A big bull moose performs the "flehmen response".
The animal behavior called a "flehmen response" is recognized when an animal lifts up its head and curls back its upper lip exposing its front teeth. As you can see in the below picture, moose are one of the species that perform this behavior.
The technical explanation is that this behavior helps scents and pheromones into an organ - "Jacobson's organ" - located in the mouth.
Moose and other animals perform the flehmen response when investigating odors. It is believed that they are interpreted as "odor messages" between animals. One of the most common reasons is that it allows the males (as in the picture below) to 'smell' whether a nearby cow moose is ready for mating and is sexually receptive.
Fine Art Prints available of your favorite wildlife images: