mass effect on thermoregulation

small animal thermal regulation

• smaller animals have a greater available tb range

• faster heat transfer rate

• lower heat stoarage capacity

transient heat transfer

• Thermal Inertia and Body Mass

  • Increasing body mass slows heating and cooling rates because larger animals have greater thermal inertia — their body temperature changes more slowly in response to environmental fluctuations.

  Thermal inertia is caused by:

  1. Increased heat capacity with mass:

     • Larger animals have more body tissue and can store more heat energy, so it takes longer to raise or lower the temperature of their entire body.

  2. Conduction rates decrease with increasing mass:

     • Heat conduction slows down because larger bodies have thicker tissues and longer distances for heat to travel internally.

  3. Convection effects:

     • The boundary layer of still air (or water) around the animal increases with the animal’s linear size, reducing the rate of heat exchange.

  4. Surface area-to-volume ratio decreases with mass:

     • Larger animals have less surface area relative to their volume, which means less area for heat to be lost or gained per unit of body mass.

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  Movement in Variable Thermal Environments

  • Small animals experience rapid changes in body temperature when moving between areas of different temperatures because of their low thermal inertia.

  • Large animals respond more slowly to environmental temperature changes and their body temperature remains more stable during movement.

  • Animals frequently move through thermal patches—microhabitats with different temperatures—to regulate their body temperature behaviorally.

  • Larger animals have a narrower range of tolerable body temperatures (Tb) because their slow thermal response limits rapid temperature adjustment.

  • Very small animals:

    • Their body temperature (Tb) is often nearly equal to the operative environmental temperature (Te) because their heat capacity is very low (approaching zero).

    • For these animals, the temperature of their immediate environment largely determines their body temperature.

mass homeothermy

Mass Homeothermy

• Increased body mass reduces body temperature (Tb) fluctuations in ectotherms.
  Larger ectothermic animals show more stable Tb because of their greater thermal inertia.

• Very large ectotherms maintain a relatively stable body temperature across varying environmental conditions without internal heat production. This phenomenon is often called mass homeothermy.

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Key Point: At What Body Mass Does Tb Become Stable?

• There is a threshold body mass beyond which the animal’s body temperature remains relatively stable despite fluctuating environmental temperatures.

• This means the animal’s large body acts like a thermal buffer, smoothing out daily or environmental temperature swings.

• In your graph (or the described data), larger ectotherms show smaller fluctuations in Tb, with their average body temperature nearly constant over time.

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Advantages and Disadvantages

• Advantages:

  • Stable Tb means predictable physiological performance (e.g., locomotion, digestion, metabolism) because temperature-sensitive processes remain within optimal ranges.

  • This predictability can be beneficial for survival and activity planning.

• Disadvantages:

  • Large ectotherms can be slow to warm up after cold periods because their large thermal mass retains cold temperatures longer.

  • If the environment is very cold, it could take a long time for the animal to become active again, reducing immediate performance or responsiveness.

  • increased predation risk

  • cannot spend time on other activities whilst basking

example of homeothermy

Leatherback Turtles

• large mass

• TB higher than environment

• facilitates migration

• why they can stay at such high temps, their body temp can attribute to it

cardiovascular mechanisms

• When the body heats up, blood vessels dilate (vasodilation) in response to heat exposure. This increases the volume of the vascular system, reducing blood pressure.

• This drop in pressure is detected by baroreceptors, and the baroreflex is triggered via the hypothalamus.

• The reflex causes the heart rate and cardiac output to increase, raising blood flow through the body and enhancing heat transfer to the environment.

• When the body cools down, the opposite occurs: blood vessels constrict (vasoconstriction), increasing resistance and decreasing cardiac output to reduce heat loss.

This is a principal mechanism in most vertebrates for managing body temperature.

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Blood Flow and Poiseuille’s Law

Flow = Pressure / Resistance

• Resistance (R) is inversely proportional to vessel diameter — meaning small changes in vessel width have large effects on flow.

Vasodilation:

• Vessel diameter increases

• Resistance (R) decreases

• Cardiac output increases

• Result: Increased heat transfer, oxygen delivery, and metabolic support

• Trigger: Nitric oxide (NO) — a short-lived gas produced by endothelial cells that relaxes smooth muscle in vessel walls, rapidly dilating arteries

Vasoconstriction:

• Vessel diameter decreases

• Resistance (R) increases

• Cardiac output decreases

• Result: Reduced heat loss

• Trigger: Angiotensin — a hormone that causes vessels to narrow, conserving heat

• Arteries are muscular and elastic, allowing dynamic changes in diameter.

