The behavior where a horse lifts its upper lip, curls it back, and holds the posture for several seconds is known as the Flehmen response. This distinctive facial expression, which can appear like a grimace or a “horse laugh,” is actually a specialized sensory mechanism for chemical communication. It serves a specific biological purpose: to transport odorants, particularly non-volatile chemical compounds, to a dedicated sensory structure within the head.
The Unique Sensory Anatomy
The purpose of the lip-curling action is to facilitate the transfer of chemical signals into the Vomeronasal Organ (VNO), also known as Jacobson’s Organ. This organ is an auxiliary part of the olfactory system, distinct from the primary nasal passages. The VNO is located in the horse’s nasal septum, just above the roof of the mouth.
When the horse executes the Flehmen response, curling the upper lip helps close the nasal passages. This closure, combined with a slight inhalation, creates a pressure change that draws air and the chemical compounds into the VNO. The compounds must be non-volatile, meaning they are typically present in liquid form, such as in urine or other bodily secretions.
Once inside the VNO, the chemical molecules are detected by specialized sensory neurons. Unlike the main olfactory system, which sends signals to the conscious areas of the brain, the VNO’s signals travel along a different neural pathway. The information is sent directly to the accessory olfactory bulb, which connects to the amygdala and hypothalamus—regions of the brain linked to instinctive and reproductive behaviors. This bypass of conscious smell perception means the horse processes these chemical messages as immediate, instinctual cues.
Recognizing Pheromonal Signals
The most frequent trigger for the Flehmen response involves the detection of pheromones, which are chemical messengers passed between individuals of the same species. The VNO is specialized in sensing these non-volatile pheromones, providing horses with information about the hormonal and social status of their herd mates. This mechanism plays a central role in equine reproductive behavior.
Stallions are the most frequent practitioners of the Flehmen response, often displaying the behavior several times an hour when investigating a mare. They typically sniff the mare’s urine or genital secretions to analyze her reproductive status. The pheromones sampled reveal whether a mare is in estrus, indicating her readiness to breed.
The Flehmen response in this context acts as a mechanism for chemosensory priming, preparing the stallion for reproductive behavior. Mares also exhibit the behavior, particularly following the birth of a foal. By sniffing the newborn foal, the mare uses the VNO to process the chemical profile, supporting maternal recognition and strengthening the mare-foal bond.
Non-Reproductive Triggers
While the primary function is pheromonal detection, the Flehmen response can also be triggered by non-reproductive chemical stimuli. Horses may curl their lips when encountering any novel, strange, or unusually pungent odor in their environment. Examples of non-social triggers include smoke, fresh paint, unfamiliar feeds, or certain plant materials.
This suggests that while the VNO is specialized, the mechanism can be activated by a generalized sensory investigation of strong chemical compounds. Any substance requiring a more detailed chemical analysis than the main olfactory system can provide may prompt the Flehmen response. Horses may also display this behavior when introduced to a new dietary supplement or a medication with a strong, unfamiliar taste.
Young horses, especially foals, frequently exhibit the Flehmen response as part of their sensory development and general curiosity. This behavior helps them learn and memorize the scent profiles of their surroundings and their mother. In some instances, a prolonged or repeated Flehmen response, especially when accompanied by other signs of discomfort, can indicate abdominal pain, such as colic.