Breaking the Rules: Museum Data Reveal Divergent Support for Classic Ecogeographical Rules in North American Flying Squirrels.

Abstract

Ecogeographic generalizations or "rules", such as Bergmann's rule and Allen's rule, propose that endotherms in colder climates evolve larger bodies and shorter appendages to minimize heat loss. These patterns are hypothesized to reflect physiological adaptations to thermal stress through reduced surface area-to-volume ratios. Although these rules have been extensively evaluated across birds and mammals, their applicability to arboreal gliders remains untested, with little consideration of how alternative ecological and evolutionary drivers may structure morphological variation in flying squirrels across environmental gradients. Using digitized museum specimens of North American flying squirrels (Glaucomys sabrinus, Glaucomys oregonensis, and Glaucomys volans), we tested these classic ecogeographic principles by examining how weight, total length, tail length, and ear length vary across a latitudinal gradient. We found significant support for Bergmann's rule, with body mass and total length increasing with latitude. In contrast, patterns in appendage morphology provided limited support for Allen's rule. Ear length exhibited a weak positive relationship with latitude, while tail length strongly increased with latitude, contradicting the prediction that appendages become shorter in colder climates. These findings suggest that thermoregulatory selection alone cannot explain morphological variation in North American flying squirrels and that functional demands, particularly those associated with gliding performance, may play a stronger role in shaping tail morphology. Overall, our results demonstrate that although classic ecogeographic rules provide a useful framework for understanding morphological variation, they do not fully capture the multiple selective pressures acting on Glaucomys species.