Extreme Temperatures and Thermal
Tolerance
• All organism have a range of tolerable body
temperatures
– Homeothermic endotherms – narrow range
– Poikilothermic ectotherms – broad range
• Exceeding limit of thermal tolerance
– DEATH!!!!!
Extreme Temperatures and Thermal
Tolerance
Factors influencing lethal exposure:
• Exposure Temperature
– Degree to which temperature exceeds limits of
tolerance
• Exposure Duration
– Length of time to which organism is exposed to
lethal temperature
• Individual Variation
Problems With High Temperature
• Denaturization of proteins
– Structural and enzymatic
• Thermal inactivation of enzymes faster than rates of
activation
• Inadequate O2 supply to meet metabolic demands
• Different temperature effects on interdependent
metabolic reactions (“reaction uncoupling”)
• Membrane structure alterations
• Increased evaporative water loss (terrestrial animals)
Problems with Low Temperatures
• Thermal inactivation of enzymes faster than rates of
activation
• Inadequate O2 supply to meet metabolic demands
• Different temperature effects on interdependent
metabolic reactions (“reaction uncoupling”)
• Membrane structure alterations
• Freezing
Freezing
• Drastic reduction in gas diffusion
– liquid water vs. solid water
• Drastic reduction in enzyme function
– Reduced molecular mobility
• Structural disruption of enzymes
• Mechanical disruption of cell membranes
• Osmotic dehydration due to freezing of
extracellular water
– Most important factor
Dealing with Subfreezing
Temperatures
• Supercooling
– Freezing point depression
• Use of antifreeze
• Freeze tolerance
Supercooling
• Water does not usually freeze at 0 °C
– Freezing involves ice crystallization
– Can occur spontaneously below 0 °C
– Water can remain liquid until crystallization
occurs
Supercooling
• Supercooling can be enhanced by addition of
solutes to an aqueous solution
–  [solutes],  freezing point
• Freezing point depression
– E.g. insects
• Produce high levels of glycerol
• Lowers freezing point
• Willow gallfly larvae can supercool to –60 °C
Antifreeze
• Antifreeze – substance that
prevents ice crystal
formation
– thermal hysteresis - lowers
freezing point but not
melting point
Freeze Tolerance
• Ability to tolerate freezing of
extracellular fluid
– Must cope with…
• potential mechanical damage
• effects of dehydration
• Cryoprotectants
– Substances that help animals
avoid damage from freezing of
body tissues
– E.g. glycerol
• appears to stabilize cell membrane
and protein structure
Freeze Tolerance
• Many freeze tolerant organisms have icenucleating agents
– Promotes ice-crystal formation in the extracellular
fluid
• Draws water out of the cells,  intracellular
concentrations and  freezing point
– Helps prevent crystal formation inside the cells
• Prevents mechanical damage
Thermal Adapation
• Different species have adapted to
differences in temperature between
species ranges
Thermal Acclimatization
• Acclimation and acclimatization
are physiological changes in
response to previous thermal
history
• Exposure to warm temperatures
increases heat tolerance,
decreases cold tolerance
• Thermal tolerance of many
species changes with seasonal
changes in temperature
Mechanisms of Thermal
Acclimatization and Adaptation
• Changes in enzyme systems
– Changes in enzyme synthesis/degradation
– Changes in use of specific isozymes
– Modulation of enzyme activity by the
intracellular environment
• Changes in membrane
phospholipids
– increase saturation of fatty acids with
increased temperature
– homeoviscous adaptation
Temperature Regulation
Approaches to thermoregulation:
• Thermal conformity (poikilothermy)
– allow body temperature to fluctuate with
environmental temperature
• Thermoregulation (homeothermy)
– Maintain body temperature at relatively
constant levels largely independent of mean
environmental temperature
Thermoregulation Methods
• Behavioral control
– Controlling body temperature by repositioning body in the
environment
• Physiological control
– Neural responses (immediate)
• E.g. modification of blood flow to skin, sweating/panting,
shivering, etc.
– Acclimation responses (long-term)
• Changes in insulation, increased capacity got metabolic heat
generation, etc.
Ectothermy
• Obtain body heat from external environment
• Environmental heat availability subject to change
– Some thermally stable environments
• vary only 1-2 °C/year
– Some highly variable environments
• 80 °C variation in one year
– Most ectotherms must deal with some degree of
temperature variation
Ectotherms and Cold
• Inactivity of enzyme systems
– Cold-adapted species have
enzymes that function at
higher rates at lower
temperatures
• Subfreezing Temperatures
– Supercooling
– Antifreezes
– Freeze Tolerance
Ectotherms and Heat
• Problems associated with heat
– Enzyme denaturization and pathway uncoupling
– Elevated energy requirements
– Reduced O2 delivery
• affinity of Hb for O2 decreases with increased temperature
• Critical Thermal Maximum (CTM)
– Body temperature over which long-term survival is no
longer possible
Ectotherms and Temperature
Regulation
• Behavioral Regulation
– Reposition body relative to heat sources
in the thermal environment
– Most widely used method
• Physiological Regulation
– Redirect blood flow for increased heat
gain-heat loss
– Pigmentation changes
• absorb/reflect radiant heat
Ectothermy vs. Endothermy
Ectothermy – low energy approach to life
• Pros
– Less food required
– Lower maintenance costs (more energy for growth and
reproduction)
– Less water required (lower rates of evaporation)
– Can be small – exploit niches endotherms cannot.
