- Metabolism is inefficient and produces heat. Endotherms use metabolic heat to keep a stable body temperature, while ectotherms do not.
- The "baseline" metabolic rate of an animal is measured as the basal metabolic rate (BMR) for an endotherm or as the standard metabolic rate (SMR) for an ectotherm.
- Among endotherms, smaller animals tend to have higher per-gram basal metabolic rates (a "hotter" metabolism) than larger animals. The same is true among ectotherms, though we can't compare between the groups.
- Metabolic rate varies with activity level. More active animals have a higher metabolic rate than less active animals.
- Some animals enter a state of torpor in which their metabolism slows. Hibernation in the winter and estivation in the summer are forms of torpor.
Metabolism and heat production
- For an endotherm, the BMR is also measured when the animal is in a thermoneutral environment, that is, one where the organism does not expend extra energy (above baseline) to maintain temperature.
- For an ectotherm, SMR will vary with temperature, so any SMR measurement is specific to the temperature at which it's taken.