Metabolic reactions are not 100% efficient in their energy transfers – a proportion of energy is always lost as heat

• Certain animals (mammals and birds) use this heat generation to maintain a constant body temperature (endotherms)
  

• Other animals (reptiles, amphibians and most fish) depend on the environment to maintain body temperatures (ectotherms)
  

The level of heat production by an animal is affected by its level of metabolic activity

• Some animals enter a state of torpor to reduce heat production (hibernation in winter, estivation in summer)