Body Mass, Structural Size, and Life-History Patterns of the Columbian Ground Squirrel

Abstract

Columbian ground squirrels (Spermophilus columbianus) exhibit an elevational cline in several life-history traits and in body mass. Body mass is a trait that might interact with the elevational changes in life histories, but it could reflect either or both of two underlying factors: structural size and physiological condition. I studied these factors in several populations of ground squirrels. Structural size was estimated from first principal component scores in an analysis of postcranial bone lengths. Physiological condition was inferred from differences in body mass that were independent of structural size. Field body mass and structural size of yearlings decreased with increasing elevation, which indicates slower growth at higher elevations. For adults, physiological condition was not measured directly, but evidence suggested that it was reflected by changes in body mass: body mass declined with increasing elevation but structural size did not, residuals of body mass on structural size showed the same patterns of change as body mass, most of the variation in body mass was orthogonal to a "general size factor" in principal components analyses, and significant changes in body mass of individuals could be stimulated by experimental supplementation of food. Life-history traits and physiological condition covaried along the elevational cline and were phenotypically plastic, but the structural size of adult ground squirrels was not an extremely plastic trait.