mammals are endothermic homeotherms maintain a constant body temperature… …using internal...
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Mammals are endothermic homeotherms
Maintain a constant body temperature…
…using internal sources of heat.
MR = C′ · (Tb – Ta)
When it’s cold outside, it takes a lot of energy to maintain a constant body temperature
Res
ting
Met
abol
ic R
ate
Energy Deficit
Energy Deficit
Energy Surplus
Food A
vailability
Jan 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
Body temperatureNo Effect
The identify of the “trigger” is still
not clear
Hibernation
summer 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
Summer Hibernating
Skeletal Muscle
Carbohydrates are the main energy source during summer, but fats are the primary metabolic fuel during hibernation.
glucose
pyruvatePyruvate Dehydrogenase
Acetyl CoA
Krebs Cycle
Fatty Acids
amino acids
proteins
muscle
Food
Ketone Bodies
Hibernators are natural models for
starvation physiology
hibernation hibernation
Fattening up: eat, eat, eat…
Saturated Fatty Acid (SFA) – stearic acid
Monounsaturated Fatty Acid (MUFA) – oleic acid
Polyunsaturated Fatty Acid (PUFA) – linoleic acid
…but not just anything!
Pro
port
ion
of H
iber
natin
g A
nim
als
Low Diet PUFA
High Diet
PUFA
Hib
erna
tion
MR
and
Tb
Low Diet PUFA
High Diet
PUFAH
iber
natio
n B
out
Leng
th
Animals cannot synthesize PUFAs, but plants can!
Low Diet PUFA
High Diet
PUFA
Melting Point
SFA MUFA
PUFA
69.6°C 13-14°C -5°C
Peroxidizability None Some Lots
PUFA
>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 brown adipocyte
Electron Transport Chain
H+
H+
ATP Synthase
ATP
Uncoupling Protein 1 (UCP1)
But only once body temperature > 15°C
ATP ADP + Pi + heat
…and shivering is the other!
Ability to rewarm using internal heat sources distinguishes hibernation from hypothermia
Social hibernation
Tb = 10°C
Solitary
Tb = 10°C
Group
Ta = 0°C
Tb = 10°C
Tb = 10°C
Tb = 10°C
Tb = 10°C
Tb = 10°C
Ta = 0°C
Tb = 10°C
Social hibernation: arousals must be synchronous…
55
60
…because synchrony affects energy expediture.
Solitary individuals
Hypothesis #1: Metabolic end-products accumulate to toxic levels
wastes
Why arouse?
wastes
Hypothesis #2: Damaged proteins accumulate during torpor
Damage
Denaturation
amino acids
Carbon backbone + NH3CO2
Urea Glutamine
Urine
Hypothesis #3: Animals cannot detect infections at low body temperature
Detection ResponseSignal Transmission
Some bacteria grow well at cold temperatures
prostaglandins
Hypothesis #4: Animals cannot sleep during hibernationS
leep
Deb
t R
epay
men
t
Sle
ep D
ebt
Rep
aym
ent
1000
kg
0.01 0.1 1 10 100
Only small mammals hibernate.
Potential energetic savings are lower for larger animals
Body Size
Mas
s-sp
ecifi
c M
R Summer Active
Hibernation
Cold environments affect larger animals less than smaller animals
10g
5kg
1kg
Ambient Temperature
Mas
s-sp
ecifi
c M
R