Thermal Relations

Thermal relations of animals require definition of two key concepts:

  • Endothermy

    • When an animal’s tissues are heated by its metabolic heat production

  • Thermoregulation

    • Maintenance of constant tissue temperature despite changes in environmental temperature

Some animals are endotherms, thermoregulators, both, or neither.

  • Animals who aren’t endotherms are called ectotherms or poikilotherms

  • Animals who both perform endothermy and thermoregulation are termed homeotherms

  • Heterotherms exhibit endothermy and thermoregulation in one part of the body, while lacking in other parts (e.g. bumblebees)

Temperature vs. Heat

  • Temperature

    • Measure of average speed of random atomic motions in a material

    • Determines the direction of heat transfer

  • Heat

    • Energy associated with the motion of each atom

    • Moves by conduction or convection

Animals exchange heat with its surroundings via

  • Conduction

    • Heat transfers via static object(s)

      • E.g. from warm end to cold end of metal bar

  • Convection

    • Heat transfers via moving substance(s)

      • E.g. via wind/flowing water

    • Much faster than convection

  • Evaporation

    • Water escapes in gas form due to absorption of heat

  • Thermal radiation

    • Emission of infrared wavelengths (=thermal radiation)

    • The warmer the object, the shorter the wavelength

      • Fire is visible thermal radiation (kind of)

Poikilothermy

  • May have behavioural control over their body temperatures, as they can move to warmer microclimates

    • Thus maintain a roughly constant body temperature

      • Must however tolerate greater spans due to e.g. bad weather (eurythermal and stenothermal)

  • When body temperature is raised in a series of steps, the metabolic rate increases exponentially

  • When the poikilotherm is kept at one body temperature for a long time, its exponential relation adapts to the constant body temperature due to acclimatisation

Acute responses

  • Occur promptly to changes in body temperature

Chronic responses

  • Changes that occur over a longer period

    • = Acclimation / Acclimatisation

When poikilotherms adapt to colder temperatures, the metabolic rate increases in a direction that puts the metabolic rate closer to the original metabolic rate prior to the drop in temperature

  • This is called compensation

Animals may acclimate to colder temperatures by

  • Increases synthesis of key rate-limiting enzymes

    • Thus increasing metabolism

  • Red muscles increase mitochondria density

The performance of a poikilotherm over body temperatures can be illustrated by the performance curve

  • Low performance at low body temperatures

  • Increases to a peak

    • Also maximal O2 delivery to blood

  • Afterwards declines rapidly

    • Pejus phase

    • Also less O2 delivery

  • Eventually dies (not sure why, temperature is lower than would cause protein denaturation)

Poikilotherms threatened with freezing may:

  • Produce antifreeze compounds

    • Lower freezing points of body fluids

    • Colligative antifreezes

      • Increases total concentration of solutes in body fluids

      • Includes glycerol, sorbitol, mannitol

    • Non-colligative antifreezes

      • Have particular chemical properties, e.g. bind to ice crystals to suppress ice growth, preventing them from joining up with other crystals

      • Lower freezing points but not melting points (=thermal hysteresis)

  • Perform supercooling

    • Cool down below freezing point, remaining unfrozen

      • If the temperature of the supercooled solution keeps falling, it may spontaneously freeze within a short time

        • Property of most liquids

    • Animals may lower supercooling point by cleansing their body fluids of ice-nucleating agents

  • Evolve tolerance of freezing

    • Can tolerate having their body fluids being completely frozen during winter

      • Does not include the intracellular body fluids

Note that the freeze-tolerance is only for extracellular body fluids. Freezing of intracellular body fluids is always fatal no matter what.

Homeothermy

  • Ability to physiologically maintain/regulate a constant body temperature independent of external environmental temperatures

  • Evolved independently in mammals and birds

    • Mammals ~37 degrees

    • Birds ~39 degrees

The body temperature is maintained by thermoreceptors in the skin, spinal cord, brain, and scrotum

  • Sensory signals are processed by thermal control centres in the hypothalamus.

The metabolic rate of a mammal/bird varies with ambient temperature.

However, within a certain range of ambient temperatures, called the thermoneutral zone (TNZ), the animal’s metabolic rate is independent of the ambient temperature.

  • In this zone, the resting BMR is constant

  • To the “left” (before the lower-critical temperature), the BMR increases (decreases until the TNZ)

  • To the “right” (above the upper-critical temperature, the BMR increases.

The metabolic rate is constant in the thermoneutral zone due to adjustment of the insulation.

  • When the ambient temperature lowers, the animal increases its insultation

    • The skin becomes erect, trapping a thicker layer of air between the skin and the surrounding air

    • Arterioles constrict, thus reducing heat loss

    • Also by curling up (mammals) or placing head under feathers (birds), heat loss is minimised

  • If the ambient temperature increases, the animal decreases its insulation

    • Arterioles can dilate, increasing the rate of blood flow to release heat

Note that TNZ tends to be narrower in smaller animals.

Dry heat transfer

  • Transfer of heat that excludes evaporation/condensation of water

    • Involves only conduction, convection, and thermal radiation

Thermogenic mechanisms

  • Increase rate of heat production when TA falls below the lower critical temperature

    • Shivering:

      • Unsynchronised high-frequency contractions and relaxations of the skeletal muscles

      • Uses ATP and liberates heat

    • Nonshivering thermogenesis (NST)

      • Most common in placental mammals

      • Occurs after long period of acclimation to low temperature (long after shivering has ceased)

      • Most important site of NST is brown fat

To restrict heat loss, many mammals reduce local temperature of the appendages

Counter-current heat exchange also helps the blood heat up on the way back to the core body.

For temperatures above thermoneutrality, there are two main strategies how to cope:

  • Active evaporative cooling

    • E.g. panting, sweating, gular fluttering

    • Extra loss of water

  • Hyperthermia

    • Birds/mammals allow their body temperatures to rise to unusually high levels

      • More heat leaves the body

    • Common in animals adapted to hot dry environments (to avoid water loss)

Both these strategies paradoxically increase the metabolic rate