1. MR = C′ · (Tb – Ta) Mammals are endothermic homeotherms
…using internal sources of heat.
Maintain a constant body temperature…
MR = C′ · (Tb – Ta)
When it’s cold outside, it takes a lot of energy to maintain a constant body temperature

2. Food Availability
Energy Deficit
Energy Surplus
Resting Metabolic Rate
Jan
Dec

3. Consider a 100 g mammal MR = C′ · (Tb – Ta)
Assume mean winter temperature is 10°C.
At 10°C, MR = 4 mL O2 / g/ hr
Therefore, MR = 400 mL O2 / hr for whole animal
= 0.4 L O2 / hr for whole animal
Assume winter lasts for 100 days (approx. 3 ½ months)
24 hrs/day X 100 days = hours
Therefore, over the entire winter, the whole animal consumes 960L O2
1 L O2 corresponds to 5 kcal of energy consumed
Therefore, over the entire winter, the whole animals consumes 4800kcal of energy
1 g of fat contains 9kcal of energy
Therefore, over the entire winter, the animal needs to metabolize 533g of fat!
That’s 533% more body mass!
MR = C′ · (Tb – Ta)

5. The identify of the “trigger” is still not clear
What triggers hibernation?
Blood Transfusion
Phenotype
Food intake
Body temperature
No Effect
The identify of the “trigger” is still not clear

6. summer
hibernation
Hibernation

7. Turn the thermostat down…

8. ….but keep the furnance on!

9. Inhibits protein synthesis
It’s more than just a passive thermal response!
Protein synthesis at 37°C
amino acid
traceable
Inhibits protein synthesis

10. Mitochondrial respiration rate at 37°C
Summer
Hibernation
Mitochondrial respiration rate at 37°C
Liver
Summer
Hibernating
Skeletal Muscle

11. Hibernators are natural models for starvation physiology
Carbohydrates are the main energy source during summer, but fats are the primary metabolic fuel during hibernation.
Food
glucose
amino acids
proteins
pyruvate
Pyruvate Dehydrogenase
muscle
Acetyl CoA
Fatty Acids
Ketone Bodies
Krebs Cycle
Hibernators are natural models for starvation physiology

12. Fattening up: eat, eat, eat…
hibernation
hibernation

13. …but not just anything! Saturated Fatty Acid (SFA) – stearic acid
Monounsaturated Fatty Acid (MUFA) – oleic acid
Polyunsaturated Fatty Acid (PUFA) – linoleic acid

14. Proportion of Hibernating Animals
Low Diet PUFA
High Diet PUFA
Low Diet PUFA
High Diet PUFA
Low Diet PUFA
High Diet PUFA
Hibernation MR and Tb
Proportion of Hibernating Animals
Hibernation Bout Length
Animals cannot synthesize PUFAs, but plants can!

15. SFA
MUFA
PUFA
Melting Point
69.6°C
13-14°C
-5°C
PUFA
Some
Lots
Peroxidizability
None

16. >80% of energy expenditure during hibernation season occurs during arousal and interbout euthermia

17. HEAT Brown adipose tissue is one main source of heat for arousal…
white adipocyte
brown adipocyte H+
ATP H+
Electron Transport
Chain
ATP
Synthase
Uncoupling
Protein 1
(UCP1)
HEAT

18. …and shivering is the other!
But only once body temperature > 15°C
ATP
ADP + Pi + heat

19. Ability to rewarm using internal heat sources distinguishes hibernation from hypothermia

20. Social hibernation

21. Solitary Group Ta = 0°C Tb = 10°C Tb = 10°C Ta = 0°C Tb = 10°C

22. Social hibernation: arousals must be synchronous…

23. …because synchrony affects energy expediture.60 55
Solitary individuals

24. Why arouse?
Hypothesis #1: Metabolic end-products accumulate to toxic levels
wastes
wastes

25. Hypothesis #2: Damaged proteins accumulate during torpor
Denaturation
amino acids
Urea Glutamine
CO2
Carbon backbone + NH3
Urine

26. Hypothesis #3: Animals cannot detect infections at low body temperature
Detection
Signal Transmission
Response
prostaglandins
Some bacteria grow well at cold temperatures

27. Hypothesis #4: Animals cannot sleep during hibernation
Sleep Debt Repayment
Sleep Debt Repayment

28. Only small mammals hibernate.kg
0.01
0.1 1 10
100
1000

29. Potential energetic savings are lower for larger animals
Summer Active
Mass-specific MR
Hibernation
Body Size

30. Cold environments affect larger animals less than smaller animals
Mass-specific MR
10g
1kg
5kg
Ambient Temperature