Environmental Factors and Co‐Occurrence Patterns Influence Dorsal Brightness in Two Jumping Mice Species in the Western United States

The phenotype is the result of interactions between an organism's genotype and its environment. One phenotypic trait of interest is dorsal coloration, which animals use to communicate, avoid detection from predators, thermoregulate, and more. Here we used standardized photography of museum specimens to explore the variation in dorsal brightness of the endangered New Mexico jumping mouse ( Zapus luteus ) in areas of allopatry and sympatry with the western jumping mouse ( Z. princeps ) in the southwestern United States. For Z. luteus we found statistically significant differences in brightness among isolated, genetically differentiated populations in Arizona and New Mexico. We additionally found significant, predictable interspecific differences in color between Z. luteus and Z. princeps where they co‐occur; however, these differences would be difficult to distinguish visually. We also found evidence that the dorsal brightness of both species is associated with geographic (elevation, latitude) and bioclimatic (precipitation, temperature) features, as well as interspecific interactions (syntopy patterns). Our findings emphasize that both biotic and abiotic factors influence dorsal coloration in free‐living organisms, and highlight the importance of using multiple lines of evidence to identify co‐occurring congeners in the field. For endangered species like Z. luteus , correct identification can have important management implications.