Black individuals sometimes occur locally. Slight aquatic modifications are developed in some species, such as the small, hairy swimming fringe on the foot. The tail is well haired, and the sole of the hind foot is generally haired. The claws on all four limbs are well developed. Both sexes possess flank glands, and females have eight mammae.
Individuals seem to grow throughout life.
The cheek teeth are ever-growing. There apparently are both aquatic and nonaquatic populations of these voles. The aquatic populations, which include most A. terrestris in the western parts of Europe, live along the banks of streams, ponds, and canals, where the water level is constant. They dive freely and swim either on the surface or underwater, sometimes near the bottom of the pool or stream, where they enter burrows that terminate below water level. These populations occupy grasslands and cultivated areas, construct extensive burrow systems, and leave conspicuous mounds on the surface.