Thermal Biology 3

Endotherms

Thermoneutral Zone

The thermoneutral zone is the range of ambient temperatures within which an endotherm's metabolic rate remains constant. Outside this zone, the metabolic rate increases as the animal must expend energy to either cool down or warm up.

  • Definition: Range of temperatures where metabolic rate doesn't change with outside temperature.

  • Critical Temperatures: Above or below these, metabolic rate rises.

  • Metabolic Cost: Cooling or warming is metabolically expensive at extreme temperatures.

Example: Homeotherm

  • Range: 5 to 38 degrees Celsius (example).

  • Thermoregulation: Animals still thermoregulate within the thermoneutral zone, but without a change in metabolic rate.

  • Heat Production: Rate remains constant in the zone.

Thermoregulation within the Thermoneutral Zone

Animals regulate temperature without altering metabolic rate through:

  • Insulation: Birds and mammals erect fur or feathers to trap air, providing insulation.

  • Blood Flow: Arterioles constrict to reduce heat loss in cold, and dilate to increase heat loss in warmth. Controlled by the nervous system at no metabolic cost.

Thermogenesis Below the Lower Critical Temperature

Birds and mammals increase metabolic rate via thermogenic mechanisms:

  • Shivering:

    • Unsynchronized contraction and relaxation of skeletal muscle motor units.

    • No mechanical work done.

    • ATP is converted to heat.

  • Non-shivering Thermogenesis:

    • Uses mechanisms other than shivering to thermoregulate.

    • Brown Adipose Tissue (BAT): Specialized fat tissue for heat production.

      • Well-vascularized with many mitochondria (containing yellow cytochrome pigments, hence the brown color).

      • Innervated by the sympathetic nervous system.

      • Cells express UCP1 (uncoupling protein 1) in the inner mitochondrial membrane.

Brown Fat vs. White Fat

  • Brown Fat Cell: Numerous mitochondria, smaller fat droplets, expresses UCP1.

  • White Fat Cell: Fewer mitochondria, larger fat droplets, doesn't express UCP1.

Mechanism of Heat Production in Brown Adipose Tissue

  1. Norepinephrine (neurotransmitter) is released.

  2. Oxidation of stored lipids increases.

  3. Oxidative phosphorylation is uncoupled from electron transport.

  4. Chemical bond energy in fat is released as heat.

Chemiosmosis and UCP1

  • Normal Chemiosmosis: Electron transport and oxidative phosphorylation are coupled via a proton pump and ATP synthase.

  • UCP1 (Thermogenin): Found in the inner mitochondrial membrane of brown fat cells.

    • Makes the membrane permeable to hydrogen ions.

    • When activated by norepinephrine, lipid oxidation increases and UCP1 is activated.

    • Mitochondria perform lipid oxidation in an uncoupled state, producing heat instead of ATP.

Occurrence of Brown Fat

Prominent in cold-acclimated adults, hibernators, and newborns.

Coping with High Temperatures

Either evaporative or non-evaporative cooling mechanisms are employed.

  • Evaporative Cooling:

    • Efficient but leads to water loss.

    • Avoided by animals in hot, dry environments except as a last resort.

  • Non-evaporative Cooling:

    • Behavioral Mechanisms: Nocturnal activity, living in cool burrows.

    • Insulation: Thick fur acts as a heat shield (e.g., camels).

    • Cycling of Body Temperature: Storing heat during the day and releasing it at night.

      • Camels: body temperatures can range from 3535 degrees C at night to 4040 degrees C during the day.

Evaporative Cooling Mechanisms

Used when heat is accumulating excessively.

  • Sweating: Fluid secreted onto the skin through sweat glands (humans, horses, camels, kangaroos).

  • Panting: Increased rate of breathing causing water evaporation from the respiratory tract.

  • Gular Fluttering: Rapid vibration of the floor of the mouth in birds to force air over moist oral membranes.

Controlled Hypothermia

Animals allow their body temperatures to fall in a controlled manner.

  • Hibernation: Drop in body temperature to near ambient for days or longer during winter (e.g., groundhog).

  • Daily Torpor: Animal allows body temperature to drop for part of each day (e.g., white-footed mouse).

    • Metabolic rate drops during torpor.

  • Advantage: Energy efficient.

Maintaining a body temperature of 3737 degrees C requires a lot of energy; allowing it to drop reduces energy requirements.

  • Estivation: Drop in body temperature in the summer (less studied than hibernation).