“Let Them Sniff” Is Not a Suggestion, but a Welfare Requirement
How dog olfaction works, how central it is to their experience of the world, and what the science says of olfactory enrichment.
Of the many senses animals can have (the most commonly known being hearing, smell, taste, sight, touch and gay), it is hypothesized that smell was probably the first one to evolve [1]. After all, being able to sense chemicals in their surroundings was probably very advantageous to the squish-creatures that early animals were, while also not all that different from the already-developed chemosensory capabilities of their single-celled ancestors.
Millions of years of diversification later, different species have now developed this ability to very different degrees, with some, like dogs, achieving an impressive level of sophistication to their olfaction that continues to captivate people. Despite this, we’ve tended to be more focused on asking how we can capitalize on the dog’s nose than on what the nose means for the dog.
The Science of Olfaction
Despite the obvious differences in our capacity to detect smells, the actual mechanism for doing it is more similar in humans and dogs than you might realize. Most of the basic structures are the same, after all: just developed a little bit differently. The short story is that we both detect smells because cells in our noses can interact with the “smelly” molecules in the air and send signals to the brain -though what is smelly varies for different species, we’ll get into that later.
Put in a more scientific language you can use to annoy everyone in the room: the olfactory epithelium -the tissue responsible for detecting smells- is covered with olfactory receptor neurons, which can interact with many different molecules [1]. The part of each neuron exposed to the air in the nasal cavity is covered in cillia (little “hairs” that increase the surface available) and filled with receptors -the proteins actually responsible for binding to smelly molecules [1, 2]. Each neuron produces only one type of receptor of the many encoded in each species’ genome [2], but that doesn’t mean each neuron reacts only to a single molecule: a receptor can bind more strongly or weakly to different molecules, and what we call “a smell” can very often be a mix of many smelly compounds. There are, after all, many more smells we can detect than there are types of receptors [1].
The process of turning this mess of signals from the neurons in the olfactory epithelium into “a smell” -that is, into a sensory experience- occurs in the brain, specifically in the olfactory bulb, which communicates intimately with other areas of the brain such as the hippocampus and the amygdala [1]. It is common knowledge that smells are connected to memory and emotions, and it seems that dogs can have similar relationships to them, with scents not only being involved in memory retrieval, but also odour preferences developing early on their lives based on what they’re exposed to [1] -a bit like the dog equivalent of smells seeming homey. Learning experiences thus can make different, previously neutral scents either pleasant or aversive.
There are, of course, some differences between the nose of a human and that of a dog -despite what certain latex-enthusiasts at Pride would have you believe. For starters, dogs and many other mammals have an extra feature, the vomeronasal organ, situated over the roof of their mouths and whose main function is to encourage dogs to lick pee in order to make people uncomfortable (and also do some other stuff, like detect pheromones [2]). If you’ve ever seen a dog do just that and get a trembling jaw, that’s the flehmen response, the vomeronasal organ in action. Congratulations. Or I’m sorry.
There’s also the differences in surface available for olfaction inside the nose, relative size of the olfactory bulb, and number of different receptors encoded in the genome, all of which are higher in dogs than humans [1]. I do have a bit of a bone to pick with articles that, to talk about dogs’ incredible sense of smell, disparage the humans’. We are not so terrible at sniffing as some people would have you believe! And smell is very important to us: the scent of our food, of each other, of the spaces we live in, etc. Have you noticed the amount of advertising done just about scents? The New Fragrance industrial complex does not exist only for famous actors struggling their way through paying for a fourth home, it’s also there because it works, to some extent. People do wanna smell like a generic Hollywood-handsome man getting into the sea with a tuxedo on. That being said, dogs do experience the world through their noses to an extent that is hard to imagine for us.
Interestingly, many of these differences do not only appear in dogs and humans: different breeds of dog can also have different surface available for olfactory receptor neurons and different variants of receptors themselves [1]. It sounds, therefore, reasonable, to assume that the nose of a German Shepherd, with several times as much surface available for receptors than that of a Pug’s, is all that better at sniffing. This assumption proved to be at least a little flawed when some differences were found in some tests [3], but not all of them1. Sometimes Pugs even outperformed the Shepherds in scent detection tasks [4].
