Annual cycles of activity and body composition in Spermophilus townsendii mollis

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Abstract
Data on activity scheduling and body composition in Spermophilus townsendii mollis Kennicott were collected over a 4-year period in northwestern Utah. Yearly variation in emergence scheduling was associated with variation in late winter warming patterns. Sex differences in patterns of growth and seasonal body composition were attributed to reproductive biology. These included (1) a decline in female lipid during reproduction, (2) a 2- to 3-week delay in the onset of female fattening relative to males, (3) a decrease in female water content during late lactation, (4) sex differences in rates of weight loss during dormancy, and (5) sexual dimorphism in juvenile growth and fattening phenology. The initiation of seasonal fattening coincided with the appearance of seeds in the diet and associated increases in dietary caloric value. Experimental supplements to the diet increased fattening rates in field populations. These observations support the hypothesis that yearly fattening and immergence schedules are influenced by variation in the quality of food resources.