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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.

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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!!!!!


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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


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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)


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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


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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


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Dealing with Subfreezing Temperatures

  • Supercooling

    • Freezing point depression

  • Use of antifreeze

  • Freeze tolerance


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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


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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


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Antifreeze

  • Antifreeze – substance that prevents ice crystal formation

    • thermal hysteresis - lowers freezing point but not melting point

Fig. 8.20


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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


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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


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Thermal Adapation

  • Different species have adapted to differences in temperature between species ranges

Figs. 8.14, 8.16b,c,

8.17, 8.18


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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


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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


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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


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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.


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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


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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


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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


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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


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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


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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


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Endotherms

  • Generate most body heat physiologically

  • Tend to be homeothermic

    • regulate body temperature (Tb) by adjusting heat production


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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


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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


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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


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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


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Endothermic Homeothermy in the Cold

  • Endotherms respond to low ambient temperatures by:

    • Increasing heat production (thermogenesis)

    • Limiting heat loss


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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


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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


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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


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Body Heat Retention

  • Increased body size

    •  surface area/volume ratio

    • Generally thicker coats

    • Bergmann’s Rule

      •  size w/  latitude


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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


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Endothermic Homeothermy in the Heat

  • Endotherms respond to high ambient temperatures by:

    • Limiting heat gain

    • Increasing heat dissipation


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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


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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


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Evaporative Cooling

  • Sweating

    • Extrusion of water through sweat glands onto the skin

  • Panting

    • Evaporative cooling through the respiratory system surfaces


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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


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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