The late prehistoric hunters of bison in what would become the plains of the United States typically slaughtered the cows, and left the remains of bulls behind, hunting in the fall and winter. But at the Gamsey Bison Kill Site, in southeastern New Mexico, the reverse seems to be true: the hunting seems to have taken place in late spring, and the remains of cows were left behind, suggesting it was the bulls which were slaughtered and packed away to the tribe's camps. The author shows that this anomaloy makes sense, because the bison cows that were pregnant or had calves would be leaner in spring, and the hunters were selecting those bison with the most fat, in an effort to stave off an annual food shortage.