The prairies of North America. This rich pasture once supported the greatest herds ever seen on our planet. There were once 60 million bison but no animal is immune to intensive hunting by man or the destruction of its habitat and a century ago the bison were reduced to barely 1,000. Now, thanks to rigorous protection, the species is recovering. The growing season is long and the grass here can support herds all year round. Male bison weigh in at one tonne. In high summer the bulls are fat from the rich grazing and in prime condition but only a few will mate. Exactly which few is about to be decided.