• Veins have valves to prevent backflow, and a more limited capacity for diameter change.

• During exercise, similar cardiovascular mechanisms are triggered: the baroreflex and nitric oxide increase blood flow to working muscles, supporting both oxygen delivery and temperature regulation.

the heart

. Pacemaker Cells

• The heart has pacemaker cells (specialized muscle cells) that spontaneously contract and set the base level of heart rate.

• These cells do not require input from the nervous system to function — they generate electrical impulses on their own.

2. Nervous System Regulation

• While the heart beats independently, it is regulated by the autonomic nervous system (ANS), part of the central nervous system:

  • Sympathetic nervous system: Increases heart rate and enhances blood flow (e.g., during stress or heat).

  • Parasympathetic nervous system: Decreases heart rate (e.g., during rest or cooling).

• The heart and blood vessels have receptors that detect signals and respond accordingly.

optimal temp and heart rate control

• Heart rate increases before Tb rises

  • When an animal enters a warmer environment, fH increases in advance of any rise in Tb.

  • This proactive increase in heart rate helps to:

    • Accelerate heat uptake by enhancing blood circulation.

    • Increase the rate of heating, especially to internal organs and muscles.

• Heart rate decreases as Tb reaches preferred levels

  • As body temperature approaches the animal’s optimal or preferred Tb, fH decreases.

  • This slows down further heat gain and helps stabilize Tb within the optimal range.

• Heart rate remains low even when Tb is still relatively high

  • When the animal moves into a cooler environment, fH stays low even though Tb hasn't dropped yet.

  • This low heart rate:

    • Slows the rate of cooling by reducing blood flow and heat loss to the environment.

    • Helps the animal retain body heat longer, maintaining performance for extended periods.

thermal neutral zone

Thermal Neutral Zone (species specific)

• range of environmental temperatures over which metabolic rate remains constant

• if temp drops past metabolic heat production capacity would be reached, and ultimatly we can’t control body temp anymore and go hyperthermic

• when really hot tends to increase as metabolic rates increase and go hypothermic

• thermoregulation by cardiovascular changes

• core and internal organs are at the thermal neutral zone and peripherals are colder

• metabolic rate increases below TNZ to increase heat production

• Tb changes beyond critical temperatures

desert bird adaptations

Main Mechanism: Evaporative Cooling

• Primary way desert birds cool down.

• Evaporation allows them to dump excess heat, but:

  • Increases thermal tolerance (they can survive in hotter conditions).

  • Depends heavily on water availability.

• 🟰 Trade-off:

  • Use less water → reduced cooling.

  • Use more water → better heat tolerance, but risk dehydration.

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Heat Tolerance Limits

• Birds must keep brain and body temperature (Tb) within survivable limits.

• Exceeding critical temperature (Tc) can result in brain damage or death.

• During heat waves, many birds can't keep up with the thermal load → mass die-offs.

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🔧 Key Thermoregulatory Adaptations in Desert BirdsA. Heat Dissipation – Beak as Radiator

• Large beaks (e.g. toucans) can act like radiators.

• Blood is sent to the beak to release heat into the air.

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B. Rete Ophthalmicum – Brain Cooling

• A vascular countercurrent system near the eyes.

• Cools brain by directing cooler blood (from evaporative surfaces) toward it.

• Prevents neurological failure during high Tb.

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C. Microvascular Adjustments

• Birds have specialised endothelial structures in skin and tissues.

• These adjust blood flow to enhance evaporative surfaces when needed.

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D. Gular Flutter

• A rapid vibration of the throat area (gular region).

• Increases airflow across moist membranes → enhances evaporative cooling.

• Uses less energy than panting, but still costs water and energy.

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E. Hyperthermia (Adaptive)

• Birds can intentionally allow Tb to rise.

• This reduces the temperature gradient between body and environment → slows heat gain.

• Controlled hyperthermia is risky but effective short-term.

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⚠ Heatwaves & Mortality

• During extreme heat, cooling mechanisms become overwhelmed.

• Birds cannot evaporate enough heat (especially if water is scarce).

• Leads to mass mortality events, especially in arid environments.