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Have you ever seen a horse curl his upper lip and show his teeth and thought he was smiling?
What you are observing is known as the flehmen response and like most behaviours it's done for a reason.
The flehmen response is a specialised action in which a horse curls its upper lip, lifts its head, and holds that frozen, slightly exaggerated pose while drawing scent molecules deep into a sensory structure called the vomeronasal organ. This organ sits between the nasal passages and the roof of the mouth and is designed to detect pheromones and complex chemical signals that ordinary sniffing cannot fully interpret. The lip curl helps trap the scent, the raised head shifts airflow, and the horse effectively pauses to 'pull' the odour into a place where it can be analysed more precisely.
Horses use the flehmen response when they encounter smells that carry important information. Stallions often do it after smelling a mare's urine or scent because it helps them assess her reproductive status. Mares may flehmen after giving birth as they investigate the scent of their newborn foal. Young horses do it frequently because the world is full of unfamiliar smells worth decoding, and even everyday things such as new bedding, medicines, fresh manure, or a human wearing strong perfume can trigger the behaviour. In each case, the horse is not being dramatic or amused; it is gathering data.
The posture itself is functional rather than expressive. The curled lip temporarily blocks normal airflow, trapping the scent and holding it in place while the vomeronasal organ processes the information. The exposed teeth are not a threat display, and the lifted head is not a smile. It is simply the mechanical position that allows the horse to analyse the scent most effectively. Once the horse has processed the information, the head drops, the lip relaxes, and the moment passes.
Although the flehmen response is the most recognisable lip‑curling behaviour, horses show their teeth for other reasons that look different in context and posture. Foals may perform a submissive 'clacking' gesture, opening and closing their mouths to signal harmlessness. Youngsters may mouth playfully during social interactions. Aggressive displays involve pinned ears, tension, and a very different facial expression. These are easy to distinguish from the still, concentrated look of a flehmen response.
The flehmen response reveals how deeply horses rely on scent to understand their world. It helps them identify other horses, assess safety, interpret reproductive cues, and make sense of new or puzzling situations. When a horse pauses, curls its lip, and seems to contemplate the air, it is not being comical; it is reading the world in a way humans cannot.
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