• Cons
– Reduced ability to regulate temperature
– Reduced aerobic capacity – cannot sustain high levels of activity
Ectothermy vs. Endothermy
Endothermy – high energy approach to life
• Pros
– Maintain high body temperature in narrow ranges
– Sustain high body temperature in cold environments
– High aerobic capacity – sustain high levels of activity
• Cons
–
–
–
–
Need more food (energy expenditure 17x that of ectotherms)
More needed for maintenance, less for growth and reproduction
Need more water (higher evaporative water loss)
Must be big
Endotherms
• Generate most body heat physiologically
• Tend to be homeothermic
– regulate body temperature (Tb) by adjusting
heat production
Regional Homeothermy
• Core body temperature
– Temperature at the interior of the
body (thoracic and abdominal
cavity, brain, etc.)
– Maintained within narrow
margins
• Peripheral body temperature
– Temperature of integument,
limbs, etc.
– Tends to vary considerably
Metabolism vs. Ambient
Temperature
• Thermal Neutral Zone
– basal rate of heat production balances
heat loss
– No additional energy required to
regulate temperature, just modification
of thermal conductance
• Lower Critical Temperature
– Temperature below which basal
metabolism does not produce enough
heat to balance heat loss
• Upper Critical Temperature
– Temperature above which modifying
thermal conductance cannot balance net
heat gain
Below the Lower Critical
Temperature…
• Zone of Metabolic Regulation
– Increase in metabolism to increase
heat production to balance
increased heat loss
– Shivering, BAT, etc.
• Hypothermia
– Increased metabolic production
cannot compensate for heat loss
– Tb decreases (as does metabolism)
Above the Upper Critical
Temperature…
• Zone of Active Heat Dissipation
– Animal increases activity to
increase heat loss
– Evaporative cooling
• Hyperthermia
– Evaporative cooling cannot
counteract heat gain
– Tb rises (as does metabolism)
towards CTM
Endothermic Homeothermy in
the Cold
• Endotherms respond to low ambient
temperatures by:
– Increasing heat production (thermogenesis)
– Limiting heat loss
Thermogenesis
• Shivering
– Rapid contractions in groups of antagonistic muscles
– No useful work generated
– Heat liberated by hydrolysis of ATP
• Non-shivering Thermogenesis
– Enzyme systems activated that oxidize fats to
produce heat
– Virtually no ATP production
Non-shivering Thermogenesis
• Brown Adipose Tissue (BAT)
– Highly vascularized, with large
numbers of mitochondria
– Inner mitochondrial membranes
contain thermogenin
• Allows H+ to bypass ATP synthase
• Protons re-enter mitochondrial matrix
and bind to O2, generating heat and
water
– Heat absorbed by blood in vasculature
and distributed throughout the body
Body Heat Retention
• Insulation
– Fur/hair/feathers (pelage)
• Reduce effects of convection
– Fat/blubber
• Lower thermal conductivity of integument
• Low metabolic activity (low perfusion needed)
– Aggregration
• Reduce convection effects
Body Heat Retention
• Increased body size
–  surface area/volume ratio
– Generally thicker coats
– Bergmann’s Rule
•  size w/  latitude
Body Heat Retention
• Circulation
– Reduced skin perfusion
• Limit heat loss from blood
– Countercurrent Exchange
• Heat transferred from arteries to veins
• Limit heat loss from extremities
Endothermic Homeothermy in the
Heat
•
Endotherms respond to high ambient
temperatures by:
1. Limiting heat gain
2. Increasing heat dissipation
Limiting Heat Gain
• Increased Size
– Large animals have large heat capacities and
low surface area/volume ratios
• Take longer to heat up
– Large animals tend to have thicker pelage
• Insulate body from external heating
Increasing Heat Dissipation
• Specific heat exchange surfaces
– Enable heat loss through
conduction/convection/radiation
– Thin cuticle
– Highly vascularized
– Lightly insulated
– Large surface areas
– Allen’s Rule
• The warmer the climate, the larger the
size of appendages
Evaporative Cooling
• Sweating
– Extrusion of water through sweat
glands onto the skin
• Panting
– Evaporative cooling through the
respiratory system surfaces
Sweating vs. Panting
• Sweating
–
–
–
–
Passive (little energy expenditure)
High salt loss
No convection
No effect on blood pH
• Panting
–
–
–
–
Active (requires muscle contraction)
No salt loss
Convection – increases cooling
Increased ventilation  pH
Panting and Brain Cooling
• Panting can cool brain during
high levels of activity
– Rete mirabile
• heat exchange between warm
arterial blood and cooled venous
blood from nasal cavity
– Maintain brain temperature
despite abnormally high body
temperature