Does your cat flehmen--sometimes make a weird sneering face? Cats lift their lips after a particularly long sniff, and hold the mouth slightly open to sniff cat pheromones. Flehmen--a German word that means "lip curl"--might be mistaken for an aggressive snarl but is a perfectly normal and peaceful cat behavior.
How Cats Sniff Pheromones
Cats have a second scenting mechanism called Jacobson's organs, or vomeronasal organs. These are found between the hard palate of the mouth and the septum of the nose. Jacobson's organs link to the hypothalamus in the brain that serves as sort of a switchboard to direct information to other areas. Tiny ducts connect them to openings behind kitty's teeth in the roof of the mouth.
Cats flehmen (grimace) when the tongue traps pheromones, then flick-transfers to the duct in the roof of the mouth. Pheromones are chemicals unique to a particular critter. For instances, cheek pheromones rubbed against objects can tell other cats that they are friendly or that the territory is owned. Pheromones in urine notify other cats if the cat is intact or fixed, and whether a boy or a girl.
All cats use this second sniff-mechanism to analyze pheromones but boy cats show the flehmen kitty sneer most often. That may be because they are particularly attuned to checking out sex-related information even if they're neutered. Boys will be boys, eh?