Bison have been present in the Great Plains for at least a hundred thousand years. Their constant presence and the presumed gradual changes in the shape and size of their horns made them an ideal chronometer for some paleontologists to estimate the age of certain fossil assemblages. Although ubiquitous during the Late Pleistocene, they typically made up only a small portion of an entire faunal assemblage. This all changed about twelve thousand years ago, when the other Great Plains megafauna went extinct. With the loss of large herbivores, such as mammoths, horses, and camels, bison populations boomed. Sites at this time included dozens, sometimes hundreds, of animals and were often associated with evidence indicating a long-term, sophisticated use of bison by Indigenous communities.
Much of this drama also played out in the Sandhills, where bison bones are commonly found in archaeological sites. At one rare locality near Burwell, tracks are preserved in a buried lakebed. Despite long-held assumptions suggesting bison migrated often and for long distances, historic records of bison movements have been ambiguous. Some early chroniclers observed the year-round presence of small bison herds, whereas others provide convincing accounts of seasonal, long-distance movements. Both scenarios could be correct, but the chemical record of bison behavior in teeth and bones from paleontological and archaeological sites has provided a more nuanced picture of bison movement at some localities.
Over the last few decades, scientists have begun to explore the potential of certain isotopes for tracking the impact of animals within ecosystems. Isotopes of some elements in teeth and bones correspond with animal behavior. For instance, the ratio of 12C to 13C (notated as δ13C) reflects the amount of warm season grasses (C4 plants) relative to cool season grasses, herbs, shrubs, and trees (C3 plants) in an animal’s diet. This relationship has been very important in understanding the timing of the spread of grassland ecosystems during the Miocene and Pliocene. The ratio of 87Sr to 86Sr (notated as 87Sr/86Sr), on the other hand, reflects the underlying geology of an area that is grazed by an animal. Through careful sampling of rapidly growing teeth, scientists can reconstruct the movement of an animal across different geological substrates. This has led to surprising results when applied to bison from the Great Plains, suggesting that bison were local herbivores with a flexible diet throughout much of the late Pleistocene and Holocene. Long-distance, seasonal migration seems to be a behavior that only existed during the narrow window of time when early writers documented it. That is not to say that bison never moved. A ten-thousand-year-old herd in the southern Sandhills seasonally shifted its range between the Platte River valley and neighboring uplands over the course of a multiyear record. Over many years, the home range of a herd might drift a significant distance, or young male bison might disperse from a natal herd to a new range. These studies paint a picture of bison as highly flexible and able to adapt to rapidly changing environments, characteristics that would have served them well when other megafauna went extinct.