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 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

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 Fig. 8.20

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

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 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

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

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

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 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

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 Fig. 8.7

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 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

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

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

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

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

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

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

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. 8.33-8.34

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

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

Endothermic Homeothermy in the Heat Endotherms respond to high ambient temperatures by: Limiting heat gain 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 Fig 8.28

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

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

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