Population Control in Arctic Ground Squirrels

Unlike most tundra rodents, the arctic ground squirrel, Spermophilus undulatus, maintains a relatively constant population. The squirrels exist in two types of groupings: (1) breeding colonies, which cannot expand because their members are territorial, the females reacting to the availability of burrow sites (an environmental constant), and (2) refugee populations, which cannot expand because they cannot breed and, living in habitat only periodically suitable for squirrels, are killed off twice each year. The breeding populations have two distinct types of territories: breeding territories from May to August and prehibernation territories from August to November. During each episode, surplus animals are driven from the colonies and enter the refugee populations, where they are subjected to heavy predation by foxes and bears.