Both emphasize unit conversions and interpretation of numerical data. A teaching commentary follows each activity. 

Activity #1: Suppose a hyperphagic, opportunistic bear quietly sneaked into a McDonald's. How many Big Macs would the bear need to eat for that day to supply its hyperphagia requirement of 20,000kcal?

Commentary about Activity #1: This straight forward in- or out of- class chemistry 'math' activity emphasizes basic chemistry mathematics and decoding thermochemical (energy-mass) information from a table. To successfully complete the table, students need to figure out:

1. what information is needed to correctly answer a question with the correct units? 

2. is that information already available or does it need to be found?, and

2. what calculations are required to get the correct numerical value for the question asked?

Note that students must convert between units (grams to pounds, calorie to Joule) in order to supply the unit requested in the table above. I do not tell students that they need to be aware of unit interconversions. Nor do I tell them the conversion coefficient between grams and pounds and calories and Joules. I expect them to know it already or, if need be, look it up. Nor do I tell them that all the info needed is already available on the exercise/activity sheet. They need to figure out- based on the questions asked- if more information is needed. If students go off on a wild goose chase trying to find- in the end- unnecessary information, then I let them do it. It is a learning opportunity. I am not a fan of pre-packaging all the requisite information in a question. Part of my teaching philosophy, adjusted over 25 years doing this, is that students need to learn how to find information, need to learn how to figure out if the information at hand is relevant to the question asked, and need to learn how to discriminate between useful and useless information relative to the question asked. Regarding useful/useless information, knowing that Abe Lincoln was the 16th President of the US is irrelevant to these thermochemical inquiries but certainly relevant and useful in a discussion about the Civil War. Or a student may ask, "What is the gross energy value (GEV) in the table?" My response, "The units define what it is." And of course for this exercise one can use any other type of food- hot dogs, sirloin steak, smoked salmon, blueberries, and so on.

Below are the answers with respect to the Big Mac activity above. They are based on the following presumptions (to keep calculations simple):

  • all food mass in the Big Mac is 100% digestible,
  • all nutrient food energy in the Big Mac can be metabolized, and
  • the 540Cal/Big Mac comes from nutrients (carbs, fat, and/or protein) only. 

Answers (left to right in table above): 37 [(20,000kcal/day)/(540kcal/Big Mac)]; 17.5lbs (37 Big Macs x 215g/Big Mac x 1lb/454g); 37.2kJ/g (= 8.9kcal/g = 250kcal from fat/28g fat where 4.18J/cal); ~54% [(energy from carbs + proteins)/total energy) = (total energy - energy from fats)/total energy = {(540 - 250)/540}Cal x 100]; and 21.4% [(mass carbs/total mass Big Mac) x 100 = (46g carbs/215g Big Mac) x 100].

Variations to the above questions could include variations to the presumptions. For example, the presumption that all the food mass in a Big Mac is digestible to a bear is just that, a presumption. Reality indicates otherwise. Only a certain percentage of food is digestible and hence only a certain percentage of energy in food can be metabolized. This is similar to the actual product yield for a reaction carried out in a lab versus the stoichiometric dependent theoretical yield. It is never 100%.

Activity #2: Not all foods are the same in energy content, digestibility, and composition. Table 2 below gives an assortment of information for two hypothetical food types available to bears in Yellowstone National Park. Answer questions (Q) 1-4. Gross Energy Value (GEV) is the total amount of chemical energy in a food per mass of food as determined by complete thermal combustion in a constant-volume bomb calorimeter. GEV is the energy available (but not necessarily used) to a bear if the food is 100% digestible.  

Commentary about Activity #2: Activity #2 lends itself to the use of the high engagement teaching technique,Think-Pair-Share (TPS). Either individually or as a group (no more than 3 students/group is recommended) students first answer a question and then discuss their answer with another student or group. If answers differ, then students need to reconcile the differences. Basically, one student or group tries to convince the other that she/he (or they, if groups are used) is (are) correct. Of course, both may be wrong. At the end of an allotted period of time, the faculty reveals the correct answer. Wrong answers are discussed as to why they are wrong. White boards also can be used to report out to the rest of the class the answers. The References section lists some TPS resources if the reader is unfamiliar with or wants to learn more about how other faculty use TPS to augment student engagement.

