Extreme Temperatures and Thermal Tolerance

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 • 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 • 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 • 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 • 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 • 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 • 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. 8.33-8.34

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

33. 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 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 • Extrusion of water through sweat glands onto the skin • Panting • Evaporative cooling through the respiratory system surfaces

38. 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 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