_MG_2740 - On the run. ©Jerry Mercier by jerry mercier on Flickr. A bull bison runs after a cow and calf. These American bison were part of a herd of at least two hundred animals. The rut was on and the herd was moving from the slopes of Specimen Ridge down towards the Lamar River. Large bulls were grunting and snorting and pushing the cows and calves. I assume this cow was in estrus as the bull was very attentive and never left her side.