1. 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!!!!!

2. 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

3. 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)

4. 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

5. Freezing Fig. 8.19 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
Fig. 8.19

6. Dealing with Subfreezing Temperatures
Supercooling
Freezing point depression
Use of antifreeze
Freeze tolerance

7. 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

8. 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

9. Antifreeze
Antifreeze – substance that prevents ice crystal formation
thermal hysteresis - lowers freezing point but not melting point
Fig. 8.20

10. 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
Fig. 8.21

11. 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

12. Thermal Adapation Figs. 8.14, 8.16b,c, 8.17, 8.18
Different species have adapted to differences in temperature between species ranges
Figs. 8.14, 8.16b,c,
8.17, 8.18

13. 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
Figs 8.10,
8.13, 8.20

14. 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
Figs 8.16 b,c,
Fig 8.18

15. 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

16. 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.

17. 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

18. Ectotherms and Cold Fig. 8.16b Inactivity of enzyme systems
Cold-adapted species have enzymes that function at higher rates at lower temperatures
Subfreezing Temperatures
Supercooling
Antifreezes
Freeze Tolerance
Fig. 8.16b

19. 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

20. 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
Fig. 8.7

21. 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

22. 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

23. Endotherms Generate most body heat physiologically
Tend to be homeothermic
regulate body temperature (Tb) by adjusting heat production

24. Regional Homeothermy Figs. 8.26, 8.27 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
Figs. 8.26, 8.27

25. 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
Fig. 8.22

26. 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)
Fig. 8.22

27. 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
Fig. 8.22

28. Endothermic Homeothermy in the Cold
Endotherms respond to low ambient temperatures by:
Increasing heat production (thermogenesis)
Limiting heat loss

29. Thermogenesis Shivering Non-shivering Thermogenesis
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

30. 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
Fig. 8.25

31. Body Heat Retention Figs. 8.33-8.34 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
Figs

32. Body Heat Retention Increased body size  surface area/volume ratio
Generally thicker coats
Bergmann’s Rule
 size w/  latitude

33. Body Heat Retention Figs. 8.26, 8.29, 8.30 Circulation
Reduced skin perfusion
Limit heat loss from blood
Countercurrent Exchange
Heat transferred from arteries to veins
Limit heat loss from extremities
Figs. 8.26,
8.29, 8.30

34. Endothermic Homeothermy in the Heat
Endotherms respond to high ambient temperatures by:
Limiting heat gain
Increasing heat dissipation

35. 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

36. 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
Fig 8.28

37. Evaporative Cooling Sweating Panting
Extrusion of water through sweat glands onto the skin
Panting
Evaporative cooling through the respiratory system surfaces

38. Sweating vs. Panting Figs 8.24, 8.32 Sweating Panting
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
Figs 8.24, 8.32

39. 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
Fig. 8.31