Geographic Size Variation in Tamiasciurus douglasii: Significance in Relation to Conifer Cone Morphology

Multivariate analysis of cranial characters of the tree squirrel, Tamiasciurus douglasii, was employed in an evaluation of geographic variation. Principal components analysis yielded a strong size component. Regression and correlation analyses identified conifer cone size, toughness, and caloric content as good predictors of variation in size of squirrels. A hypothesis that squirrel morphology is influenced by pressures associated with maintaining efficiency in conifer cone caching and winter cone utilization is examined. It suggests that with geographic changes in cone morphology, squirrel size has undergone selection. The smallest squirrels were found in spruce, hemlock, and redwood forests, where trees have small cones with little energy per cone. Larger squirrels were from forests with predominantely large, energy rich pine cones. It appears that populations of T. douglasii have developed feeding tactics based on morphology which minimize loss of net energy available from their primary winter food source by increasing their efficiency in handling time. Large squirrels utilize large cones that give a high net energy/handling time ratio in areas where food must be consumed quickly. Small squirrels utilize small, probably lower ratio cones in less demanding environments.