Thermal Physiology of Endotherms

Overview of Thermal Physiology

• Thermal energy influences chemical interactions affecting macromolecular structure and biochemical reactions.
• Thermal strategy involves behavioral, biochemical, and physiological responses ensuring body temperature (Tb) is within an acceptable limit.
• Ambient temperature (Ta) is the temperature of the animal's surroundings, and is the most important environmental influence on an animal's thermal strategy.
• Animals must be able to survive extreme thermal conditions and changes specific to their environment.
• Different environments exhibit varying thermal conditions, including hot springs (high Ta), hot deserts (with daily variations), alpine regions (extreme cold), temperate zones (seasonal variations), intertidal zones (rapid Ta variations), lakes (thermal stratification and winter freezing), subterranean refuges (moderate and stable Ta), epipelagic zone (variable Ta), mesopelagic zone (stable Ta), and bathypelagic zone (cold, stable Tb), as well as hydrothermal vents (>100°C).

Thermal Strategies

• Two major thermal strategies exist: tolerance and regulation.
• Tolerance involves allowing body temperature to vary with ambient temperature.
• Regulation keeps body temperature constant, regardless of ambient temperature.
• Both strategies have associated costs and benefits.

Heat Fluxes

• Body temperature reflects the thermal energy of the body's molecules.
• Thermal energy can transfer from the animal to the environment or vice versa, moving down a temperature gradient.
• Many sources and sinks of thermal energy exist.
• The total thermal energy (ΔHtotal) is a dynamic equation comprised of: metabolic (ΔHmetab), conductive (ΔHconduct), convective (ΔHconvec), radiative (ΔHrad), and evaporative (ΔHevap.) components.

Types of Heat Flux

• Convection: The transfer of thermal energy between an object and a moving external medium. The rate of heat exchange depends on the temperature gradient and the rate of fluid flow, as well as the fluid's conductivity.
• Radiation: Emission of electromagnetic radiation. The sun is the primary source of radiant heat. Darker colors absorb more radiant heat (basking).
• Evaporation: Water molecules absorbing thermal energy from a surface during evaporation. Evaporative cooling occurs and heat loss depends on the volume of water and the heat of vaporization. Salt increases the heat of vaporization.
• Conduction: Transfer of thermal energy between one object or fluid to another directly. Heat flux (Q) is the rate of heat transfer from hotter to colder.

Fourier's Law: Quantifying Thermal Conduction

• Q = -kA(ΔT/d), where Q is rate of heat flow (in Watts), k is thermal conductivity, ΔT is temperature gradient, and d is distance of flux.

Conduction

• Thermal conductivity varies based on material type and geometry. Water has high thermal conductivity.

Insulation

• Insulation is a layer of material that reduces thermal exchange. Internal insulation (e.g., blubber) and external insulation (e.g., hair, feathers, air, water) are significant for temperature regulation in animals. Effectiveness depends on thickness.

Surface Area to Volume Ratio

• Large animals exchange heat more slowly than smaller animals.
• Bergmann's Rule: Larger animals tend to be found in colder environments.
• Allen's Rule: Animals in colder climates tend to have shorter extremities.
• Behavioral adjustments like posture changes and huddling alter surface area exposure.

Thermal Strategies (cont.)

• Poikilotherm: Variable body temperature.
• Homeotherm: Stable body temperature.

Thermal Strategies (cont. 2)

• Ectotherm: Environmental temperature determines body temperature.
• Endotherm: Animal generates internal heat to maintain body temperature.

Temporal and Regional Endothermy

• Hypometabolic phase: A period with a decrease in body temperature. Example: hibernation or torpor.
• Hibernation/estivation: Extended periods of dormancy, usually longer than a day.
• Torpor: Periods of reduced metabolic activity, often lasting less than a day. Duration varies among animals and dormancy types.

Homeothermy is Relative

• Circadian rhythm influences metabolic rate and body temperature.
• Relative temperature variations exist across species and within a single species. Animal breeding depends on accurately predicting fertilization windows.

