Forest Preserve District of Cook County, Illinois
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Animals that Hide Underground
Nature Bulletin No. 733   November 23, 1963
Forest Preserve District of Cook County
Seymour Simon, President
David H. Thompson, Senior Naturalist

A hole in the ground has an air of mystery about it that rouses our curiosity. No matter whether it is so small that only a worm could squeeze into it, or large enough for a fox den, our questions are much the same. What animal dug the hole? Is it down there now? What is it doing? When will it come out?

An underground burrow has several advantages for an animal. In it, many kinds find safety from enemies for themselves and their young. For others, it is an air-conditioned escape from the burning sun of summer and a snug retreat away from the winds and cold of winter. The moist atmosphere of a subterranean home allows the prolonged survival of a wide variety of lower animals which, above the surface, would soon perish from drying.

The woodchuck or groundhog is a famous excavator, digging numerous burrows on gravelly slopes, roadsides and in open fields. Each burrow is a wide branching tunnel with two or more entrances. In it they sleep at night, rear their young, and hibernate in a torpid condition from late October until March. These woodchuck homes are frequently taken over by foxes and skunks to rear their own families and, in winter, they also are used by raccoons, opossums, mink and rabbits.

Several smaller native rodents live in burrows. Like the woodchuck, the 13-striped ground squirrel and Franklin's ground squirrel sleep through the winter below the frost line. In his burrow a chipmunk stores seeds, nuts and grain for winter rations. Many muskrats, instead of building winter houses of water plants, dig tunnels with underwater entrances into the banks of streams and ponds. There they are safe from all invaders except the bloodthirsty mink.

The great majority of burrowers divide their time above and below ground. The mole is an exception. They can be born, live out their lives and die without ever coming out into the open. They could not see anything if they did, because their degenerate eyes are completely covered with skin. Their presence is revealed by the ridges they push up in lawns, gardens and fields as they forage for earthworms and insects.

In this region kingfishers and colonies of bank swallows dig deep holes in steep sand or gravel banks in which they incubate their eggs and bring up their fledglings. Most of us have heard of the little owl that lives in prairie dog burrows in the western states.

Farmers, fishermen, bird watchers and gardeners are familiar with earthworms but few people realize their importance as earth movers and mixers. On an acre of fertile soil they may number into the millions and have a total weight of one-half ton -- more than all other underground animal life combined. Earthworms literally eat their way through the soil. Part is digested and the remainder is passed out through the gut and deposited as a ring of granules around the opening of each burrow. Soil is improved by the bits of dead grass, leaves, and stems that they pull beneath the surface, as well as by the air and water that seep into their tunnels.

Insects -- hundreds of species of them -- pass the winter months in soil either as eggs, larvae, pupae or adults. With the arrival of warm weather, most kinds come up into the open air to carry on their active lives. Among the exceptions is the white grub or "grubworm" which hatches from a buried egg, spends two or three years feeding on grass roots. Then, after a pupal stage, emerges as our familiar June beetle.

Watch a busy ant colony on a warm afternoon. Columns of workers hustle in and out of their network of subterranean galleries, some bringing food, others carrying out granules of soil as the ant hill is enlarged. Sometimes you see an ant war as one species raids the colony of another to capture slaves.

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