Olfaction (smell) is important for horse’s survival. Smell helps horse select what they eat (horses are sensitive to poisonous plants, moldy forage and grain, dirty water, etc.). Horses use smell to detect other horses by smelling feces, urine, and body odors. Smell is very important in detecting the sex and stage of estrous in mares.
Horses only breathe through their noses, or they can’t breathe through their mouths. The term for this is an obligate nose breather. Horses have a large nasal cavity with structures called turbinate bones. Within the turbinate’s, inhaled air is mixed, warmed and distributes scents. Olfactory receptors are positioned towards the top of the nasal cavity. Olfactory epithelium lines the inside of the upper nasal cavity and connects olfactory neurons held in the turbinate’s to olfactory bulbs in the horse’s brain. The olfactory bulbs are relatively large in size and include numerous folds that increase the surface area over the receptor cells. Horses are thought to have about 300 million olfactory receptors, which is considerably higher than humans (five or six million olfactory receptors).
|The horse's nasal cavity|
Horses have an accessory olfactory system known as the vomeronasal organ that detects pheromones and volatile odors. When a horse breathes in strong odors, the vomeronasal organ expands, contracts, and sends the aroma to the brain. In response, the horse will display the flehmen response where they extend their neck, raise their nose, open their mouth slightly and curl the upper lip. The flehmen response is commonly demonstrated by stallions, but mares and geldings may also demonstrate the response. The flehmen behavior can appear as early as the first day of life in foals.
|A horse displaying the flehmen response|
Remember, odors can be used to smell predators, stage of estrous cycle, and social recognition of other horses and even people. It is beneficial to allow the horse to smell your hand, tack, or whatever is causing them to be anxious when working around a horse.
- Beaver, B. 2020. Equine Behavioral Medicine. Academic Press.
- Rorvang, M V., B L Nielsen, and A N McLean. Sensory Abilities of Horses and Their Importance for Equitation Science. Front. Vet. Sci 2020 7:633