Body size in the fourth dimension: A paleoecological test of Bergmann's Rule

Background/Question/Methods

Bergmann’s Rule, which posits a causal, negative relationship between temperature and body size, is one of the oldest hypotheses in ecology, but subsequent research suggesting that other climatic and biotic variables may play a role in shaping body size evolution have made it the subject of a great deal of debate. As with many areas of ecology, the fossil and paleoenvironmental records provide a valuable perspective on this debate by allowing observation of trends through time and of organisms and climatic conditions without modern precedent. Because of its rich fossil record and well-resolved paleoclimatic record, the Oligo-Miocene (30-5 Ma) of western North America represents an ideal natural laboratory in which to conduct a paleoecological test of Bergmann’s Rule. If the predictions of Bergmann’s Rule sensu stricto are correct, mammal body size should increase during periods of cooling and should decrease during periods of warming; further, latitudinal body size gradients should be strongest during cold intervals, which are characterized by steeper climatic gradients. In order to test these predictions, we have tracked body size in three representative families of mammals (equids, canids, and sciurids) through the Oligo-Miocene, both through time in the Northwest United States and along the West Coast between Washington and Oaxaca. These results were regressed against existing paleoclimatic data derived from paleosol chemistry, allowing body size to be compared directly to temperature, as well as to precipitation and seasonality, two other climatic variables that have been suggested to play a role in shaping body mass trends in modern ecosystems.

Results/Conclusions

Within the Northwest, there is no evidence for a causal relationship between Oligo-Miocene climate and body mass in the mammalian taxa studied. Not only are significant correlations between any climatic variable and body size nearly absent, but also each group shows extremely different body size trends through time, suggesting the absence of ecosystem-wide body mass drivers. Likewise, body size gradients along the West Coast do not track changes in climate, and are, in fact, largely absent within the included taxa. Bergmann’s Rule, then, does not appear to hold for much of the Cenozoic. We suggest that the latitudinal body mass gradients upon which the model is predicated are a relatively recent phenomenon, perhaps brought on by the onset of extreme cooling and climatic variability in the Pliocene and Pleistocene.