Intra- and interspecific phenotypic variation in mammals and its effect on biodiversity under climate warming

Global average temperature is rapidly increasing and is currently the warmest it has been in the Northern Hemisphere over the past centuries, driving species to local extirpation or extinction, to shift their distributional range or to adapt locally to the warming climate. A given species' sensitivity to changes in its environment may be related with patterns and amount of variation in its phenotype. As such, a decrease in body size has been proposed as the third universal ecological response to global warming, following distributional shifts and changes in phenology. Across their geographic ranges, mammals are thought to vary in body size following Bergmann's rule, which predicts that populations within a species found at higher latitudes, or colder climates, are larger in size compared to populations occurring farther south in warmer temperatures. While Bergmann's rule is well supported by empirical data among mammals, there is more and more evidence for exceptions to the rule, with species either decreasing in size with latitude or displaying no apparent relation between latitude and size. The patterns of variation in various morphological traits (skull length, skull width, tooth row length, and body mass) of 17 small and midsize mammalian hosts of Lyme disease were analyzed to determine if body size variation occurred in a predictable manner through space and time at an interspecific and intraspecific level. Results suggest little evidence of a generalizable pattern supporting Bergmann's rule within and among species at both a broad spatial and temporal scale. The effect of latitude or time on each of the morphological traits studied were highly variable leading to three types of responses: increases in size, decreases in size, or no changes in size across space and time. Overall, size trends were detected more often in space than through time, as size variation in space was studied over a significantly larger temperature gradient than the recent change in temperature that occurred over the past 120 years. Additionally, large-bodied species were not more likely to conform to Bergmann's rule than small-bodied species, in contrast to what was previously reported in the literature; in fact, this study showed that small mammals were found to vary more in size with latitude or time than midsize mammals. Contrary to predictions, size trends related to cranial measurements were detected as more likely to conform to Bergmann's rule than body mass, indicating the importance of simultaneously comparing metrics in studies on body size variation across species' ranges. However, body mass was found to have increased amounts of trait variability compared to cranial measurements; along a latitudinal gradient, the direction and magnitude of the variability depended on the size category of the species. Climate change is expected to cause the mammalian hosts of Lyme disease to expand their geographic ranges northward, facilitating the establishment of the bacteria and tick populations in southern Canada. For the mammalian hosts of Lyme disease, studies on phenotypic variation can help determine which host species have an increased sensitivity to climate and should be integrated into future species distribution models to increase the model's predictive power. More accurate projections of a host species' future distributional shifts into southern Canada will help determine the human populations most at-risk for Lyme disease in the future.

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