Allen’s rule is an ecogeographical hypothesis that states the length of appendages in endotherms relative to body size is reduced in cooler parts of a species range. Traditionally this hypothesis received little attention, but a flurry of investigators has explored this relationship in the last decade. As part of an investigation into morphological variation in the deermouse (Peromyscus maniculatus), I explored the validity of Allen’s rule. After constructing a data base of approximate 6,000 deermice based on museum specimens, I then found climate records from weather stations within 50 kilometers of capture site. Further limiting the sample to mice whose body length was more than 106 mm as well as a tail length more than 46 mm resulted in a pool of approximately 4,000 deermice. Tail length has been demonstrated to correlate with vegetation (forest-dwelling deermice have longer tails than grassland deermice). Would Allen’s rule apply to forest-dwelling deermice?
Results/Conclusions
Peromyscus maniculatus borealis is a subspecies of deermouse that lives in forest, grassland, and an ecotonal zone (referred to as parkland). When the tails of just the forest-dwelling deermice are compared with the average January minimum temperature, a significant correlation is found. The colder the temperature, the shorter the tail, which would be a prediction of Allen’s rule. Further investigation has found that other subspecies are not as strongly correlated. With respect to other appendages, the length of ears is positively correlated with altitude (higher elevations have mice with larger ears), but negatively correlated with June’s average temperature. Neither of these results was expected. The length of feet is positively correlated with January average temperature, but negatively correlated with the average temperature of May. These results suggest that summer temperatures also play a role. The length of appendages in deermice appears to be under differing selective pressures depending on the appendage but also the environment.