Grinnell had extensive contact with the terrain, animals and Native Americans of the northern plains, starting with being part of the last great hunt of the Pawnee in 1872. He spent many years studying the natural history of the region. As a graduate student, he accompanied Lieutenant Colonel George Armstrong Custer’s 1874 Black Hills expedition as a naturalist. He declined a similar appointment to the ill-fated 1876 Little Big Horn expedition.