Well, this massive elk bull was not really smiling but actually taking a big whiff of the air.
This image was captured mid-September in Rocky Mountain National Park at the height of the annual rut. Hormones were raging and the bulls were fighting for dominance. The bull was guarding a nice sized harem of cows and calves.
Ungulates like elk and some other mammals do what is called the flehmen response. They lift their lips and open their mouths to take in more of a scent, just as you or I do when taking a big whiff of something.