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

****:ANIMALS THAT HIDE UNDERGROUND

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.