Geographic Size and Non-Size Variation in Rocky Mountain Tamiasciurus hudsonicus: Significance in Relation to Allen's Rule and Vicariant Biogeography

Abstract

Multivariate analysis of cranial characters of Rocky Mountain populations of the red squirrel, Tamiasciurus hudsonicus , was employed in an evaluation of geographic variation. Principal components analysis yielded a relatively weak size component which was not significantly correlated with environmental variables. Homogeneous morphology of conifer cones in this area may not provide the strong pressure for size variation suggested in a similar analysis of T. douglasii . A relatively strong non-size component was not significantly correlated with indices of conifer cone morphology, was moderately correlated with temperature, and was highly correlated with latitude and humidity. Correlation of latitude with weather variables indicated the lack of an environment favorable for size variation according to Bergmann's ecogeographic rule but highly favorable to proportional variation according to Allen's rule. The vicariant biogeography of the Southwest is proposed as a means of enhancing the selective capabilities expected under Allen's rule. From this analysis it appears that, with the lack of strong biotic selective forces in the area, squirrels are being more heavily affected by abiotic pressures.