Explain thermoregulation in endotherms and ectotherms, including behavioural and physiological responses to heat and cold

What this dot point is asking

QCAA expects you to compare endothermic and ectothermic thermoregulation and to give specific behavioural and physiological responses to heat and cold. You should be able to explain the role of the hypothalamus in the negative feedback loop.

The answer

Temperature determines enzyme activity, membrane fluidity and the rate of every cellular reaction. Animals fall into two strategies for keeping body temperature within a survivable range.

Endotherms and ectotherms

Endotherms. Generate most of their body heat internally through high-rate metabolism. Maintain a roughly constant body temperature (around 37 degrees Celsius in mammals, 40 in birds) regardless of ambient temperature.

• Examples: all mammals, all birds.
• Cost: high food and oxygen requirement; small mammals can need 10 to 20 percent of their body mass in food per day.
• Benefit: high sustained activity, ability to occupy cold environments.

Ectotherms. Body temperature is set largely by the environment. Heat from metabolism is small; ectotherms rely on external heat sources.

• Examples: reptiles, amphibians, fish, invertebrates.
• Cost: activity is limited to favourable temperatures; many ectotherms become torpid in the cold.
• Benefit: very low energy demand; ectotherms can survive long periods without food.

Endotherm vs warm-blooded. "Warm-blooded" is informal and inaccurate. A basking lizard can be warmer than a small mammal in shade; the difference is the source of heat (internal vs external), not the temperature itself.

Heat exchange between organism and environment

Heat moves by four mechanisms.

• Radiation. Electromagnetic energy to or from any object. A basking lizard absorbs solar radiation; a warm body radiates heat to a cooler environment.
• Conduction. Direct contact between objects of different temperatures.
• Convection. Heat carried by moving air or water. Wind chill is convective heat loss.
• Evaporation. Liquid water absorbs heat as it becomes vapour. Sweat and panting use this; transpiration cools plant leaves.

Responses to heat (in endotherms)

The hypothalamus heat-loss centre is activated.

• Sweating (or panting). Evaporative cooling. Sweat across most of the human body; panting in dogs (small surface area lacking sweat glands) and many birds.
• Vasodilation of skin arterioles. Skin flushes red. More blood reaches the surface; heat is lost by radiation and convection.
• Behavioural. Move to shade, reduce activity, drink cold fluids, postural changes to maximise surface area exposed to air.

Responses to cold (in endotherms)

The hypothalamus heat-promoting centre is activated.

• Vasoconstriction of skin arterioles. Skin pales. Less blood reaches the surface; heat is retained in the core.
• Shivering. Rapid involuntary skeletal muscle contractions generate heat as a by-product of ATP hydrolysis.
• Non-shivering thermogenesis. Brown adipose tissue (rich in mitochondria with uncoupling protein) generates heat directly. Important in infants and hibernating mammals.
• Piloerection. Arrector pili muscles erect hair to trap an insulating air layer. Effective in fur-covered mammals; vestigial "goose bumps" in humans.
• Hormonal. Adrenaline (short-term) and thyroxine (long-term) raise basal metabolic rate.
• Behavioural. Add clothing, curl up to reduce surface area, seek shelter, shiver consciously.

Responses in ectotherms

Ectotherms rely mostly on behavioural responses.

• Basking. Position on warm rocks or in sun to absorb radiative heat.
• Sheltering. Move to shade or burrows to avoid overheating.
• Body orientation. A lizard turning broadside to the sun maximises heat gain; turning end-on minimises it.
• Colour change. Some reptiles and amphibians darken to absorb more heat.
• Limited physiological responses. Some lizards adjust blood flow to skin or alter heart rate to control heat exchange.

Many ectotherms have wide tolerance ranges. Reptiles operate efficiently across a body temperature range of 30 to 40 degrees Celsius, much wider than a mammal's narrow set point.

Australian context

• Red kangaroos. Endotherms with low metabolic rates for their size. Use heavily vascularised forelimbs to dissipate heat by licking and evaporation; rest in shade during the day.
• Bearded dragons (Pogona). Ectotherms that bask in the morning sun, retreat to shade in the heat of the day, and shelter at night.
• Echidnas. Endotherms with unusually low core temperatures (around 32 degrees Celsius); enter torpor in cold weather to save energy.

Common traps

Calling all reptiles "cold-blooded". They are ectothermic. A lizard in the sun can be warmer than a human.

Forgetting that humans regulate by both behaviour and physiology. Choice of clothing and shade is part of the answer.

Confusing vasoconstriction with vasodilation. Cold: vasoconstrict (skin pale, conserve heat). Hot: vasodilate (skin flush, dump heat).

Treating shivering and sweating as the only mechanisms. Piloerection, non-shivering thermogenesis, behavioural responses and hormonal changes are also examinable.

Cross-link to Year 12 assessment

The thermoregulation loop is the canonical example used to test the homeostasis framework in EA short-response questions. Tolerance to temperature is a key abiotic factor in Unit 3 ecosystem dot points (see abiotic and biotic factors) and shapes population distribution in IA1 stimulus.

In one sentence

Endotherms maintain a stable body temperature by generating heat internally and using physiological responses (sweating, shivering, vasomotor changes, hormonal control) coordinated by the hypothalamus, while ectotherms depend mostly on behavioural responses (basking, sheltering, orientation) to absorb or avoid external heat.