While we don’t think of a horse’s ability to smell as impressive as that of a dog’s, it is still umpteen times better than a human’s. Horses easily detect medication in feed, even if covered in molasses, recognize familiar people and other horses from a considerable distance by smell, and even, according to a 2011 study, recognize the manure of horses they’ve met before.
1. Horses Smell Better
“Umpteen” is code for in the ballpark of 50 times more than humans – oddly, there isn’t much research available on comparing scenting ability between various animals. Genetically speaking, however, humans have 350 olfactory receptor (OR) genes, while horses have 1,066 OR genes. The OR gene’s code for various olfactory receptors that detect different smells. Inside a human nose there are about five or six million olfactory receptors. While this sounds high, it is believed long-nosed animals like cattle and horses have considerably more, but somewhat fewer than dogs and rats that boast about 300 million such receptors.
2. Super Sniffers
Their sense of smell is not based on olfactory receptors alone, however. The horse’s large, flexible nostrils will flare to pull in as much air as possible when faced with a potentially significant scent array, useful for finding water, finding mates, identifying their offspring and alerting to approaching predators or other environmental changes. By sniffing, the horse can intensify the currents of air in the nasal passages, providing more contact between the odour molecules and the receptor cells, and more time for analysis.
3. Let it Linger
This increased volume of air flows into the large nasal cavity and over the nerve-rich mucosa of the nasal turbinates. These convoluted structures are responsible for warming, humidifying and filtering the inhaled air. This function makes odour molecules bind more easily and quickly to the olfactory receptors, which send out sensory nerve impulses to the brain.
4. Directional Smelling
The twin olfactory bulbs, distinct areas of the brain which are responsible for identifying scents, are located at the very front of the cerebrum – one on each lobe – and are connected via the main olfactory nerves to the receptors in the nasal passages. The olfactory bulbs are one of the only brain structures that do not cross over; the receptors in the left nostril are directly connected with the left olfactory bulb, and the right with the right, allowing horses to smell in “stereo.” They can quickly identify the direction from which a scent is originating.
5. Thank Jacobson
Horses, like many other animals, have a vomeronasal organ (VNO), also known as the olfactory organ or Jacobson’s organ, that helps increase smelling ability. It is positioned at the base of the nasal cavity, within the roof of the mouth, and is separated into two parts by the nasal septum. The VNOs in horses are tubular and cartilaginous and about 12 centimetres long. Despite their size, they’re so carefully concealed that anatomists prior to Ludvig Jacobson completely missed them. They’re lined with mucous membranes, they contain more sensory fibers of the olfactory nerve and they’re connected to the main nasal passages by a duct called the nasopalatine duct.
6. An Extra Nose
In some animals, the nasopalatine duct also makes a connection with the mouth, making it possible for scents to be drawn in through more than one entrance, but in horses, which aren’t mouth breathers, the VNO communicates only with the nasal passages. VNOs seem to expand and contract like a pump with stimulation from strong odours, and they have their own pathways to the brain, functioning almost as completely separate sensory organs
7. The Medium is the Message
The VNO’s receptors and sensory neurons are specific to identifying odour compounds that emanate from other animals and, most significantly, are used to detect and analyze pheromones. These sex hormones indicate an animal’s sexual status. So their VNO helps stallions identify when a mare is in heat and receptive to breeding, when she is out of season and likely to reject his advances and when there might be a rival stallion nearby. In some species, horses included, stimulation of the VNO has a profound influence on the animal’s endocrine system. Depending on the message the pheromones bear, a horse might significantly adjust its reproductive behavior, including timing its estrus cycle.
8. Not a Smile
Some mammals, particularly felids (cats) and ungulates (which includes horses), use a distinctive facial movement called the flehmen response to direct inhaled compounds to the VNO. The animal lifts its head after discovering a scent, wrinkles its nose while lifting its lips and ceases to breathe momentarily.
9. Sorry Geldings
Stallions are the most enthusiastic equine practitioners of the flehmen response. In the presence of a mare in estrus, for example, they might flehmen several times an hour. Mares also will flehmen, although not as frequently; the smell of birthing fluids on a newborn foal often triggers the response. Geldings seem to flehmen the least. In fact, it’s theorized that castration seems to compromise a male’s ability to detect and analyze pheromones, making him sexually ineffectual in more ways than one.
10. That Stinks
But while pheromones are definitely the most likely flehmen trigger, they’re not the only one. Occasionally, horses also will react with an upper lip curl when they come in contact with an unusually strange or pungent inorganic odour such as perfumes or gasoline. Some horses may react fearfully to strong, unknown odours, being unable to identify them as friend or foe.