SMELL ME SOMETHING I DON'T KNOW!
2 SCENTS ON EQUINE OLFACTION
BY KATE THORNTON
“But there’s nothing there boy” you say to your horse as his head shoots up and he looks off into the distance at goodness knows what – nothing you can see, that’s for sure! Perhaps if he could speak your language he’d say “are you crazy?! Can’t you smell that??” Olfaction is of great importance to equines, and one of the senses that they rely heavily on. This, along with the other four sensory modalities, forms a rich part of the horse’s sensory integration, affecting the emotional experience, conditioned responses, and formed memories. Studies on equine olfaction are still relatively sparse, however sensory experience and ability is an important consideration for their welfare and our understanding this information can benefit them, just as our lack of knowledge may be detrimental. Here, we review some of the available information and look at ways in which we can use it to enhance the experiences of our equines.
As horses don’t breathe through their mouths like us, every breath passes through their highly sensitive olfactory system. As air is taken in through their (sometimes expanding) nostrils, it is filtered through the nasal cavities and epithelium by mucous membrane and tiny hairs, called cilia. Pathogens detected are ‘vacuumed’ up to the pharynx and either swallowed or coughed out. Whilst in the nasal cavities, air is warmed and humidified, enabling better binding to olfactory receptors. The receptors then send sensory (afferent) nerve impulses to the olfactory bulbs, the tip of which align approximately with the ‘whorl’ on the horse’s forehead (Merkies, Paraschou and McGreevy, 2020).
Some interesting stats: whilst the olfactory ability of the equine is considered inferior when compared to dogs, it is still vastly more sensitive than ours. We have around 5-6 million olfactory receptors. That sounds a lot, but dogs have a whopping 300 million! Equines fall somewhere in between, but with 1066 intact olfactory genes compared to our 350, it is likely that they are far superior to us! The size of the epithelium is a good indicator of the quantity of olfactory receptor cells, therefore long-nosed mammals likely possess a high number, aiding their abilities to detect thousands of different odours in their environment (Padodara, 2014). In terms olfactory bulb size, horses outdo us by far
again; ours is around 11x3.5mm compared to their 33x23mm! (Draaisma, 2021). In fact, equines are amongst the privileged species which have twin olfactory bulbs – one for each nostril. This enables them to smell ‘in stereo’ – extremely beneficial when odours tell them so much about their environment. Smelling in different directions concurrently is pretty amazing. (Apfel, 2019).
You may have heard of the vomeronasal organ, or the Jacobson’s organ (named after Ludvig Levin Jacobson who studied this amazing structure in the early nineteenth century). It is present in equines, dogs and cats as well as many other animals, amphibians and reptiles. In the horse, it is approximately 12 cm long, and is situated in the upper nasal cavity (Briggs, 2013). The vomeronasal organ, or VNO, comprises millions of nerve cells. These are able to analyse low and non-volatile moisture-borne odours as they pass through, sending information via the VNO’s own neural pathway to the brain (Rørvang, Neilson and MacLean, 2020). So independent in its function, it’s almost like a second olfactory system.
The VNO, ‘specialises’ in pheromones. Each individual, including us, has their own ‘pheromone profile’, formed by bodily excretions which are then detectable by the VNO – not something we, as humans, are able to detect in the same way (although we do possess a VNO). With this, we often think instantly of sexual behaviour, and certainly the VNO is heavily utilised for gaining information about conspecifics via these sensitive chemoreceptors. It is thought, however, that it has less efficacy in the gelding – almost as a side-effect of castration, the sensitivity of the VNO appears to be lessened (Apfel, 2019). In fact, there have been contrasting studies carried out on the ability of equines to differentiate between faeces from various individuals. Whilst some results indicate that stallions have a good ability to identify males vs females, others suggest that horses aren’t able to sort between the faeces of known vs unknown conspecifics but are able to pick out their own faeces (Jezierski et al, 2018; Krueger and Flauger, 2011). It is thought, though, that olfaction is an important element in the social interaction of equines and mares in particular, are able to identify other pregnant mares or geldings via chemosignals in urine (Hothershall et al, 2010). Perhaps the findings that stallions were not particularly effective in discriminating between mares who were in oestrus and those who were not suggests that this ability may have been at least reduced during the evolutionary process, and that they rely on behavioural cues from mares, who do use their sense of smell to identify potential mates. Authors of these studies do conclude that both sexes are able to identify potential competitors within their herd (Rørvang, Neilson and MacLean, 2020).
