At last they have reached the gate to the pen.  The corrals are just out of view to the right.  Once the Bison are confined, park personel go to work on the herd.  In order to keep the herd size within the carrying capacity of the park, excess animals are auctioned off each year.  Before they are allowed to be sold they must be tested for brucellosis and tuberculosis to prevent the spread of these diseases to domestic stock.  All calves are branded, and all cows are vaccinated.   Most of the Bison are released right back out to the park,  and most of the culled stock is sold to private ranchers.  We found out later that this roundup included about 1400 head.  Of course the biggest bulls are not included in the roundup, they are just too cantankerous.