When Climate Wags the Dog

In light of modern, anthropogenic global warming, the question of how (or whether) climate change influences biological change is becoming increasingly important. John Orcutt’s research focuses on a particular aspect of this question: the relationship between paleoclimate and mammalian body size. The classical explanation, known as Bergmann’s rule, holds that temperature is the primary determining factor of body size; larger individuals will be favored in cold climates because they are better able to retain heat. However, more recent work has suggested that other climatic variables (in particular seasonality and aridity) or biotic interactions (such as competition or migration) play a more important role. John is examining body size trends over the course of the Miocene in three ecologically distinct families of mammals: horses, canids, and squirrels. The Miocene is an excellent interval in which to assess the effects of paleoclimate climate change, as it encompasses periods of both global warming and global cooling. By comparing body size profiles along geographic transects and through time to climate curves and similar trends in other groups of mammals, John hopes to be able to determine which climatic or biotic variables most strongly influence body mass.

Preliminary results from the Pacific Northwest show exactly the trends predicted by Bergmann’s rule: individuals from the cool, dry, and seasonal late Miocene tend to be larger than their counterparts in the warmer, wetter, and less seasonal mid-Miocene. This pattern is most clearly visible among the different groups of horses (as seen in the chart on the right). A more detailed analysis of local paleoclimatic conditions should allow a determination of which climatic factors track body size most closely.