Effects of food quality on growth and survival of juvenile Columbian ground squirrels (Spermophilus columbianus)

Within a cohort of Columbian ground squirrels (Spermophilus columbianus), most deaths during hibernation usually occur in the first winter. Over-winter survivorship of juvenile Columbian ground squirrels is positively related to prehibernation body mass, which is primarily determined by the growth rate after emergence from natal burrows. This study examined how food quality influences growth and survival of juvenile Columbian ground squirrels. Growth and survival of juveniles in three colonies was measured, and biomass, composition (forb versus grass content), gross energy, crude protein, and digestibility of the summer forage supply were assessed. Differences in biomass and gross energy among colonies did not explain differences in growth and survival of resident ground squirrels. Growth and survival of juveniles was enhanced on sites with higher forb content in the forage supply. Forbs contained 15-40% more protein and were 30-50% more digestible than grasses, but protein levels on all sites were probably in excess of ground squirrels' requirements. This suggests that digestibility was limiting growth and was a more important food-quality indicator for juvenile ground squirrels than either energy or protein.