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