A darker coloured region of adipose tissue found in newborn and hibernating animals (in which it may also be called the hibernating gland). Adult humans also have small amounts of brown fat distributed around the body, the amounts of which seem to be inversely related to total body mass. Compared to normal white fat, deposits of brown fat are more richly supplied with blood vessels and have numerous mitochondria (hence the brown colour, due to the high concentrations of cytochrome oxidase). They can also be more rapidly converted to heat energy—a process that takes place in the fat cells themselves—especially during arousal from hibernation and during cold stress in young animals (see uncoupling protein). Since the deposits are strategically placed near major blood vessels, the heat they generate warms the blood returning to the heart. Brown fat may serve to burn excess calories, hence the suggestion that some types of obesity in humans may be linked to a lack of brown fat. See thermogenesis.