Natural Electric Blanket

For a very long time scientists have puzzled over what was first called the brown gland, a substance in animal bodies that later became known as brown fat. Ordinary fat is white, but as far back as 1551, Konrad von Gesner, the Swiss naturalist, noticed when he was dissecting the carcase of a marmot a mass of brown tissue between the shoulder blades. Since that time many other scientists have found the same kind of brown tissue in the bodies of animals, especially in those which, like the marmot, hibernate. So this brown tissue was also called the hibernating gland, although what role it played in hibernation was a mystery. The puzzle was made the more perplexing when it was found in animals that do not hibernate, such as the brown rat. To make matters worse, it was later found that brown fat was a conspicuous feature of new-born animals.

One theory put forward was that the brown fat supplied corpuscles to the blood. Another guess was that it constituted a ductless gland that pours hormones into the blood. Both theories are now known to be wide of the truth. All the same, nobody had succeeded in shedding any light on this mystery. Then in 1963, scientists at Oxford University, in England, and at Harvard Medical School, in the United States, made a thorough investigation and discovered that the brown fat is a kind of electric blanket.

When a human baby is born it leaves the warmth of the womb and comes into the atmosphere which is at a lower temperature. Except under conditions of modern civilization, with the advantages of artificial heating, this would mean that the baby would quickly cool at a critical moment of its life, unless there were some means of keeping its body warm. The same is true for any new-born animal. The adult warm-blooded animal exposed to cold can increase the heat in its body by shivering. A new-born baby and new-born animals cannot shiver, so they need something to take its place. In the human baby there is a layer of brown fat between the shoulder blades and around the neck, and smaller areas behind the breastbone and along the spine. A newborn European rabbit also has brown fat between the shoulders and around the neck, and other animal babies have much the same. Layers of white fat, which is what we are talking about when we say an animal is fat, help to insulate the body and keep it warm. But there is a marked difference between the way it works and the way brown fat works. White fat is made up of cells each containing only one droplet of fat. The cells of brown fat are not only larger but each contains several droplets of fat. More important, the cells of brown fat contain many mitochondria, whereas the cells of white fat contain only a few. Mitochondria are very small bodies, visible only with high powers of the microscope, that burn up the fat to produce heat, and the more there are the more quickly the fat droplets can be converted to heat. The cells of brown fat produce heat twenty times as quickly as white fat. Moreover, their heat production increases rapidly as the temperature of the surrounding air drops. This dark colored fat behaves, therefore, like a thermostatically controlled electric blanket.

Although most animals lose their brown fat by the time they are adult, some, like the rat, retain it throughout their lifetime, making them better able to stand up to extremes of cold. The house mouse is another animal that keeps its brown fat as it grows up. It has always been something of a puzzle why mice should be able to live and breed in the refrigerated meat stores. Now we know why.

Brown fat has one great advantage over the electric blanket: as fast as it wears out it is replenished from white fat elsewhere in the body.

This is how the brown fat works. When the temperature falls sense-cells send messages to a temperature-regulating centre in the brain. From this, impulses go out along the sympathetic nerves to the brown fat, setting it in motion.

Although the brown gland is not a hibernating gland, found only in hibernating animals, nevertheless it carries two advantages for animals during their winter sleep. When the temperature of the surrounding air falls it warms up the sleeping animal just enough to prevent it dying of cold. Moreover, the more the temperature falls the more the body of the sleeping animal warms up until finally it wakes, becomes active for a while, and so can seek a spot more sheltered from the cold. This is one reason why we sometimes see hibernating animals, such as bats, out and about on very cold nights.

Finally, it is the brown fat which wakes the hibernating animal up, at the end of the winter, and warms it so that it quickly becomes active once more. This has been studied in an American bat (Eptesicus fuscus). When arousing from hibernation the brown fat in this bat is 3°C warmer than the rest of the body. This 'hot spot' imparts its heat to the rest of the body and in little more than a quarter of an hour the bat will have warmed up almost to its summer normal.