Mammals have developed several adaptations for coping with cold environments, including increasing insulation through fur and fat, decreasing heat loss through mechanisms like vasoconstriction, and increasing metabolism. Some mammals use torpor or hibernation, which involves lowering their body temperature and metabolic rate to conserve energy. True hibernators, like marmots, undergo multi-day bouts of torpor over winter where their body temperature drops to near the ambient temperature. During hibernation, animals rely solely on fat reserves built up in preparation. Hibernation allows animals to survive periods of low food availability and is an ancestral condition that evolved separately in some birds and mammals.

1. Response to cold
environments: Freeze
avoidance vs. Freeze
tolerance

2. Dealing with cold—
Mammals




Increase insulation
Decrease heat
loss
Increase
metabolism
Decrease
metabolic costs

3. Increase insulation




Fur
Fat
Size
Burrowing

4. Decrease heat loss

Vasoconstriction

Piloerection

Countercurrent exchange

5. Increase metabolism

Physiological Thermogenesis:
– Basal Metabolic Rate (BMR)

Produces heat and ATP
– Shivering thermogenesis

Non-synchronous contraction of skeletal muscle
– Non-shivering thermogenesis (NST)


Brown adipose tissue (BAT)
Metabolismmax = BMR + NST + shivering

6. Temperature and
metabolic rate

7. Torpor and Hibernation


Torpor =naturally occurring drop in Tb (body
temp) with resulting drop in metabolism,
circulation, and respiratory rates
Who uses torpor?
– Birds, reptiles, amphibians, mammals

How is torpor different from hypothermia?
– Core temperature defended at low level in torpor
– Time taken to arouse
– Euthermia=normal warm Tb

8. Torpor and Hibernation
(cont)


Estivation=torpor bouts in summer
Hibernation =multi-day torpor bouts in
winter
– Endothermy NOT abandoned—regulate temp
at lower level

9. Hibernation

How developed?
–
–

Ancestral condition
Evolved separately in birds and mammals
Why do only some members of each group
hibernate?
– Size
– Resource availability

‘True’ hibernation vs. ‘winter lethargy’
– True hibernators are smaller, drop Tb to close to
ambient, bouts last several days
– Winter lethargy (bears, carnivores)—temp drops
a few degrees, bouts last weeks-months

10. The Hibernation Cycle:
Fall (Aug-Oct):
Major fat-building
(lipogenic) period-pack on as much
fat as possible,
prepare burrows
for winter
Winter (Oct-Mar):
Animals enter
hibernation, drop
Tb to near Ta,
drop metabolic
rate, start burning
fat reserves
Summer (MayAug): Continue
feeding to build
fat, sunning to
help store fat,
juveniles mature
Spring (MarMay): Arousal—
males first, then
females; mating,
parturition, start
replacing body
mass lost

11. Hibernation (cont)

A typical torpor bout (marmot):
– Animal drops Tb to near Ta, slows metabolism
(2-6 hours)


Respiration, heart, metabolic rates drop
Synapses separate in brain—brain dead!
– Animal remains at low tissue temp (6-8 days)

Metabolic process occur at very low levels
– Animal returns to euthermic temperature (1-2
hours)

Synapses re-grow, metabolic & cardiovascular rates
speed up, animal becomes active

12. Date
03/27/2006
03/17/2006
03/08/2006
C
02/27/2006
02/18/2006
02/09/2006
01/31/2006
01/22/2006
01/13/2006
B
01/04/2006
12/26/2005
12/17/2005
A
12/08/2005
11/28/2005
11/19/2005
11/10/2005
11/01/2005
10/23/2005
45
40
35
30
25
20
15
10
5
0
10/14/2005
Body Temp (c)
Typical hibernation season
GSM-5-05 Cold
D

13. Obesity and hibernation

Obesity=excessive accumulation of
body fat
– Generally considered pathogenic
– Diabetes, atherosclerosis, osteoarthritis

Hibernating animals become fat
without associated pathologies
– Hormone pathway blocking
– Time span/seasonality of obesity

14. Model Animals

Golden-mantled ground
squirrels (GMGS)
–
–
–

Spermophilus lateralis
Summer weight 150 g
Winter weight 350 g
Yellow-bellied marmots
– Marmota flaviventris
– Summer weight 2 kilos
– Winter weight 5 kilos
– Marmota monax
– Summer weight 2 kilos
– Winter weight 5 kilos

15. What can we measure?




Body temperature (implanted data
loggers, temp tracking radio collars)
Hormone levels (factors in obesity,
food intake)
Dietary choices (energy levels, fatty
acids for membrane fluidity)
Behavior (constrained by food intake,
dietary choices)