Identification of white-footed mice (Peromyscus leucopus) and deer mice (P. maniculatus) through comparison of allozymes for salivary amylase

Background/Question/Methods

Two of the most common and abundant small mammals in North America are the white-footed mouse (Peromyscus leucopus) and deer mouse (P. maniculatus). These two species have been identified as significant seed dispersers, predators of pest insects, prey items of both avian and mammalian predators, and both established disease vectors and reservoirs. Recent studies have also identified possible ecological replacement of P. maniculatus by P. leucopus across a large geographic area. While well-studied, these two species are extremely difficult to tell apart in the field, even for experienced observers, and are often found occupying the same microhabitats. Morphological measurements are typically used for identification, but are inconsistent even within a local area. Reliable, non-lethal, identification is possible through detection of different allozymes for salivary amylase.  We collected saliva and used cellulose acetate electrophoresis to identify these two species from a variety of sites throughout northwestern Minnesota between 2004 and 2012. We compared the results with morphological data (tail length, hind foot length, ear length, and observation of tail pigmentation). Our objective was to definitively identify Peromyscus to species in a region with high habitat overlap and to examine how well morphological predictors compared to the salivary amylase identification.    

Results/Conclusions

We managed to get useful salivary amylase samples from 615 Peromyscus from a variety of forest and prairie habitats in northwestern Minnesota. After relative stability in 2005 – 2007 with P. maniculatus being the more common species, the densities of the two species have fluctuated widely, with dominance switching repeatedly. We found that 33% of the captured mice could not be definitively identified by measurements because of extensive overlap.  Additionally, depending on year, we found 15 to 25% of identifications based on morphology to be incorrect when compared to results from electrophoresis of amylase. In sum, the cumulative error when trapping Peromyscus results in up to 48% of the mice we captured being unidentifiable or incorrectly assigned to species. We found that the tail pigmentation criterion – i.e., P. maniculatus tends to have a distinctly bi-colored tail whereas P. leucopus does not – to be accurate approximately 73% of the time. We caution against the overuse of the tail pigmentation criterion because we found this to be subjective. Considering their potential as disease vectors and reservoirs, as well as the apparent ecological replacement of P. maniculatus, these two species require diligent monitoring of population trends and accurate distribution determination.