As VNO activity is stimulated by these odour molecules, this triggers the flehmen response, which we’ve all seen in equines, whereby the animal raises their head, curls the upper lip upwards, thereby close off the nostrils with force. This captures the compounds for careful analysis. This is perhaps not always done with reproductive information in mind, as the flehmen response is sometimes triggered in the company of humans. Where we may enter a room and detect a strong odour, we seem to ‘get used’ to it, and are less able to smell it after a period of time. However, animals with highly developed and receptive VNOs can keep the smell ‘fresh’ for much longer. Our own pheromone profile together with our signature scent is useful to the horse for the purpose of identifying our presence. This is also true of conspecifics, and the outcomes of interactions between individuals likely creates conditioned emotions, aiding the horse in judging which future interactions are likely to be beneficial – an element of cognitive appraisal perhaps. This adaptive ability is something that, as humans, we must take into consideration when we are near horses. The scent profiles from all the horses we come into contact with are likely to remain on our hands, hair and clothes, so we carry them around and introduce them unwittingly to each horse we are near. Depending on how an individual feels about others, this may affect them and alter the behavioural output we see from them. Apparent refusal to ‘cooperate’ may, therefore, be related to odours which we bring into a horse’s environment. One example of how many factors we need to take into account when trying to determine causes of behaviour, and this does a good job of highlighting the importance of how our horses feel at a given moment.
Indeed, various odours within the environment can affect the emotional experience of a horse, as amongst other reasons (finding food, for example) they use their highly tuned ability to detect potential predators – part of their strong and highly adapted survival mechanism. Not only predators, however - as discussed, previous interactions with individuals (of various species) contribute to a formation of ‘smell memories’ which advise the horse
about situations in which he/she may feel a certain way. Avoiding those who have previously shown aggressive behaviour and identifying them via chemoreception – whether horse or human – is an understandable coping strategy. An example of an odour memory is that which is developed by a mare when she breathes in the scent from her newly born foal. The emotional response and memory which is triggered by this helps her to ensure future identification of her offspring amongst the herd. We know ourselves how a certain smell can make us feel – even taking us on a trip back to our childhood. The emotions which were felt at the time are again evoked, whether positive or negative. This sensory and cognitive ability is also experienced by equines, again, for clues about beneficial or detrimental situations and intercommunication (Higgins, 2012).
Or should it be ‘odour-reacting’? The way in which we, as humans, perceive equine behaviour has its flaws. In relation to the senses, if we aren’t able to detect a stimuli, but our horse is reacting to something, we really do need to believe them. It’s not their imagination! Sure, their perception of a particular stimulus may be maladaptive (believing danger where there is no likelihood of it in reality), but to them it’s very real, and very adaptive. If they weren’t able to sense so acutely, chances are they wouldn’t have evolved to the stage they have. Survival is a very powerful motivator! On detection of a particular odour, or group of odours, there are many different scenarios available to the horse;
• This smell conjures up memories and conditions emotions of…….. (could be positive, negative or
• This is a new smell, approach with caution….. not sure, avoid…… seems interesting and safe, approach
for closer examination
• I know this smell means danger, avoid (this could be real or perceived)
• I enjoy this smell
• I find this smell repulsive
• This smells toxic
• This smells edible
So how can we work with them on this?...
• Refer to the available resources (some listed at the bottom of the page) for further information about the
horse’s sensory abilities
• Spend time observing equines, from a distance and up close. Watch how they approach or avoid
different stimuli, and social interactions. What does their body language say when they encounter
something? How long do they spend analysing something in their environment? Then crucially, allow
them agency to respond as they wish. Gaining an insight into how they feel about different things enables
us to better understand their responses in future and give them the time and space they need in order to
be comfortable with things they’re unsure about, and enjoy the things they love
• Learn which odours they are attracted to the most – each equine is an individual and will have their own
preference. Experiment with natural odours – forage, the pheromones of other herd members, of
yourself and yard mates. Certain diluted essential oils may be appealing (resource at the end of the
article – please take steps to ensure each is safe for horses and use quality sources). Odours which your
enjoys being around may be useful in situations when he/she is feeling uncomfortable or anxious.
• In addition to engineering olfactory stimuli, take your equine for a stroll and let him/her take in whichever
odours they choose, for as long as they choose to. Take mental notes!
• If your equine does suddenly look at something, or wants to turn their head to get a view, never prohibit
this, and wherever possible, give them time to digest and assess. Pushing them on in the presence of a
worrying odour may cause heightened anxieties and future issues when presented with the smell in the
• Never punish an equine (any animal) for their response to something (verbally or physically). They are
highly adapted creatures who respond according to their individual and species-specific needs. All
behaviour has a reason behind it which is very real to the animal, and is never with a cunning plan to
annoy a human.
• Patience and compassion are always a great start
Another extremely beneficial way in which we can use olfaction and scentwork with equines is through re-socialisation programmes, which take smell into account. This can be done by building enriched environments or doing scentwork exercises as described in “Scentwork For Horses”, the unmissable book by Rachael Draaisma
A buzz-word right now, and rightly so. The horse world, along with the dog world, is getting to grips with learning how to improve the quality of experiences for our animals. Enrichment doesn’t always mean food games though. Far from it in fact. It encompasses a variety of considerations;
• Safety and security
• Good health (mental and physical)hygiene
• Opportunities to soak up information via the various sensory modalities
• Providing outlets for natural behaviours – species and breed considerations as well as preferences of each
• Appropriate nutrition for the individual
• Social opportunity where welcomed
The list is probably not exhaustive, but taking each of these and really dissecting what they mean for each individual animal we care for or spend time around can really improve their rich emotional lives as well as their physical wellbeing.
