Abstract
Climate change is expected to induce many ecological and evolutionary changes. Among these is the hypothesis that climate warming will cause a reduction in body size. This hypothesis stems from Bergmann's rule, a trend whereby species exhibit a smaller body size in warmer climates, and larger body size under colder conditions in endotherms. The mechanisms behind this rule are still debated, and it is not clear whether Bergmann's rule can be extended to predict the effects of climate change through time. We reviewed the primary literature for evidence (i) of a decrease in body size in response to climate warming, (ii) that changing body size is an adaptive response and (iii) that these responses are evolutionary or plastic. We found weak evidence for changes in body size through time as predicted by Bergmann's rule. Only three studies investigated the adaptive nature of these size decreases. Of these, none reported evidence of selection for smaller size or of a genetic basis for the size change, suggesting that size decreases could be due to nonadaptive plasticity in response to changing environmental conditions. More studies are needed before firm conclusions can be drawn about the underlying causes of these changes in body size in response to a warming climate.
Highlights
Biological responses to global change fall into three categories: extinction, change in distribution and adaptation to the new environmental conditions
The inability to make simple, clear predictions about body size changes in response to climate change further emphasizes that the adaptive nature of a size change cannot be inferred from patterns only, and that estimating selection pressures acting on size is key to understand these trends
There is no direct evidence that body size decreases in birds and mammals are an adaptive response to climate change
Summary
Biological responses to global change fall into three categories: extinction, change in distribution and adaptation (genetic or plastic) to the new environmental conditions. We review the evidence that (i) decreases in body size through time have occurred with recent climate warming in accordance with Bergmann’s rule, (ii) these size trends are adaptive in nature, with smaller individuals being selected and (iii) these trends have either a plastic or a genetic basis. The inability to make simple, clear predictions about body size changes in response to climate change further emphasizes that the adaptive nature of a size change cannot be inferred from patterns only (conformity or not to Bergmann’s rule), and that estimating selection pressures acting on size is key to understand these trends. Studies reporting evidence of selection for smaller body size are still very sparse, and no overall conclusion can be drawn on the adaptive nature of temporal size trends in the context of climate warming. Of the 19 studies showing a size trend, only two formally investigated the plastic or evolutionary basis of size change in response to climate change (Table 2)
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