This doesn’t mean that there are no differences in how different breeds experience scents, but it certainly points towards us having to check some of our assumptions and never dismiss the importance of sniffing for any dog, brachycephalic or otherwise.
Olfaction can also change with age, health status, nose microbiome, possibly with hormones [1]… though how this impacts the lives of dogs is not clear. What we do know is that sniffing is a central part of their welfare.
What We Know about Olfaction and Welfare
Dogs live in a world of scent. It is the main way they experience the world and each other. They are innately motivated to sniff, and devote a huge portion of their time and body resources to it. It’s no surprise that sniffing is intimately tied to well-being for such a nose-y species.
Despite that, one of the many things a lot of domestic dogs typically aren’t allowed a say in is precisely olfaction. We have a bit of a conflict of interest with them, as a species that largely dislikes rolling in rotting animal carcasses and finding excrement to eat. We want dogs that smell as little as possible and, often, that don’t sniff much either. Dogs living inside the home are mostly unable to decide when to go out, and, even when out, people are often encouraged to walk on a straight line from point A to point B and back, completely undermining the dog’s ability to explore through their nose.
There’s even dog trainers who propose “structured walks” -ones where the dog stays mostly in a heel position and otherwise obeys their owner’s commands- as the main form of exercise for them. They claim this to be both a preventative and solution to several behaviours people consider problems -with, of course, no evidence to back it up.
This is a serious attack on their welfare. Sniffing is not just a natural dog behaviour, but possibly one of the most important ones in their repertoire [5]. As such, denying or seriously restricting opportunities to freely sniff violates one of the Five Freedoms2 -which formulate but the most basic welfare requirements-, the freedom to express normal behaviour. A dog who isn’t allowed to sniff is a dog whose agency is seriously compromised, or non-existent.
Furthermore, any attempt at making a dog “better behaved” that heavily restricts when dogs are allowed to sniff is pretty much shooting itself in the foot. Dogs use smells to gather information to feel secure in the world. Some scents seem to be inherently soothing, and others can become positive through learning [1]: scents can become familiar to dogs through exposure. Since smell is heavily associated with memory retrieval, this explains how the presence of familiar odours associated with positive situations could to help calm dogs down, whereas the opposite can be said of scents associated with stress. More than likely, allowing a dog to explore any environment they’re in through their nose is fundamental for them to feel secure in it. There is, it would seem, something to that old advice of leaving dogs used clothes when they’re to be in an unfamiliar environment [8].
The extent to which sniffing can improve welfare was further proved by a study by Duranton and Horowitz (2019) which used a cognitive bias test in dogs divided into two groups: one practicing heel-work, a common activity in obedience classes3, and the other engaging in a scent-focused foraging activity. Cognitive bias tests are used in several species to measure their attitudes when approaching neutral or ambiguous stimuli, which researchers often characterize as a measure of their “optimism” -a more “optimistic” animal will approach ambiguous stimuli more readily. The study found dogs became more optimistic compared to their baseline only after the sniffing activity, not the obedience one [9]. This is particularly interesting because both were trained using only positive reinforcement, which we often associate with creating, precisely, optimistic learners.
To be sure, it’s not that the study found any welfare detriment to the obedience task4, simply that those dogs’ attitude towards the cognitive bias test did not significantly change the way it did in dogs that spent time in foraging activities. Because the dogs had time to learn how the test worked before being assigned to either group, it is not very likely that the scent-work dogs were better prepared to look for the food in the test, so it is reasonable to assume the differences in performance do reflect a change in the dogs’ emotional state.
This, to me, is a hallmark study that should inform what we prioritize in our time with dogs. There are so many sports, games and simple ways to pass the time together, and, as long as the dog is not being coerced and their health and safety are considered, it’s hard to go wrong. But they’re not all equal. Activities that center the dog’s way of seeing the world seem to have a higher potential at maximizing their well-being, which in a way shouldn’t be surprising considering they are, probably, the most enriching.
