All of the ground-dwelling squirrels are diurnal. Temperate ground squirrels tend to have bimodal daily activity pat-

Young black-tailed prairie dogs (Cynomys ludovicianus) play in Tucson, Arizona, USA. (Photo by © George D. Lepp/Corbis. Reproduced by permission.)

tems during the hot summer months. During the hot periods above-ground activities tend to be concentrated in the mornings and late afternoons to early evening. The animals avoid the heat of the day by escaping below ground to their cool burrows.

The degree of sociality varies widely in the ground-dwelling squirrels from the solitary chipmunks to the highly social marmots. Although there are exceptions, the degree of sociality tends to increase with body size and shortness of the growing season where they live. Large-bodied squirrels, such as the marmots, living at high elevations with short growing seasons tend to be highly social. For example, the hoary marmot (Marmota caligata) lives in large family groups of up to 35 animals that include an adult male, several females, and several offspring cohorts. The short growing season prevents large-bodied squirrels from reaching adult size for several years and therefore they tend to delay the age at which they disperse until they reach a sufficient size that improves their success. Other factors influence grouping behavior in squirrels as well. The Cape ground squirrels (Xerus inauris) of Africa are unusual among the ground-dwelling squirrels in that the males form groups despite the increase in competition for females. Waterman in 1997 suggested that males live in groups to increase their safety from predators.

Hibernation is a feature that is prominent in many of the ground-dwelling squirrels. However, the degree of hibernation varies from those that do not hibernate at all such as the Cape ground squirrel (Xerus inauris), those that undergo estivation in summer such as Nelson's antelope squirrel, to the majority of ground-dwelling squirrels that hibernate for up to nine months. Although marmots are the largest true hibernating mammal, the arctic ground squirrel is the most extreme. According to Barnes in 1989, the arctic ground squirrel can survive a core body temperature as low as 26.8°F (—2.9°C).

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