The flehmen response is a behaviour in which your horse curls back her upper lip exposing its front teeth, inhales with the nostrils usually closed, and then tyically holds this position for several seconds.
The nostrils of your horse flare to draw more scents into the long, cavernous nasal passages that are made for sucking in large quantities of air so that your horse can decode all the chemical messages in the air.
Your horse’s olfactory (smelling) receptors are located in the mucous membranes in the upper portion of the nasal cavity. The receptors are made up of millions of elongated nerve cells that specialise in analysing smells.
When odour molecules in the air come into contact with the mucous membranes, they interact with the microscopic tufts of hair protruding from the receptor cells.
By sniffing your horse intensifies the currents of air in the nasal passages and provides more contact between the odour molecules and the receptor cells and gives them more time to analyse the odour.
Two branches stretch from the olfactory cells; one branch extends over the surface of the olfactory mucous membrane and another goes directly to the brain.
Your horse has two distinct areas of the brain that are responsible for identifying scents. They are called the olfactory bulbs and are located at the very front of the cerebrum – one on each side.
They are connected by the scent detection nerves to the scent receptors in the nasal passages.
Your horse has two olfactory systems
A second pair of olfactory organs sit just below the floor of your horse’s nasal cavity. These are called the vomeronasal organs.
The word vomeronasal comes from the organ being close to the vomer bone in the nasal cavity. The Danish anatomist Ludvig Jacobson was the first one to describe the organs in 1813, so they’re sometimes also called Jacobson’s organs.
The vomeronasal organs can be found in all snakes and lizards as well as in many mammals, like cats, dogs, horses, cattle, pigs. In humans, it’s present but is vestigial (meaning that it has become functionless during the course of our evolution) and so, is non-functional.
The flehmen response is commonly used by all animals who have vomeronasal organs, and you can, for instance, catch your cat doing it after discovering an interesting smell that needs to be investigated further.
The vomeronasal organs in your horse are tubular and made or cartilage. Despite being about 12 cm long (4,5 in) they’re carefully concealed and it’s not surprising that many anatomists before Jacobson missed them entirely.
The organs are lined with mucous membranes and contain more sensory fibres of the olfactory nerve. They’re connected to the main nasal passages by a duct called the nasopalatine duct (nasopalatine meaning to connect to the nose and the palate).
The vomeronasal organs expand and contract like pumps when they’re stimulated with strong odours. They have their own pathways to the brain meaning that they function as independent sensory organs.
Why does your horse need two systems to decode smells?
The job description of the vomeronasal organs is different than for the main sensory system for smells.
The vomeronasal organs’ main purpose is to detect and analyse pheromones.
Pheromones are chemicals that can act like hormones outside of the body (of the individual secreting them). Their purpose is to impact the behaviour of the individuals receiving them and are designed to have an effect on members of the same species.
The main purpose of pheromones is to indicate an individual’s sexual status.
In this way, the vomeronasal organ is really a sex organ, designed to help stallions identify when a mare is in heat and receptive to breeding.
Stallions will use this information to determine when a mare is out of season and likely to reject any attempts at mating as well as to tell when there is a rival stallion nearby that might steal his mares away.
In many animals, horses included, the stimulation of the vomeronasal organ can have a profound effect on the endocrine system.
The endocrine system is a collection of glands that produce hormones to regulate things like metabolism, growth, development, tissue function, sexual function, reproduction, sleep, and mood.
For example, in bees, the queen exudes pheromones to stall the sexual development of all surrounding female bees (a hive only has one queen to produce offspring).
And the pheromones of male mice can promote the sexual maturation of young female mice, or even induce abortion (presumably as a form of genetic competitiveness with other breeding males).
The flehmen grimace
The word has its origins in German, flehmen, to bare the upper teeth, and Upper Saxon German flemmen, to look spiteful. It does kind of look like your horse is laughing at a particularly good joke, doesn’t it?
When your horse wants to inspect something more closely she’ll usually smell it thoroughly all over if she can (or dares) get close to it.
Especially interesting scents are further investigated by lifting the head up high and curling the upper lip in a flehmen grimace. This appears to trap pheromone scents in the vomeronasal organs so that they can be analysed more closely.
After inhaling the odour, your horse will curl the upper lip and temporarily close the nasal passages to trap the odour particles inside. The upward head tilt seems to help airborne molecules linger for longer in the vomeronasal organs.
When a horse does the flehmen response you’re seeing an outward demonstration of her vomeronasal organs being stimulated.
Both sexes use the flehmen, but stallions are by far the ones who do it the most. If a stallion is in the presence of a mare in oestrous, he can flehmen several times in just 10 minutes, as he walks around smelling her behind and flanks, her urine and her faeces.
Mares will flehmen, especially when smelling the birthing fluids of their newborn foals (or those of others’ foals).
Geldings tend to flehmen the least (although they do still do it). It has been theorised that the process of gelding compromises the male’s ability to detect and analyse pheromones – thus making him sexually ineffective in more ways than one.
Pheromones are present in horses themselves, but also on their bodily fluids and their manure
Horses use urine and droppings to advertise their sexual status to other horses and to mark their sexual territories.
A stallion will create a “stud pile” by successive passings of manure that are carefully piled on top of each other to tell other stallions in the are that this is his turf.
Mares will pee several times an hour when they’re in heat because the pungent odour of the urine is like a billboard ad to any sexually active stallion in the area. Sometimes geldings can also get very excited when a mare is in oestrous.
Studies have shown that stallions cannot tell if a mare is ready to mate by the smell alone. Visual an vocal cues play an important role in equine courtship.
Stallions that are in the presence of already pregnant mares will flehmen a lot less. This is most likely because mares in oestrous pee all the time, triggering the flehmen response in stallions.
Pheromones are unquestionably the most likely flehmen trigger, but they aren’t the only ones
Your horse will also react by curling her upper lip when she comes in contact with an unusually strange or strong smell.
Smoke from a fire, fresh paint and essential oils can all easily make your horse flehmen.
There is a lot of research into the vomeronasal organ, but most of it isn’t equine specific. There is still a lot we don’t understand about how pheromones are secreted and in what ways they influence behaviour.
Read more about your horse’s sense of smell here.