The Relation of Social Evolution and Dispersal in Ungulates During the Pleistocene, with Emphasis on the old World Deer and the GenusBison

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Abstract
The evolution of Pleistocene mammals is characterized by giantism and in the ungulates by the appearance of huge and often bizarre horns, antlers, ossicones, and tusks. An earlier study of sheep (Ovis) led to a theory explaining these phenomena. This theory, termed the “dispersal theory,” is described briefly and then applied to the evolution of Old World deer (Cervinea, Baird, 1857). It is shown that the zoogeography and physical characteristics of Old World deer follow the predictions of this theory. Attention is drawn to the parallels in zoogeography and convergent evolutionary changes between Old World deer and other ungulates such as New World deer, the goats and sheep and the genusBison. The application of the “dispersal theory” to the evolution of the genusBisonshows that it is compatible with known facts and that it explains aspects ofBisonmorphology not usually considered. It also suggested a new and testable hypothesis of the origin of present-dayBison. In general the “dispersal theory” applied to Pleistocene conditions predicts that a specialization of social behavior and social organs leading to giantism, altered body proportions and bizarre hornlike organs occurs during postglacial dispersal into new habitat. It also describes the characteristics of these early, pioneering populations. In so doing it links the disciplines of animal behavior, ecology, animal science, population dynamics, and zoogeography into a functional whole. In particular it shows the relationship between ecology and social behavior. Finally, the “dispersal theory” is shown to be a general theory, applicable to Pleistocene and pre-Pleistocene conditions. In the tropics or pre-Pleistocene conditions social evolution follows ecological specialization, while in the periglacial zones social evolution only reflects the colonization history of the species. It is shown that the “dispersal theory” unites the apparently contradictory views on evolution and zoogeography of Matthews and Darlington. Matthews' views apply largely to Pleistocene conditions and Darlington's to pre-Pleistocene conditions. A preliminary application of the “dispersal theory” to pre-Pleistocene mammals suggests that it predicts correctly.