A name that many of you will be very familiar with is Rachaël Draaisma - a pioneer in the equine sensory and communication field (as I said, her books are not to be missed). If you Google ‘scent games’ or ‘scent work with horses’ Rachaël’s name will feature almost exclusively on at least the first page! Rachaël made a transition from working with dogs to horses a number of years ago, following her time studying with the wonderful Turid Rugaas, and from there she was captured by the wide array of communicatory behaviours of equines, and how they experience and navigate their world. Since then, she has enriched our own lives by publishing two books; ‘Language Signs and Calming Signals of Horses: Recognition and Application’ and more recently ‘Scentwork for Horses’. These are both highly recommended reads and links to buy them are at the bottom of the page.
Happily, there are now more resources available which means that more and more people are taking the time to explore the equine’s sensory experiences more. In an article for Pet Dog Trainers of Europe (PDTE), Rachaël gave an insight into some comparisons of exploring olfaction and scent work between horses and dogs – a really interesting read! One snippet from the article highlights the discovery in Rachaël’s early work, that odours that are strong (even if they don’t seem so to our inferior sense of smell!) are often too overwhelming for a horse to want to explore (repeatedly), whereas smells which we would consider extremely weak, such as footsteps, proved to stay extremely workable to the equines she worked with and did scent tracking with. This is something important to bear in mind when we introduce new smells into our horses’ environments, whether we are intending to do scentwork or not – inadvertently producing odours may be aversive, so it’s worth considering if a horse avoids interaction (or other unusual behaviour for the individual).
Obviously, there are some differences between dogs and horses, however, there are many similarities. One very important one being the importance of olfaction to each animal, and how it combines with the other senses, context and environment to contribute to emotional experience, behavioural output and future perceptions. Providing the time for an equine or a dog to sniff what they want to, follow their nose literally, and process the information they retrieve contributes to the ability to not only appraise independently, but also to develop coping strategies and make decisions. Additionally, it can contribute to healthy and enjoyable relationships between us and our equines, and provide them with important mental stimulation. If you are interested in learning more about Rachaël’s work and clinics, visit www.calmingsignalsofhorses.com
Essential Oil Use with Horses
Please ensure that any oils you use are safe for horses. Whilst their odour will likely be strong to the equines you’re working with, if they are used diluted, they can be attractive. Indeed, different individuals may seek out particular oil odours – perhaps they feel a need? I highly recommend working with a certified zoopharmacognosist if you intend to explore essential oils.
Horse Talk has a useful article HERE
Links to Rachaël's Books;
Apfel, Karin (2019) Ten Amazing Facts About Equine Smell. [online] Horse Canada. Available from; https://horse-canada.com/magazine/miscellaneous/10-amazing-facts-equine-smell/ [Accessed 5 February, 2022]
Briggs, Karen (2013) Equine Sense of Smell. [online] The Horse Media Group LLC. Available from: https://thehorse.com/13971/equine-sense-of-smell/
Draaisma, R (2021) From Dogs to Horses – Mental Stimulation and Scentwork for Horses. Pet Dog Trainers of Europe.
Higgins, G (2012) Horse Anatomy for Performance. David and Charles, Exeter
Hothersall B, Harris P, Sörtoft L, Nicol CJ. Discrimination between conspecific odour samples in the horse (Equus caballus). Appl Anim Behav Sci. (2010) 126:37–44. doi: 10.1016/j.applanim.2010.05.002
Jezierski T, Jaworski Z, Sobczyńska M, Ensminger J, Górecka-Bruzda A. Do olfactory behaviour and marking responses of Konik polski stallions to faeces from conspecifics of either sex differ? Behav Processes. (2018) 155:38–42. doi: 10.1016/j.beproc.2017.09.015
Krueger K, Flauger B. Olfactory recognition of individual competitors by means of faeces in horse (Equus caballus). Anim Cogn. (2011) 14:245–57. doi: 10.1007/s10071-010-0358-1
Merkies K, Paraschou G, McGreevy PD. Morphometric characteristics of the skull in horses and donkeys—a pilot study. Animals. (2020) 10:1002. doi: 10.3390/ani10061002
Padodara, Ramesh. (2014). Olfactory Sense in Different Animals. The Indian Journal of Veterinary Science. 2. 1-14.
Rørvang M.V, Neilson B.L and MacLean A.N (2020) Sensory Abilities of Horses and Their Importance for Equitation Science. [online] Frontiers Media S.A. Available from; https://www.frontiersin.org/articles/10.3389/fvets.2020.00633/full#B86 [Accessed 5 February, 2022]
(From Top to Bottom); StockSnap, Alexas_Fotos, Rencie, Rich Dahlgren, Rachaël Draaisma)
“Oh come on, you’re over-reacting….”
BY KATE THORNTON
Kate is an Equine Massage and Manual Therapist, covering Norfolk and surrounding areas. Her training as encompassed many aspects of equine physiology and psychology, and she has a keen interest in behaviour and affective states in addition to the physiological health of horses.