Thermoregulation

Temperature influences every biochemical process in a cell — enzyme activity, membrane fluidity, diffusion rates and the rate of respiration all depend on it. Keeping body temperature within a narrow range is therefore critical for any active animal. Different animals solve this problem in different ways: ectotherms rely mainly on behaviour to exchange heat with the environment, while endotherms use metabolic heat and a suite of physiological mechanisms coordinated by the hypothalamus. This lesson covers the mechanisms of thermoregulation in both groups, with particular emphasis on the detailed mammalian responses required for OCR A-Level Biology A specification module 5.1.1(f)–(g).

Key Definitions:

• Ectotherm — an animal whose body temperature depends largely on heat from the environment.
• Endotherm — an animal that maintains body temperature primarily using metabolic heat.
• Thermoregulation — the maintenance of body temperature within a range that allows enzymes and other proteins to function.
• Hypothalamus — the region of the brain that acts as the coordinator of mammalian thermoregulation.

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Why Temperature Matters

Enzymes have an optimum temperature (around 37 °C for most human enzymes). Below the optimum, rates are slow because molecules have insufficient kinetic energy. Above the optimum, the shape of the active site becomes distorted; by about 40–45 °C, most human enzymes are denatured beyond repair. Small temperature changes therefore have disproportionate effects on metabolism. A deviation of ~5 °C from normal can be fatal in mammals.

Temperature also affects membrane fluidity: at higher temperatures, phospholipids move more freely and membranes become leakier; at lower temperatures, they become more rigid and less permeable. The Q₁₀ (temperature coefficient) of most metabolic reactions is ~2, meaning that rates double with every 10 °C rise — hence the dramatic effect of even small temperature changes on metabolism.

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Ectotherms

Ectotherms gain most of their body heat from the environment. Reptiles, amphibians, fish and invertebrates are ectothermic. They tend to have lower resting metabolic rates than endotherms of similar size, and they do not need to eat continuously to fuel heat production.

Behavioural Thermoregulation in a Lizard

Consider a lizard on a cool morning. Its body temperature is close to air temperature — perhaps 15 °C — and at this temperature it is sluggish because metabolic reactions proceed slowly. The lizard warms up by:

• Basking on a rock in direct sunlight, orienting its body to maximise the surface area exposed to the sun.
• Darkening its skin (in species that can change colour) to absorb more radiation.
• Flattening its body against the warm rock to increase conductive heat gain.

Once warmed to its preferred operating temperature (~35 °C in many lizards), it becomes active. If it starts to overheat during the day, it:

• Retreats into the shade or into a burrow.
• Orients so that only a small part of its body faces the sun.
• Pants to lose heat by evaporation.
• Climbs off hot rocks onto cooler surfaces.

flowchart LR
    A[Cold lizard] --> B[Basks in sun]
    B --> C[Body temperature rises]
    C --> D[Active lizard]
    D --> E[Overheating]
    E --> F[Retreats to shade]
    F --> G[Body temperature falls]
    G --> D

Advantages and Disadvantages of Ectothermy

Advantages:

• Less energy is spent on generating heat, so ectotherms require less food.
• A larger proportion of energy intake can be devoted to growth and reproduction.
• They can survive long periods without food (e.g., pythons).

Disadvantages:

• Activity depends on ambient conditions — early mornings and winters limit movement.
• Escape from predators may be impossible when cold.
• Ectotherms are generally confined to warmer climates.

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Endotherms

Endotherms (mammals and birds) generate most of their body heat through metabolism and maintain it using physiological mechanisms. They can remain active regardless of the ambient temperature, but pay a much higher energy cost.

The Hypothalamus as the Thermostat

The hypothalamus contains two sets of thermoreceptors:

• Central thermoreceptors — detect the temperature of the blood flowing through the hypothalamus itself. This gives a measure of core body temperature.
• Peripheral thermoreceptors — in the skin, detect external temperature changes and send signals via sensory neurones to the hypothalamus.

The hypothalamus has two functional centres:

• The heat loss centre is activated when core temperature rises. It sends impulses via the autonomic nervous system to effectors that promote heat loss.
• The heat gain centre is activated when core temperature falls, triggering effectors that conserve or generate heat.

Cooling Mechanisms (Response to Heat)

When core body temperature rises above ~37 °C: