Smell is one of the horses’ key senses and horses have a particularly well-developed sense of smell.
In scientific terms, “olfaction” is the sensation of smell that results from the detection of odorous substances aerosolized in the environment.
Smell is important to horses for a number of reasons. In the wild, horses are able to smell predators from a long way away. Sometimes, before they could see or hear them, so it’s an important early warning system. Although, of course, only if they are downwind of the predators! Horses also use smell to communicate with other horses through scent within sweat, faeces and urine. This can communicate things such as identity, reproductive status in mares and marking territory. Smell is a key part of the bonding process between mares and foals, and within a herd, a mare is able to find and identify her foal by using smell. Horses also use smell in the same way that other animals, such as dogs, which also have a very highly developed sense of smell (and both are much better than humans). Horses and dogs are often observed sniffing each other when they meet, and this can convey information, for example, about hierarchy. Horses, of course, also use their sense of smell to find and select food and water whilst at the same time avoiding poisonous plants or foul water.
When presented with new and unfamiliar smells, for example, from new stable equipment or when in new environments or unfamiliar horses, allowing horses time to smell these can help reduce anxiety.
Horses have essentially two areas for detecting smell. The nasal passages in the head are large, and the surface is lined with millions of cells capable of interacting with molecules with a “smell”. There are also millions of olfactory nerves, and these go to the olfactory bulb at the front of the brain. The horse also has another area for sensing smell in the lower frontal area of the nasal cavity known as the Vomeronasal organ or Jacobson’s organ. This is referred to as an “auxiliary” sense organ and can detect pheromones; chemical signals that animals secrete in sweat and urine to communicate with each other. Interestingly, whilst humans secrete pheromones in sweat, urine, semen, vaginal secretions, breast milk, saliva and breath humans have no Vomeronasal organ so there has been debate amongst scientists as to their importance.
Substances in the air that stimulate the vomeronasal organ trigger the horse’s Flehmen response; this is where the horse curls its upper lip back and inhales. As part of this response, the horse closes its nostrils so that the air is effectively trapped, which increases the sensitivity of detection by the vomeronasal organ. Horses are not the only animals that show the Flehmen response; cats, deer, sheep, goats, cattle, elephants, some monkeys and dogs can also show it.
The horses’ sense of smell is much more developed than ours but less developed than dogs. Horses have been estimated to have 25-30 million olfactory receptor neurons (nerves which send sensory information about smell directly to the brain), whereas we only have around 5 million, but dogs have an incredible 300 million! Horses are also able to detect subtle odours that we can’t.
Perhaps surprisingly, given how important the sense of smell is to horses, there have not been many scientific studies. But here is a summary of what has been investigated.
- Stallions did not differ in their response to mares if they were or were not in heat
- Horses can identify different individuals by smell
- Horses can identify their own faeces by their smell
- Horses in the wild show more interest in the faeces of horses that are aggressive towards them
- Each horse has its own individual “smell”
- Different breeds of horses have different smell profiles
- Mares can tell the difference between the smell of urine from pregnant mares and geldings
- Pheromone sprays and gels for calming have been found to work in some studies but not in others
- Horses can show “vigilance” behaviour when exposed to unknown smells or the odour of a predator
- Horses may be able to tell the difference between calm and anxious people from smell alone
- Stress may modulate the sense of smell; acute stress may increase the ability to use smell whilst chronic stress may cause a decrease in olfactory perception
- Horses spend longer sniffing novel odours than familiar odours
Recent research on equine smell
A recent study published in 2022 investigated the behaviour and response to smell in Icelandic horses. The authors found that horses’ odour exploration behaviour and interest in odours “varies with age and pregnancy” and “that horses naïve to the taste of a substrate may be able to link smell with taste, which has not been described before.”
Of course, smells produced by horses are not only used by other horses! Many flies use smell to locate horses. One study found that horseflies were more strongly attracted to aged horses’ urine as opposed to fresh urine, a good reason to keep your stables free from any urine smell. In contrast, a recent study on intestinal parasites in free-ranging Przewalski’s horses found they were more strongly attracted to fresh faeces.
A recent review also suggested that olfactory stimulation could be used in domesticated horses as a form of environmental enrichment, stimulating the brain by manipulating the environment.
Finally, a paper published only a few weeks ago (at the time of writing May 2024) found that when bowls of oats were placed on plates covered with gauze soaked in anise (aniseed), horses consumed twice as much as the corn-oil control. This is particularly interesting as the oats themselves were not flavoured with anise, only the environment around the bowl.
Summary
- Horses have a highly developed sense of smell; far superior to humans but not as good as a dogs
- Smell is important for social reasons, for avoiding predators and for selecting food and water
- Smell and taste likely work together, as in humans
- New and unusual smells can cause stress unless horses are allowed time to investigate them
- Each horse has its own individual smell and can recognise the smell of other horses
- Smell in the environment may influence food intake
- Using odours in the horses environment may be a novel method of environmental enrichment
References
- Baldacchino F, Manon S, Puech L, Buatois B, Dormont L, Jay-Robert P. Olfactory and behavioural responses of tabanid horseflies to octenol, phenols and aged horse urine. Med Vet Entomol. 2014 Jun;28(2):201-9. doi: 10.1111/mve.12038. Epub 2013 Nov 5. PMID: 24188243.
- Bini de Lima AC, Sebastião da Fé VC, Palermo Hernandes MS, Oliveira Dos Santos VM. Olfactory Stimulation as Environmental Enrichment for Domestic Horses-A Review. Animals (Basel). 2023 Oct 12;13(20):3180. doi: 10.3390/ani13203180. PMID: 37893904; PMCID: PMC10603683.
- Bombail V. The Role of Olfaction in Relation to Stress and Fear. In: Lindstrøm Nielsen B., editor. Olfaction in Animal Behaviour and Welfare. CABI Publishing; Wallingford, UK: 2017. pp. 136–150.
- Fureix C., Jego P., Henry S., Lansade L., Hausberger M. Towards an Ethological Animal Model of Depression? A Study on Horses. PLoS ONE. 2012;7:e39280. doi: 10.1371/journal.pone.0039280.
- Jardat P, Destrez A, Damon F, Menard-Peroy Z, Parias C, Barrière P, Keller M, Calandreau L, Lansade L. Horses discriminate human body odors between fear and joy contexts in a habituation-discrimination protocol. Sci Rep. 2023 Feb 25;13(1):3285. doi: 10.1038/s41598-023-30119-8. PMID: 36841856; PMCID: PMC9968287.
- Perry EB, Handlos GC, Fenton JM. Impacts of Olfactory Cues on Equine Feeding Behavior. J Equine Vet Sci. 2024 May 13:105096. doi: 10.1016/j.jevs.2024.105096. Epub ahead of print. PMID: 38750945.
- Rørvang MV, Nicova K, Yngvesson J. Horse odor exploration behavior is influenced by pregnancy and age. Front Behav Neurosci. 2022 Jul 28;16:941517. doi: 10.3389/fnbeh.2022.941517. PMID: 35967896; PMCID: PMC9366077.
- Rørvang MV, Nielsen BL, McLean AN. Sensory Abilities of Horses and Their Importance for Equitation Science. Front Vet Sci. 2020 Sep 9;7:633. doi: 10.3389/fvets.2020.00633. PMID: 33033724; PMCID: PMC7509108.
- Zhang K, Zhou R, Huang H, Ma W, Qi Y, Li B, Zhang D, Li K, Chu H. Host feces, olfactory beacon guiding aggregation of intestinal parasites Gasterophilus pecorum (Diptera: Gasterophilidae). Parasitol Res. 2022 Sep;121(9):2601-2613. doi: 10.1007/s00436-022-07577-6. Epub 2022 Jul 5. PMID: 35788769.