The answers to Q1 are c., d., and f. [GEVA = 396kcal/105g = 3.78kcal/g; GEVB = 570kcal/84g = 6.78kcal/g; GEVB/GEVA = (6.78/3.78)kcal/g = ~1.80:1]

The answers to Q2 are b., d., and g. (since mass used (15g) for both is the same and GEVB > GEVA). An added inquiry question to Q2 may be to ask students why a food needs to be dry in order to get an accurate measurement of a food's nutritional energy value. Short answer: It is desireable for all the energy to go into combustion of the solid food material and not into a combination of combustion of the solid material plus an energy-intensive, endothermic liquid to gas phase change for water.

The answer to Q3 is ~1.4 pounds where 18,400kJ = 4,402kcal and [4,402kcal/(570kcal/84g food B)]x(1lb/454g) and assumptions made in question. Reality is that the mass of food needed is higher by a factor of 2-3 depending on the proportions of plant and animal matter consumed. But this question focuses on math and not physiology.

Q4 may be a good opportunity to have students learn or practice using a spreadsheet and showing numerical data in visual format.

The upcoming third installment (Part 3) shifts gears a bit as it will focus on the organic chemistry of fat catabolism- a hibernating bear's life-saving defense against starvation.

Cheerio, SJD

Figure below: Seasonal availability (peak dates; solid colored rectangles) and annual variability (colored lines with circles at end) from mid-March to late October of high and low energy density foods for a bear to eat in the Greater Yellowstone Ecosystem (GYE). 

References

Fortin, J.K., Schwartz, C.C., Gunther, K.A., Teisberg, J.E., Haroldson, M.A., Evans, M.A., and Robbins, C.T. 2013. Dietary adjustability of grizzly bears and American black bears in Yellowstone National Park. The Journal of Wildlife Management 77(2):270-281. DOI: 10.1002/jwmg.483

Gunther, K.A., Shoemaker, R.R., Frey, K.L, Haroldson, M.A., Cain, S.L., van Manen, F.T., and Fortin, J.K. 2014. Dietary breadth of grizzly bears in the Greater Yellowstone Ecosystem. Ursus 25(1):60-72. DOI: 10.2192/URSUS-D-13-00008.1 

McLellan, B. 2011. Implications of a high-energy and low-protein diet on the body composition, fitness, and competitive abilities of black (Ursus americanus) and grizzly (Ursus arctos) bearsCanadian Journal of Zoology 89(6):546-558. DOI: 10.1139/z11-026

Mowat, G. and Heard, D.C. 2006. Major components of grizzly bear diet across North America. Canadian Journal of Zoology 84:473-489. DOI: 10.1139/Z06-016

Nelson, R.A. 1980. Protein and fat metabolism in hibernating bears. Federal Proceedings 39:2955-2958.

Nelson, R.A., Folk, Jr., G.E., Pfeiffer, E.W., Craighead, J.J., Jonkel, C.J., and Steiger, D.L. 1983. Behavior, biochemistry, and hibernation in black, grizzly, and polar bears. Ursus 5:284-290.

Schwartz, C.C., Fortin, J.K., Teisberg, J.E., Haroldson, M.A., Servheen, C., Robbins, C.T., and Van Manen, F.T. 2014. Body and diet composition of sympatric black and grizzly bears in the Greater Yellowstone Ecosystem. The Journal of Wildlife Management 78(1):68-78. DOI: 10.1002/jwmg.633 

For Think-Pair-Share (TPS)

Sam McKagan and Daryl McPadden. "." PhysPort (developed by AAPT, American Association of Physics Teachers), October 26, 2017; successfully retrieved November 2019 (https://www.physport.org/recommendations/Entry.cfm?ID=101319)

Heather Macdonald and Rebecca Teed. "" Science Education Research Center (SERC) at Carleton College (in Starting Point: Teaching Entry Level Geoscience); successfully retrieved November 2019 (https://serc.carleton.edu/introgeo/interactive/howto.html)

"" Science Education Research Center (SERC) at Carleton College (in Starting Point: Teaching Entry Level Geoscience); successfully retrieved November 2019 (https://serc.carleton.edu/introgeo/interactive/tpshare.html)

 

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