Nearly 74 percent of all mammalian species are terrestrial in their way of life, spending all or a large percentage of their time aboveground.

Another 20 percent or so of the total species are bats, which are aerial. About 2 percent of the species are entirely aquatic, including mammals such as whales and manatees.

Yet 4 percent of the world's mammals have opted for a way of life that is spent completely in a subterranean environment -- imagine living your entire life in a windowless basement.

The 4 percent of mammals that have evolved this subterranean or underground way of life occurs in 10 very different groups or families: marsupial moles of Australia, golden moles of Africa, true moles of North America and Europe, and seven different rodent families from around the world.

Three species of moles occur in Pennsylvania: eastern, hairy-tailed and star-nosed.

Moles are mouse-like in size and can easily be confused with mice. They are cylindrical in shape, allowing them to easily move within an underground or subterranean burrow system. Their snout is typically long and naked, their eyes are very small and no external ears are present. Their fur is soft and velvety, permitting the hair to lie flat in either direction as a mole makes its way through a burrow system. The front feet of moles are distinctive -- the "hand" is broad and turned permanently outward for digging into soil.

Moles spend virtually all of their time in underground burrow systems. The familiar "molehills" are formed as the mole pushes excess subterranean soil out of the burrow system.

Of the three mole species in our region, eastern moles are most responsible for damage to golf courses, lawns and flowerbeds as they tunnel for food, such as insects and earthworms, within shallow burrow systems. Not all surface tunnels can be attributed to moles, however, because some rodent and shrew species also construct subterranean tunnels just below the soil surface.

Regardless of species, subterranean mammals share some characteristics. None have long appendages or protruding body parts that would make rapid back-and-forth movements in burrow systems difficult. Instead, they all are relatively small, sausage-shaped animals, with reduced appendages and eyes and no external ears. Smell, hearing and touch are well-developed in subterranean mammals.

Virtually all of these species are solitary and territorial, and few young are produced per litter. Compared with similar-sized terrestrial mammals, movements in subterranean mammals are considerably reduced.

Why did some mammals evolve a subterranean way of life? As the climate became more arid millions of years ago in certain parts of the world, some forests converted to grasslands.

In these grasslands, ungulate (hoofed mammals) and kangaroo species flourished and competed for plant food resources. Food, such as insects and plant roots, was available underground and free for the taking by species adapted to a subterranean way of life.

Hence, ancestors of the three extant families of moles chose an underground insectivorous feeding strategy, while ancestors of the seven families of rodents selected an underground herbivorous strategy.

An underground environment provides conditions that are very different from those aboveground. Underground, temperatures are predictable and relatively constant, and predation risks are low.

Because the underground environment is typically stable and uniform, little genetic variation is found in subterranean mammals compared with their aboveground counterparts. Thus, in early evolutionary time, the subterranean environment represented an untapped resource for certain types of small, cylindrical mammals.

The subterranean environment has changed little over time and continues to remain important to moles and other small, sausage-shaped mammals that are perfectly adapted to a "basement without windows."

But today, this underground environment may be impacted by habitat alterations, such as those created by urbanization or agricultural practices, which can influence the distributional patterns of subterranean mammals.

As an aside, what type of midterm exams do student moles like best? Mole-tiple choice tests.