The cows come into "Estrus" about every 3 weeks during rutting season. Here you can see the bull "sniffing" her because he can smell when she is in estrus and may be receptive to him.
These bulls are fighting for dominance in a herd. They often break antlers and even kill each other. At the end of rutting season the bulls are worn out, hungry and may be too weak to make it through the winter. They look for a place to rest and some high quality food. The bulls and the cows do not hang out together at any other time of the year. The cows have a "lead cow" for each herd. Gestation is about 9 months and the calves are born in the spring. The calf will stay with his mother and the rest of her herd until she is ready to mate again, then she will push him away.