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Front
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Back
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Overview of Thermal Physiology
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- Ambient temperature (TA)
- Temperature of the animal's surroundings
- It is the most important environmental influence on animal's thermal strategy
- Animals must be able to survive thermal extremes & thermal change
- Thermal strategy: Combination of behavioral, biochemical, & physiological responses that ensure body temperature is within an acceptable limit
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Thermal Energy
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- Thermal energy moves down a temp. gradient from the animal to environment or vice versa
- Total thermal energy = Sum of each of the following:
- Metabolism
- Conduction: Transfer of thermal energy from an object or fluid to another
- Varies w/ the type of material & geometry
- Water has very high thermal conductivity
- Convection: Transfer of thermal energy b/n an object & external medium that is moving
- Radiation - Emission of electromagnetic radiation
- Evaporation - Water molecules absorb thermal energy from a surface when making the transition from liquid to vapor
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- Tolerance
- Body temperature is allowed to vary w/ ambient temperature
- Regulation
- Body temperature does not vary w/ ambient temperature
- Both strategies have costs & benefits
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Thermal Tolerance of Animals
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- Eurytherm: Can tolerate a wide range of ambient temperatures
- Stenotherm: Can tolerate only a narrow range of ambient temperatures
- Therefore, eurytherms can occupy a greater # of thermal niches than stenotherms
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Source of thermal energy: - Ectotherm: Environment determines body temperature
- Endotherm: Animal generates internal heat to maintain body temperature
Poikilotherm: - Variable body temperature
- Get heat from environment
- Cold-blooded: Reptiles, amphibians
Homeotherm: - Stable body temperature
- Warm-blooded: Birds, mammals
Heterotherm: - Switches between poikilothermic & homeothermic strategies
- Invertebrates & lower vertebrates
- Temporal heterotherms
- Changes over time (hibernating animals)
- Regional heterotherms
- Body temperature varies in regions of the body (tuna retain heat in red muscle)
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Thermogenesis: How Do We Produce Heat?
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- Heat is a by-product of metabolic processes
- Energy metabolism
- Digestion
- Muscle activity
- Both endotherms & ectotherms produce metabolic heat
- Only endotherms have the ability to retain enough heat to elevate body temperature above environmental temperature
- Endotherms possess futile cycles: Metabolic reactions whose sole purpose is to produce heat
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Surface Area to Volume Ratio (1)
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- Ratio increases as body size decreases
- Large animals exchange heat more slowly than small animals
- Bergmann's rule: Animals living in cold environments tend to be larger
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Surface Area to Volume Ratio (2)
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- Allen's rule: Animals in colder climates have smaller extremities
- Body posture can alter exposed surface area
- Huddling behavior reduces effective surface area
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Insulation
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- Layer of material that reduces thermal exchange
- Internal insulation (under the skin)
- External insulation (on the body surface)
- Effectiveness of insulation depends on its thickness
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Piloerection
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- Hair & feathers act as insulation
- Reduce thermal conductivity
- Efficiency of insulation depends on thickness
- Hair & feathers are pulled perpendicular by smooth muscles (erector muscles) attached at their base
- Mammals & birds get fluffier when it's cold
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Regulation of Body Temperature
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- Coordination of multiple physiological systems
- Internal thermostat
- Mammals
- Info. from central & peripheral thermal sensors is integrated in hypothalamus
- Hypothalamus sends signal to body to alter rates of heat production & dissipation
- Birds: Thermostat is located in the spinal cord
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- Unique to birds & mammals
- Uncoordinated myofiber (multinucleated muscle cell) contraction
- Works for short periods of time
- Muscles are rapidly depleted of nutrients & become exhausted
- Prevents animal from using locomotory muscle for foraging or predator avoidance
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Sweating
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- Used primarily by large animals
- Sweat is controlled by hypothalamus & reduces body temperature by evaporative cooling
- NaCl in sweat raises heat of vaporization, causing a greater heat loss than evaporation of pure water
- To minimize ionic and osmotic problems, the amount of NaCl in sweat decreases during long periods of heat exposure
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Brown Adipose Tissue (BAT)
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- Used for non-shivering thermogenesis
- Important in thermogenesis for small mammals & newborns in cold areas
- Located near the back & shoulder region
- Have high levels of mitochondria
- High rate of fatty acid oxidation
- Energy is released as heat
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Strategies for Surviving Freezing Temperatures
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- Freeze-tolerance: Animals can allow their tissues to freeze
- Extracellular fluid freezes, but intracellular fluid remains liquid
- Produce intracellular solutes to counter the movement of water
- Freeze-avoidance: Use behavioral & physiological mechanisms to prevent ice crystal formation
- Solutes depress the freezing point of a liquid
- Antifreeze macromolecules:
- Proteins or glycoproteins that depress the freezing point
- Disrupt ice crystal formation by binding to small ice crystal & preventing growth
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