Bison Mobility, Wildness & Range Size

Elsewhere on this website, the role of human hunting in past evolution of modern bison is discussed. At the end of the ice age, natural selection by large and dangerous predators was replaced by human predation. Evolution responded by altering the predator-accommodation strategy of North American bison. Once large bison, with large and/or aggressively shaped horns and a predator-evasion strategy of “stand and defend”, were replaced by smaller bison with a strategy based on great mobility. Our modern bison species developed from this process. When attacked, bison herds would travel great distances, beyond the ability of pedestrian predators to follow.

Consequently, mobility is a basic characteristic of modern bison. Notably, our bison are smaller than their ice-age ancestors. But all other characteristics of bison anatomy, physiology and behavior must be compatible with, or supportive of, great mobility. Presumably, bison stamina based on skeletal, muscular and cardio-vascular anatomy and physiology, have developed to enhance mobility. Once established as a predator-evasion strategy, great mobility also influenced how bison have evolved to effectively utilize a large and diverse environment. Bison appear to learn and remember characteristics of a large environment; and their social behavior is designed to pass that information across generations. A wild bison herd, established on a large range with topographic and vegetative diversity can range widely to best exploit its environment in response to seasonal and among-years variation in weather, vegetation and availability of water.

In the evolutionary past of bison, animals that best developed these characteristics achieved higher rates of survival and reproduction – enhancing or maintaining the genetic composition of the herd across generations. However, south of Canada, few of today’s bison live on large ranges.

In a survey of 44 conservation herds of plains bison south of Canada (Bailey, 2013), only 1 herd had a range larger than 110 square miles. Bison could cross a range of 110 square miles in less than a day. Only 6 herds had ranges of 51-110 square miles. Worse, 37 of 44 bison conservation herds had ranges of 50 or fewer square miles.

Thus, great mobility is neither useful nor necessary for almost all of today’s conservation herds. Natural selection to maintain characteristics associated with great mobility has ended for these bison. It is a major part of the gradual process of domestication – simplifying and disorganizing the wild genome.

To maintain wild bison, the Coalition recommends at least 1000 bison on at least 100 square miles of diverse habitat. Larger herds on much larger ranges would be much better. But anything less than these minimums would contribute little or nothing to forestalling the evolutionary decline of wild bison. We already have many small conservation herds on small ranges.

Bailey, J. A. 2013. American Plains Bison: Rewilding an Icon. Sweetgrass Books, Helena, MT.

 

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