Download 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