Forest Preserve District of Cook County, Illinois
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Nature Bulletin No. 658-A   December 3, 1977
Forest Preserve District of Cook County 
George W. Dunne, President
Roland F. Eisenbeis, Supt. of Conservation

Man is the only warm-blooded creature that is able to survive freezing temperatures without growing his own coat of fur or feathers. Far back in prehistoric times he learned to live in regions with cold winters by wrapping himself in the skins of animals. Strictly speaking, we do not endure the cold. Like the Eskimo inside his suit of fur, we really live in a small tropical climate that we carry around with us.

Mammals are commonly covered with two types of hair, a thick soft underfur and a layer of longer coarser guard hair which forms a protective outer coat. Air trapped among the fibers of the dense underfur insulates the skin from winter cold and holds in the body heat. For example, the Arctic fox lives on the wind-swept Greenland icecap, often at 60 degrees below zero Fahrenheit, without affecting its internal temperature.

The guard hair is somewhat oily and readily sheds water. In our local muskrat, mink and beaver, which swim long distances under water, even under ice in winter, the underfur and skin remain warm and dry. Animals living in warm climates have mostly guard hair with scant underfur. The opposite is the case in cold climates. Our woodchucks and ground squirrels, which burrow down below the frost line and become cold-blooded during the winter months, need little insulation and have only coarse hair.

A mammal's hair grows, is molted, and then renewed at least once each year. Among the commercial fur-bearers which are trapped or reared on fur farms for their valuable skins there is a season when their fur is at its best. The shedding of the underfur begins in spring and continues throughout the summer. At this same time, the old guard hairs gradually drop out and are replaced by new ones, giving a thin sleek summer coat. With the onset of cold weather, the new underfur develops and the pigment granules stored in each hair root migrate up into the hair shaft. When the underfur is fully grown, the guard hair stands on end, the pelt is said to be prime and ready to be harvested. All of the hairs are now dead and from this time on the fur begins to fade and fray and lose value. This is why, in the northern zone of Illinois, for example, the legal trapping season is from November 20 to December 31.

The big brown snowshoe hare which we sometimes see on our summer vacations in northern Wisconsin or Michigan grows a white furry coat in autumn, then changes back to a brown one in spring. Likewise, in fall, here in Cook County, a few of our weasels turn from brown to white except for the black tips of their tails. Then their skins are sold as ermine. Farther north, all of them turn white and, farther south, none of them. It has been learned that with 18 hours of artificial illumination per day they stay brown the year-round. With only nine hours of light they turn white, even in summer.

Fur has played a long and fascinating part in human history. Its first mention as an ornament seems to be the description of the protective coverings of the "holy of holies " in the portable tabernacle as recorded in the Book of Exodus of the Old Testament.

More than gold, timber or rich soil, the demand for fur was responsible for the opening up of the interior of the North American continent. Beaver pelts were sought most of all. Their fur made the felt for the tall beaver hats so popular both here and abroad. Chicago got its start as a fur-trading post because the portage at this point was a connecting link between the Great Lakes-St. Lawrence water route and the rivers that stretch westward to and beyond the Mississippi.

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