Transcript hibernation
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 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) What triggers hibernation? Blood Transfusion Phenotype Food intake No Effect The identify of the “trigger” is still not clear Body temperature summer hibernation Hibernation Turn the 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 CoA muscle 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 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 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 (UCP1) …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 Ta = 0°C Tb = 10°C Tb = 10°C Tb = 10°C Tb = 10°C Tb = 10°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 wastes Hypothesis #2: Damaged proteins accumulate during torpor Damage Denaturation amino acids CO2 Carbon backbone + NH3 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 Sleep Debt Repayment Hypothesis #4: Animals cannot sleep during hibernation Only small mammals hibernate. kg 0.01 0.1 1 10 100 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 10g 1kg 5kg Ambient Temperature