When We Talk Sniffing We Talk Enrichment (Mostly)
For dogs, sniffing and enrichment are inextricably linked: when providing dogs opportunities to engage in natural behaviour -that is, when enriching their environments- sniffing is almost certainly going to be part of how they choose to behave. In turn, providing opportunities to sniff is a requirement for any enrichment plan of a species whose world revolves around the nose. That’s why conversations about the common “let them sniff” slogan are never only about sniffing in and of itself, or the physiological processes associated to it, but about the agency of the animals.
Some researchers do mix up enrichment and sniffing completely, though, mostly out of a misunderstanding of what enrichment actually is -in much the same way the general public tends to misunderstand it. The name “enrichment” seems to suggest to some that “adding” anything to an environment to make it less barren is inherently enriching. As such, there are several studies that have basically sprayed different odours in shelter dogs’ kennels and called it enrichment [10, 11, 12]. That’s not just a misuse of the term, but one that is dangerous for welfare: there is no enrichment without an increase of agency [5]. These studies might sometimes find benefits to using this or that essential oil in kennels, but we still mustn’t call them enrichment studies, as they aren’t providing any sort of choice for the animals as to when or how they interact with the new stimulus. The information they provide is only whether dogs tend to react positively, negatively or neutrally to the presence of certain scents.
By contrast, an enrichment study will give us information about what dogs tend to choose to do when the opportunity to sniff is presented -but not forced upon them-, as well as the subsequent behavioural and health changes that might take place thanks to having the choice. Examples of experimental setups that have been used to study olfactory enrichment in dogs include providing several toys with different scents and observing their behavior [13] or comparing the sniffing behaviour of dogs with varying degrees of freedom to move around [14].
See the difference there? One type of study asks what happens when dogs are exposed to scents, whereas the other is about dogs engaging with them on their own terms. This is why claims that dogs benefit from sniffing this or that essential oil must be taken with a grain of salt, as they often come from the first kind of study, not the second. It’s not that exposure to certain smells can’t be beneficial for dogs, far from it, just that some studies ignore the part choice plays in welfare -sometimes for practical reasons, but that doesn’t mean we should do the same. After all, nothing stops you from doing a little experiment of your own and simply giving your dog choices, noting their responses.
Conclusion: What to Do With All of This
There are two important lessons to be learned here: one, to always be curious about how differently dogs experience the world compared to us, and two, to make agency a priority in domestic dog welfare.
Asking questions about dogs’ interiority can give us surprising and insightful answers, as proved by the study by Duranton and Horowitz (2019). We might think any and all activities trained with positive reinforcement where dogs spend time with their people are equally beneficial for welfare, but it seems dogs would beg to differ. That doesn’t mean we can’t do things like trick training or obedience. After all, I do believe activities that people find fun and enjoyable are important as well, and dogs can have fun doing things we like to do. Still, being aware of where the dog’s priorities lie can help us compromise and make sure they’re not denied opportunities to do what they enjoy the most.
At the same time, curiosity about the dog’s nose shouldn’t be made into prescriptive approaches that deny the animal’s agency, nor to unjustified panic. Studies showing some benefit to this or that scent (usually lavender or Dog-Appeasing Pheromone [10, 11, 12]) do not justify spraying every item we own with it in the name of helping dogs “relax”. Dogs deserve choice over their surroundings, and that includes their olfactory environment. If we’re introducing any kind of aromatherapy, or if we simply want to enrich a dog’s environment, we must prioritize the dog’s agency. This is easier than it might sound and it can give you information about the dog’s preferences -something that just wildly spraying odours over an environment cannot do. It’s as simple as providing multiple choices of scented toys, beds, (safe) plants, etc, and observing their reactions. Beware, as well, of anyone trying to sell you magic products they claim dogs will innately prefer. It’s not that they will necessarily be harmful, but there’s no reason to assume a dog will like this or that scent when we can just ask them.
While aromatherapy is worth exploring in many cases, remember that you don’t really need to get fancy to enrich your dog’s environment through smell. You know which scents dogs tend to love? Mulch. Other dogs’ pee. The scent left behind by other animals. Letting dogs explore on walks, preferably in quiet areas if possible, is exponentially more enriching for them than the limited odours we can artificially add to their environments. The latter should be just a nice plus.
