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 ice-nucleating 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