Houses aren’t the only place where insulation can…
Houses aren’t the only place where insulation can be seen in our world. Two kinds of animals — birds and mammals — maintain a constant body temperature despite the temperature of their surroundings. Both have evolved methods to control the flow of heat into and out of their bodies. Part of these strategies involve the use of insulating materials — fat, feathers, and fur — that serve to slow down the heat flow. Because most of the time an animal’s body is warmer than the environment, the most common situation is one in which the insulation works to keep heat in. Whales, walruses, and seals are examples of animals that have thick layers of fat to insulate them from the cold arctic waters in which they swim. Fat is a poor conductor of heat and plays much the same role in their bodies as the fiberglass insulation in your attic. Feathers are another kind of insulation. They are made of light, hollow tubes connected to each other by an array of small interlocking spikes. They have some insulating properties themselves, but their main effect comes from the fact that they trap air next to the body. This stationary air is a rather good insulator. For instance, in winter, a house sparrow has about 3,500 feathers, which maintain the bird’s normal temperature even in below-freezing weather. Birds often react to extreme cold by contracting muscles in their skin so that the feathers fluff out. This increases the thickness — and hence the insulating power — of the layer of trapped air. Incidentally, birds need insulation more than we do because their normal body temperature is 106°F. Hair (or fur) is actually made up of dead cells similar to those in the outer layer of the skin. Like feathers, hair serves as an insulator in its own right and traps a layer of air near the body. In some animals (for example, polar bears) the insulating power of the hair is increased because each hair contains tiny bubbles of trapped air. The reflection of light from these bubbles makes polar bear fur appear white — the strands of hair are actually semitransparent. Hair grows from follicles in the skin, and small muscles allow animals to make their hair stand up to increase its insulating power. Human beings, who evolved in a warm climate, have lost much of their body hair as well as the ability to make most of it stand up. There is a reminder of our mammalian nature, however, in the phenomenon of “goose bumps,” which is the attempt by muscles in the skin to make the nonexistent hair stand up. As used in Sentence 4 (in Paragraph 3), the word stationary means