INTO THE TROPICS: A QUANTITATIVE STUDY OF MAMMALS IN THE GREAT AMERICAN BIOTIC INTERCHANGE

For almost 100 million years, North and South America were isolated from each other. This long period of geologic separation led to the evolution of strikingly different mammalian faunas: marsupials were prevalent in South America, while ecosystems in North America were composed of placental mammals. Roughly 3 Mya, a land bridge formed between the two continents leading to an accelerated exchange of mammalian fauna. The Great American Biotic Interchange (or GABI, as this has come to be called,) led to the successful colonization of many North American species, but few South American species. The highly asymmetrical nature of the faunal exchange has puzzled paleontologists for more than a century. We examined the GABI using an End-Pleistocene dataset of non-volant mammals and their associated ecological characteristics. We employed classification trees to quantify the relative importance of traits that led to successful colonization. Our analysis suggests the most important factor was body size. For species originating in North America, small body size was beneficial, v but the pattern was reversed for those moving north from South America. We believe this morphological asymmetry was due to the different climate regimes present in North and South America. We propose an Into the Tropics\u27 model of colonization for taxa that follow Bergmann\u27s Rule. We find that dietary and environmental niche, or mode of birth (placental vs marsupial) were less important than suggested by previous studies. Our study strongly supports the role of climate in determining the composition of mammal communities