The flehmen response is a behaviour in which a horse curls its upper lip back and stretches its neck upward, appearing to grimace. It is triggered by exposure to an unusual or particularly interesting scent and allows the horse to direct odour molecules to the vomeronasal organ — a chemoreceptor located in the roof of the mouth that detects chemical signals beyond what the regular olfactory system processes.
Why Horses Do It
The vomeronasal organ, also known as Jacobson’s organ, is especially sensitive to pheromones and other chemical signals that standard smell receptors do not register effectively. By curling the lip and inhaling through the mouth rather than the nostrils, the horse draws air directly over this organ, allowing far more detailed chemical analysis of the scent.
In practical terms, you will most commonly see the flehmen response when a horse encounters: urine from another horse — particularly a mare in oestrus; a novel or strong-smelling substance such as certain medications, chemicals, or feeds; another horse’s dung in unfamiliar territory; and occasionally after tasting something unexpected.
Stallions and Mares
Stallions perform the flehmen response most frequently, particularly in response to mare urine as a means of assessing reproductive status. A stallion will often smell the urine from a mare, then perform an extended flehmen response, holding the posture for several seconds while processing the information. Geldings and mares also exhibit the behaviour, though typically less frequently and with less intensity.
During the breeding season, stallion handlers should be aware that the flehmen response indicates heightened arousal and interest — a horse in the middle of a prolonged flehmen response following contact with a mare’s scent may be distracted or more excitable than usual.
Is the Flehmen Response a Sign of Pain?
This is a common point of confusion. An isolated flehmen response in response to an interesting smell is entirely normal behaviour. However, repeated or sustained flehmen responses outside of typical triggers — particularly in combination with other signs such as pawing, rolling, reluctance to move, or looking at the flank — can be associated with gastrointestinal discomfort, including colic. The mechanism is thought to relate to visceral pain stimulating a reflexive response via the same neural pathways.
If you observe a horse performing the flehmen response repeatedly without an obvious scent trigger, monitor closely for other signs of colic or abdominal pain and contact a veterinary surgeon if you have any concerns. Early intervention in colic cases significantly improves outcomes. For related conditions, see Colic.
Other Species
The flehmen response is not unique to horses. It is observed across a wide range of mammals including cats, dogs, cattle, deer, and big cats. In all species, the mechanism is the same — directing chemical signals to the vomeronasal organ for more detailed analysis. In horses, the exaggerated upper lip curl makes it particularly visible and it is often noticed and remarked upon by people new to the species.
Frequently Asked Questions
Is it normal for a gelding to show the flehmen response?
Yes, completely. While stallions exhibit it most frequently, geldings retain the vomeronasal organ and will still respond to interesting or unusual scents. It is not a sign of hormonal issues in a gelding — simply normal chemical investigation behaviour. Some geldings are noticeably more prone to it than others, reflecting individual variation rather than anything clinically significant.
My mare does the flehmen response when I groom her — should I be concerned?
Not necessarily. Some mares exhibit the response to physical sensation as well as scent, and grooming can occasionally trigger it. If it is a consistent response to a specific area of the body — particularly the belly or flanks — it may be worth monitoring for other signs of discomfort. If she appears otherwise well and comfortable, it is likely normal individual behaviour. When in doubt, consult your vet.
How long does a normal flehmen response last?
Typically a few seconds to around thirty seconds. A stallion investigating a mare’s urine may hold the posture longer than a horse briefly responding to a novel smell. Sustained or repeated responses over several minutes, particularly without an obvious scent trigger, warrant closer observation and potentially veterinary attention.