Q. My backyard soil is so rocky that I often need a pickax to dig a hole, yet chipmunks and moles burrow through with apparent ease. How do they do it?

A. Some burrowing mammals have specially evolved bones and other structures for moving earth, and some, notably chipmunks, may reuse tunnels cut by others. In fact, most burrowers choose their habitat for its favorable soil conditions and may time their excavations for periods when digging may be easier, like after rain.

The familiar eastern mole, Scalopus aquaticus, digs two kinds of tunnels. The ones often seen near the surface are used for hunting subterranean foods like insects and grubs. Deeper tunnels, perhaps 12 inches down, are to counter seasonal temperature fluctuations and are used for shelter and hunting.

Moles use a digging technique called sand swimming in loose, sandy soil, but they also have the equipment for heavy lifting and shifting in rocky, root-clogged soil.