However, it is especially with the rodents, the ungulata, and the ruminants that we find a highly developed practice of mutual aid. The squirrels are individualist to a great extent. Each of them builds its own comfortable nest, and accumulates its own provision. Their inclinations are towards family life, and Brehm found that a family of squirrels is never so happy as when the two broods of the same year can join together with their parents in a remote corner of a forest. And yet they maintain social relations. The inhabitants of the separate nests remain in a close intercourse, and when the pine-cones become rare in the forest they inhabit, they emigrate in bands. As to the black squirrels of the Far West, they are eminently sociable. Apart from the few hours given every day to foraging, they spend their lives in playing in numerous parties. And when they multiply too rapidly in a region, they assemble in bands, almost as numerous as those of locusts, and move southwards, devastating the forests, the fields, and the gardens; while foxes, polecats, falcons, and nocturnal birds of prey follow their thick columns and live upon the individuals remaining behind. The ground-squirrel--a closely-akin genus--is still more sociable. It is given to hoarding, and stores up in its subterranean halls large amounts of edible roots and nuts, usually plundered by man in the autumn. According to some observers, it must know something of the joys of a miser. And yet it remains sociable. It always lives in large villages, and Audubon, who opened some dwellings of the hackee in the winter, found several individuals in the same apartment; they must have stored it with common efforts.