American Bison / Buffalo - male vocalizing (bellowing) during rut, side view (Bison bison). Wyoming, USA. Males weigh up to 2000 pounds. They are the heaviest land mammal in North America. Mates in the summer-Bull enters the matriarchal herd and displays 'flehmen' and 'tending behavior' -Flehmen consists of curling the lip back and extending the neck and is thought to enhance the sense of smell- Male 'tends' a female by remaining between her and the herd in an attempt to keep the cow isolated - Males threaten and can battle other contenders which can result in butting and horn-locking.