Finally: never listen to men doing frontal facing videos on tiktok where they show the thin thread holding their whole masculinity together is controlling every breath another living being takes. In this case, literally.
Sources
1 - Lazarowski, L. (2023). Olfactory Research in Dogs. Springer.
2 - Kokocińska-Kusiak, A., Woszczyło, M., Zybala, M., Maciocha, J., Barłowska, K., & Dzięcioł, M. (2021). Canine olfaction: physiology, behavior, and possibilities for practical applications. Animals, 11(8), 2463.
3 - Polgar, Z., Kinnunen, M., Újváry, D., Miklósi, Á., & Gácsi, M. (2016). A test of canine olfactory capacity: comparing various dog breeds and wolves in a natural detection task. PloS one, 11(5), e0154087.
4 - Hall, N. J., Glenn, K., Smith, D. W., & Wynne, C. D. (2015). Performance of Pugs, German Shepherds, and Greyhounds (Canis lupus familiaris) on an odor-discrimination task. Journal of Comparative Psychology, 129(3), 237.
5 - Young, R. J. (2013). Environmental enrichment for captive animals. John Wiley & Sons.
6 - Farm Animal Welfare Council (FAWC). Second Report on Priorities for Research and Development in Farm Animal Welfare; DEFRA: London, UK, 1993
7 - Webster, J. (2016). Animal welfare: Freedoms, dominions and “a life worth living”. Animals, 6(6), 35.
8 - Shin, Y. J., & Shin, N. S. (2016). Evaluation of effects of olfactory and auditory stimulation on separation anxiety by salivary cortisol measurement in dogs. Journal of veterinary science, 17(2), 153-158.
9 - Duranton, C., & Horowitz, A. (2019). Let me sniff! Nosework induces positive judgment bias in pet dogs. Applied Animal Behaviour Science, 211, 61-66.
10 - Amaya, V., Paterson, M. B., Descovich, K., & Phillips, C. J. (2020). Effects of olfactory and auditory enrichment on heart rate variability in shelter dogs. Animals, 10(8), 1385.
11 - Amaya, V., Paterson, M. B., & Phillips, C. J. (2020). Effects of olfactory and auditory enrichment on the behaviour of shelter dogs. Animals, 10(4), 581.
12 - Pattillo, M. J., Mitchell, L. N., Catchpole, J. A., & Martin, A. L. (2021). The Effects of Olfactory Enrichment on Shelter Dog Behavior. The Kennesaw Journal of Undergraduate Research, 8(1), 1.
13 - Murtagh, K., Farnworth, M. J., & Brilot, B. O. (2020). The scent of enrichment: Exploring the effect of odour and biological salience on behaviour during enrichment of kennelled dogs. Applied Animal Behaviour Science, 223, 104917.
14 - Budzinski, C., & Budzinski, A. (2019). Pulse Study “At the Heart of the Walk.” DogFieldStudy. https://www.dogfieldstudy.com/en/study/pulse-study-at-the-heart-of-the-walk
An important feature of studies measuring the olfactory capabilities dogs is that they must be motivated to find whatever it is the study is testing. This means we’re not just measuring olfaction, but motivation to smell as well.
As per the Farm Animal Welfare Council [6]:
Freedom from thirst, hunger and malnutrition, by ready access to a diet to maintain full health and vigour
Freedom from thermal and physical discomfort, by providing a suitable environment including shelter and a comfortable resting area
Freedom from pain, injury and disease, by prevention or rapid diagnosis and treatment
Freedom from fear and distress, by providing sufficient space, proper facilities and the company of the animal’s own kind
Freedom to express normal behaviour, by ensuring conditions which avoid mental suffering
This framework emerged as a response to the poor conditions animals are kept in because of industrial farming, and it’s meant to conceptualize the bare minimum requirements for animal welfare [7]. It’s in some ways a flawed system, but still to this day the legal basis for a lot of animal welfare legislation, and thus, in my opinion, worth knowing like the back of your hand.
And, according to some trainers, the solution to about 95% of behaviour issues (the other 5% can be solved through sheer masculinity).
That is, when trained without punishment. Obedience classes that do rely on aversives and negative punishment are a different can of worms.