Rapid morphological divergence in two closely related and co-occurring species over the last 50 years

We studied morphological variation in two closely related and ecologically similar species of mice of the genus Peromyscus, the deer mouse (P. maniculatus) and white-footed mouse (P. leucopus), over the last 50 years in Southern Quebec. We found that contemporary populations of the two species are distinct in morphology and interpret this differentiation as a reflection of resource partitioning, a mechanism favouring their local coexistence. While there was no size trend, geographic or temporal, both species displayed a concomitant change in the shape of their skull over the last 50 years, although this change was much more apparent in the white-footed mouse. As a result, the two species diverged over time and became more distinct in their morphology. The observed changes in morphology are large given the short time scale. During this period, there was also a shift in abundance of the two species in Southern Quebec, consistent with the northern displacement of the range of the white-footed mouse in the last 15 years. Our study thus reports the changes in morphology of two co-occurring mammal species that were accompanied by changes in distribution and local abundance, potentially in response to rapid climate change.