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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
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 tolerance2
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
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
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
  • 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
Dealing with Subfreezing Temperatures
  • Supercooling
    • Freezing point depression
  • Use of antifreeze
  • Freeze tolerance
  • 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 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 – substance that prevents ice crystal formation
    • thermal hysteresis - lowers freezing point but not melting point

Fig. 8.20

freeze tolerance
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 tolerance11
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
Thermal Adapation
  • Different species have adapted to differences in temperature between species ranges

Figs. 8.14, 8.16b,c,

8.17, 8.18

thermal acclimatization
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
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
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
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.
  • 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
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

ectotherms and heat
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
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 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 endothermy22
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
  • Generate most body heat physiologically
  • Tend to be homeothermic
    • regulate body temperature (Tb) by adjusting heat production
regional homeothermy
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

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

body heat retention32
Body Heat Retention
  • Increased body size
    •  surface area/volume ratio
    • Generally thicker coats
    • Bergmann’s Rule
      •  size w/  latitude
body heat retention33
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

endothermic homeothermy in the heat
Endothermic Homeothermy in the Heat
  • Endotherms respond to high ambient temperatures by:
    • Limiting heat gain
    • Increasing heat dissipation
limiting heat gain
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
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
Evaporative Cooling
  • Sweating
    • Extrusion of water through sweat glands onto the skin
  • Panting
    • Evaporative cooling through the respiratory system surfaces
sweating vs panting
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

panting and brain cooling
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