Effects of unpredictable, variable rainfall in arid systems on diets of adults and lambs of desert bighorn sheep (Ovis canadensis) are poorly understood. We determined associations based on analysis of feces between amounts of rainfall and composition and similarity (overlap and diversity) of diets of adults and lambs, and relationships between diets of adults and lambs, in the Sonoran Desert of central Arizona during 1999–2003. Drought occurred in 2 of 3 years during sampling of feces of adults, and in 1 of 3 years during sampling of feces of lambs. Composition and diversity of diets of adults were comparable during years of drought and normal rainfall, except that diets were not correlated between years when severe drought (31% of long-term average rainfall) followed normal rainfall. Composition and diversity of diets of lambs also were independent of amounts of rainfall, but dietary overlap between and within age groups was greater during years of increasing rainfall compared to previous years. We hypothesized that amounts of rainfall moderately influenced diets of adults and lambs of desert bighorn sheep, but diets generally were closely linked among years and between age classes, and associations tended to persist independently of amounts of rainfall. Improved quality of diets and higher production, particularly of forbs, were more important proximate factors than composition of diets in affecting differences in nutritional status of adults and lambs between drier and wetter years. Furthermore, selection of forage by lambs likely is learned, at least in part, through associations, particularly with their mothers, prior to weaning.
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Vol. 52 • No. 4