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

Food Availability Energy Deficit Energy Surplus Resting Metabolic Rate Jan Dec

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 = 2400 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)

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

summer hibernation Hibernation

Turn the thermostat down…

….but keep the furnance on!

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

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

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

Fattening up: eat, eat, eat… hibernation hibernation

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

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!

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

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

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

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

Ability to rewarm using internal heat sources distinguishes hibernation from hypothermia

Social hibernation

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

Social hibernation: arousals must be synchronous…

…because synchrony affects energy expediture. 60 55 Solitary individuals

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

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

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

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

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

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

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