Mammals are endothermic homeotherms using internal sources of

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

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

Consider a 100 g mammal Assume mean winter temperature is 10°C. At 10°C, MR = 4 m. L O 2 / g/ hr Therefore, MR = 400 m. L O 2 / hr for whole animal = 0. 4 L O 2 / 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 960 L O 2 1 L O 2 corresponds to 5 kcal of energy consumed Therefore, over the entire winter, the whole animals consumes 4800 kcal of energy 1 g of fat contains 9 kcal of energy Therefore, over the entire winter, the animal needs to metabolize 533 g of fat! That’s 533% more body mass! MR = C′ · (Tb – Ta)

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

summer hibernation Hibernation

Turn thermostat down…

…. but keep the furnance on!

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 Liver Summer Hibernation Skeletal Muscle Summer Hibernating

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 Acetyl Co. A muscle Fatty Acids Ketone Bodies Krebs Cycle Hibernators are natural models for starvation physiology

Fattening up: eat, eat… hibernation

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

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

MUFA PUFA 69. 6°C 13 -14°C -5°C None Some PUFA Lots SFA Melting Point Peroxidizability

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

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

…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

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

Hypothesis #2: Damaged proteins accumulate during torpor Damage Denaturation amino acids CO 2 Carbon backbone + NH 3 Urea Glutamine Urine

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

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

Only small mammals hibernate. kg 0. 01 0. 1 1 10 1000

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

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