Temperature effects: Biochemistry and Physiology

• Proteins and lipids are affected by temperature.
• Hydrogen bonds and van der Walls forces are disrupted by high temperature.
• Hydrophobic interactions are stabilized at high temperatures.

Macro-molecule Structure

• Proteins are affected by temperature over normal ranges.
• Hydrophobic interactions are stable at higher temperatures.

Membrane Fluidity

• Membrane fluidity is temperature dependent.
• Low temperatures cause membrane lipids to solidify, affecting protein function. High temperatures increase fluidity.
• Increased protein movement correlated with increased membrane fluidity.

Homeoviscous Adaptation

• Homeoviscous adaptation maintains membrane fluidity at differing temperatures by adjusting membrane lipid content (e.g., fatty acid chain length and saturation).

Homeoviscous Adaptation (cont)

• Phospholipid classes (e.g., PC, PE) have different effects on membrane fluidity.
• Cholesterol content can prevent solidification at cooler temperatures.

Thermoregulation and Acclimatization

• Ectothermic animals remodel tissues in response to long-term temperature changes by altering metabolic "machinery". This could be qualitative or quantitative.
• Qualitative adaptations involve changes in the type of metabolic "machinery," such as differing myosin isoforms across seasons (e.g., winter vs. summer).
• Quantitative adaptations involve more metabolic "machinery" (i.e., more mitochondria).

Cold Adaptation

• Psychrotrophs thrive in low temperatures.
• Enzyme activity can be affected by weakened bonds at low temperatures.
• Cold-adapted enzymes can be more susceptible to temperature dependent unfolding.

Freeze-tolerance/avoidance

• Freeze-tolerance: Animals allow tissues to freeze in a controlled manner.
• Freeze-avoidance: Physiological mechanisms to prevent ice crystal formation.
• Supercooling: Ice crystal formation requires a trigger. Intracellular freezing avoidance or tolerance usually involves producing solutes.
• Antifreeze macromolecules disrupt ice crystal formation by binding to small crystals, preventing their growth.

Maintaining a Constant Body Temperature

• Endotherms use high metabolic rates for thermogenesis.
• High metabolic rates provide energy for growth, development, digestion, and biosynthesis.
• Endotherms often maintain a constant body temperature higher than the environmental temperature.
• Futile cycles are metabolic processes designed to produce heat.

Regulation of Body Temperature

• Heat regulation coordinates multiple physiological systems, primarily through the hypothalamus (for mammals).
• Coordination signals alteration of heat production and dissipation (e.g., shivering)
• Some species have spinal cord based thermoregulatory centers (e.g., birds).

Shivering Thermogenesis

• Shivering thermogenesis is a coordinated muscle contraction.
• Muscles undergo rapid nutrient depletion, leading to costs.
• Important for maintaining body temperature.

Brown Adipose Tissue (BAT)

• BAT is a tissue used for nonshivering thermogenesis in small mammals and newborns.
• BAT is located near the back and shoulder regions.
• BAT has higher mitochondrial levels and produces the protein UCP-1(uncoupling protein-1) to uncouple mitochondrial electron transport from ATP synthesis, thus releasing energy as heat.
• BAT is activated by sympathetic nervous system input that activates fatty acid beta oxidation and upregulates UCP protein translation/transciption.

Ground Squirrel Orbital Fat Depot

• Similar morphology and composition to BAT.
• Potentially thermogenic, but lacks UCP1/3.
• The thermogenic mechanism is yet to be fully understood.

Torpor

• Shallow torpor: Moderate reduction in metabolic rate and body temperature that adjusts with ambient temperature.
• Deep torpor: High reduction in metabolic rate and body temperature that is more strongly correlated with environmental temperature.

Explore the fascinating mechanisms of thermoregulation in endothermic animals. This quiz covers key concepts such as thermogenesis, body temperature regulation, and the role of behavioral strategies. Test your knowledge on how mammals and birds adapt to their environments through thermal physiology.