AP Syllabus focus:
‘Endotherms and ectotherms use different strategies to regulate body temperature and metabolic rate.’
Thermoregulation links physiology to ecology: maintaining body temperature affects enzyme function, activity, and survival. Animals differ in where their heat comes from and the costs and limits of keeping temperature within a functional range.
Core idea: temperature affects metabolism
Temperature changes alter reaction rates and membrane fluidity, so most animals must keep internal conditions compatible with cellular function.
Thermoregulation: the set of behavioral and physiological mechanisms that maintain body temperature within a tolerable range despite environmental temperature change.
Heat exchange with the environment occurs through predictable physical routes, so organisms evolve structures and responses that control heat gain and loss.
Heat exchange pathways (targets for regulation)
Radiation: heat transfer via electromagnetic waves (sunlight warms; infrared loss cools)
Conduction: direct heat transfer to/from surfaces (rock, soil, ice)
Convection: heat transfer via moving air/water (wind or currents increase loss)
Evaporation: heat loss when water evaporates from skin or respiratory surfaces
Endotherms vs ectotherms
Animals are often described by their primary heat source and their typical relationship between body temperature and ambient temperature.
Endotherm: an animal that generates a substantial fraction of body heat from internal metabolic processes and uses physiological controls to maintain relatively stable body temperature.
Endothermy commonly supports sustained activity across variable climates but demands high energy intake.
Ectotherm: an animal whose body temperature is largely determined by external heat sources, with body temperature typically varying more with environmental conditions and usually lower baseline metabolic costs.
Ectothermy reduces energy requirements but can constrain performance when environmental temperatures are unfavorable.
Metabolic rate and temperature regulation (key comparison)
This set of graphs summarizes how body temperature (Tb) and metabolic rate (MR) change with ambient temperature (Ta) across major thermoregulatory strategies. It highlights that endotherms can keep Tb relatively stable by adjusting MR, whereas ectotherms’ MR and Tb track environmental temperature more closely. The visual also helps students recognize the idea of a thermoneutral zone (minimal MR across a middle range of Ta) in endotherms. Source
Endotherms
Use metabolic heat production to defend a set point (or narrow range)
Often increase metabolic rate in cold conditions to offset heat loss
Risk: overheating during high activity or in hot environments unless heat can be dissipated
Ectotherms
Rely more on behavioral control of heat gain/loss (microhabitat choice)
Metabolic rate is more temperature-dependent, often dropping in cold and rising in warmth
Risk: reduced movement, feeding, and escape responses outside optimal temperatures
Thermoregulation strategies
Strategies are best understood as controls on heat exchange pathways and on internal heat production.
Endotherm strategies (physiological emphasis)
Insulation (reduces heat loss)
Fur, feathers, blubber reduce conduction and convection
Seasonal changes in coat thickness can shift heat balance
Vasomotor control (adjusts heat transfer to the environment)
This flow chart shows thermoregulation as a negative-feedback system: deviations from normal body temperature trigger effectors that restore the set point. It emphasizes vasodilation and sweating as heat-dissipating responses to overheating, and vasoconstriction and shivering as heat-conserving/heat-producing responses to cold. The diagram helps connect vascular changes directly to changes in heat exchange with the environment. Source
Vasoconstriction reduces blood flow to skin, decreasing heat loss
Vasodilation increases skin blood flow, increasing heat loss
Evaporative cooling (increases heat loss when hot)
Sweating or panting increases evaporation; effective but can cause dehydration
Thermogenesis (increases internal heat production when cold)
Shivering: muscle contractions generate heat
Nonshivering thermogenesis: increased metabolic heat without muscle activity (common in small mammals and infants)
Countercurrent heat exchange (regional control)
Close arterial–venous pairing in limbs reduces heat loss, maintaining core temperature while allowing cooler extremities
Ectotherm strategies (behavioral emphasis, limited physiology)
Behavioral thermoregulation
Basking to increase radiation gain; flattening body increases surface exposure
Seeking shade, burrows, or water to reduce radiation and convection-driven heating
Adjusting posture and orientation to wind/sun to tune convection and radiation
Physiological tuning (typically smaller, but important)
Circulatory adjustments can change heat distribution (e.g., diverting blood away from skin)
Some ectotherms show short-term metabolic adjustments or dormancy-like states to reduce energy demand when temperatures are poor
Trade-offs that shape which strategy works
Water balance vs cooling: evaporation removes heat but costs water
Energy intake vs heat production: endotherms must fuel higher metabolic rates
Body size effects: small animals lose heat faster (high surface area-to-volume), often requiring stronger endothermic defenses; larger animals retain heat but may struggle to dissipate it
FAQ
They allow proton leak across the inner mitochondrial membrane, reducing ATP production efficiency and releasing energy as heat, especially in brown adipose tissue.
Some body regions (e.g., swimming muscles) are kept warmer than the environment using countercurrent heat exchangers, improving power output without warming the whole body.
They may operate at higher thermal optima and use rapid shuttling between microhabitats (sun/shade/burrows) plus timing of activity to avoid peak temperatures.
It describes how physiological performance changes with temperature, typically rising to an optimum then dropping sharply; it predicts when thermoregulation is most critical.
High humidity lowers the vapour pressure gradient, slowing evaporation and reducing cooling efficiency; this increases overheating risk even when sweating or panting rates rise.
Practice Questions
State two differences between an endotherm and an ectotherm in how they regulate body temperature and metabolic rate. (2 marks)
Endotherm: produces substantial internal metabolic heat and maintains relatively constant body temperature (1)
Ectotherm: relies mainly on external heat sources; body temperature varies more with environment and metabolic rate is more temperature-dependent (1)
Explain how an animal can reduce heat loss in a cold environment and increase heat loss in a hot environment. Include at least two physiological mechanisms and one behavioural mechanism, and link them to heat exchange pathways. (6 marks)
Any two physiological mechanisms for reducing heat loss, linked to pathway (2 max), e.g.:
Insulation reduces conduction/convection (1)
Vasoconstriction reduces heat loss via reduced skin blood flow (1)
Countercurrent exchange reduces heat loss from limbs (1)
Any one physiological mechanism for increasing heat loss, linked to pathway (1), e.g.:
Sweating/panting increases evaporative cooling (1)
Vasodilation increases heat loss via radiation/convection (1)
One behavioural mechanism, linked to pathway (1), e.g.:
Basking increases radiation gain (1) OR seeking shade reduces radiation gain (1)
Clear link to endotherm/ectotherm strategy emphasis or metabolic implication (1)
