Your Dog Is Reading the World Through Smell

You have walked past this lamppost hundreds of times. There is nothing new about it, nothing particularly interesting to look at and no obvious reason to stand beside it for another thirty seconds. Your dog disagrees.

Their nose moves across the pavement, up the base of the post and back towards one particular patch of grass. They pause, return to something they nearly missed, then add a small contribution of their own. From the other end of the lead, it can feel as though the walk has stalled. From the dog’s perspective, this may be one of the most informative parts of it.

We tend to understand a walk through distance, destination and time. A dog encounters a route that has already changed since the last visit. Another animal has crossed the path, rain has lifted smells from the ground and the wind has carried others elsewhere. Even the most familiar park presents a slightly different set of clues each time.

A nose built to investigate

Estimates of the number of scent receptors in a dog’s nose vary considerably, partly because breeds differ so much. Published figures commonly range from around 125 million to 300 million, with the highest estimate usually associated with Bloodhounds. Humans have around five or six million. The comparison is imperfect, but it gives some sense of how heavily canine perception is weighted towards smell.

Inside the nose, folded structures create a large area of tissue on which odour molecules can be detected. A considerable part of the canine brain is devoted to processing the signals that follow. Sniffing draws scent-carrying air across this tissue, and the pattern changes as a dog tries to locate or examine a smell.

This is one reason scent can be so closely connected to recognition, memory and emotional responses. It helps dogs find food, recognise individuals, detect potential danger, choose mates and navigate social encounters. We may think of a dog’s nose as an impressive detection device because humans have trained dogs to locate explosives, missing people and disease samples. Those remarkable working abilities grow from a sensory system dogs use throughout ordinary daily life, deciding where to go, what to avoid and what deserves another sniff.

Who has been here?

Urine is only one of the ways dogs leave scent behind. Faeces, glandular secretions, body odour and traces from their paws can all contribute. Other dogs pause over what remains and may place a mark beside it or directly over the top.

Research into exactly what domestic dogs obtain from scent marks is still developing. There is evidence that odour can convey individual identity and reproductive information, and that dogs respond differently according to characteristics such as the sex or reproductive status of the animal that left it. Experiments have also found that dogs recognise scent collected from the feet of other dogs and spend different amounts of time investigating urine from neutered and unneutered animals.

This does not mean every sniff gives a dog a complete profile of the animal responsible. A scent mark is not a written sentence with a fixed translation. Its meaning may depend on the dog investigating it, their previous experience and all the other information present at the same time. Even so, the sniffing is purposeful. Something has been left there, and the dog is trying to understand it.

The information can shape what happens next. A dog may move on, investigate for longer, follow the scent away from the spot or leave a mark of their own. What looks like a lengthy fascination with somebody else’s wee is part of a communication system happening largely beyond us.

Can dogs really smell the past?

In a limited sense, yes. Odour remains after its source has gone, which allows dogs to follow trails, recognise people who are no longer present and find traces that humans cannot detect. Trained dogs have located remarkably old residual scents under controlled and operational conditions.

The poetic version is that your dog can smell who stood there last week. Real life is less exact. Once left behind, an odour does not remain fixed. Its compounds disperse while weather, temperature, the surface beneath it and the movement of people and animals continue to alter what is there. Dogs can work with old scent and may distinguish a newer trail from an older one, but that does not mean a pet dog can attach a precise time to everything they find around the neighbourhood.

What remains along a route is therefore a mixture: a fresh mark here, a fading trace beneath it, something washed from the pavement and another scent held in damp soil. Wind redistributes odour, warmth causes some compounds to disappear more quickly, and moisture can make others easier to detect. The scent picture is continually being rearranged.

No wonder the same corner deserves another inspection the following morning. The houses and trees are where they were, but the visitors and traces have changed.

They know the smell of you

Recognition happens through scent too. A person’s characteristic odour is shaped by genetics, skin secretions, microorganisms, diet, health and whatever they have encountered that day. Clothes and surroundings add further smells, yet dogs can still distinguish the familiar person within the mixture.

In one brain imaging study, awake dogs were presented with scents from familiar and unfamiliar humans and dogs. In the scanner, the scent of a familiar person prompted the strongest activity in the caudate, a brain region involved in reward and positive expectation. The person who supplied the sample was elsewhere during the test. The finding does not translate neatly into a word such as love, but it shows that familiar human scent held particular significance for the dogs.

This ability extends beyond recognising identity. Human stress changes the volatile compounds present in breath and sweat, and dogs have successfully distinguished samples collected from people before and after a stressful task. A 2024 study then asked whether an unfamiliar person’s stress odour could affect a dog’s behaviour. Eighteen dogs completed a test in which they decided whether to approach bowls placed in ambiguous positions. When exposed to stress odour, they became less likely to approach one of those positions, suggesting a more cautious response.

It was a small study, the effect appeared at one of three ambiguous locations, and the order of testing also influenced the results. It nevertheless offers an intriguing glimpse of information passing between species without a word, gesture or facial expression. A dog may notice a chemical change in us before we have outwardly shown how we feel.

On any given day, a dog is also responding to pain or illness, sleep, their surroundings, previous experiences and whatever has just happened. Smelling stress in a human does not make that person responsible for everything the dog does.

Can dogs smell changes in our health?

Illness can change the mixture of volatile compounds released through breath, sweat, urine and other bodily materials. Researchers have taught detection dogs to identify samples from people with certain cancers or infections, while other dogs have been trained to alert to physiological changes such as low blood sugar. Some studies have produced striking results, although accuracy varies and many experiments have been small or difficult to compare.

Medical detection requires careful selection, extensive training and tightly controlled testing. Dogs must learn which particular odour predicts the target condition while ignoring countless irrelevant differences between samples. Researchers also need to prevent handlers from unintentionally guiding them and test whether success in a training environment carries over into real clinical settings.

An untrained dog may still notice that somebody smells different. There are accounts of pets repeatedly investigating one part of a person’s body before a health problem was discovered, and those stories helped inspire some areas of medical detection research. They do not turn a pet’s attention into a diagnostic test. Dogs sniff, lick, stare or become clingy for many reasons, most of which have nothing to do with serious illness.

The extraordinary part is their ability to detect chemical differences that remain invisible to us. Working out what those differences mean is a separate task, and one that still belongs to medical professionals.

Can a dog smell a storm coming?

Some dogs begin pacing, hiding or seeking their owners before people have noticed any sign of bad weather. They may be responding to distant thunder beyond the range of human hearing, changes in wind, darkening light, shifts in atmospheric pressure, static electricity or odours carried through the air.

Smell may contribute, but saying that dogs can simply smell a storm makes a complicated process sound more certain than the evidence allows. Researchers have proposed several possible cues, and dogs may use more than one. Their previous experience matters too. A dog who has learned that particular changes predict frightening thunder may react to the earliest cue available.

If a dog repeatedly becomes unsettled before storms, the behaviour deserves attention even when we cannot identify what they noticed first. Their experience begins when the warning signs become perceptible to them, which may be well before the storm becomes obvious to us.

Why sniffing deserves space in a walk

Walking provides movement, opportunities to toilet, contact with the outside world and, for many dogs, a great deal of scent investigation. The balance will vary. Some dogs move quickly between smells, while others study a few spots in remarkable depth. Age, health, breed, confidence, arousal and familiarity with the area can all affect how a dog uses their nose.

There is some evidence that scent based activity can influence welfare. In one study, pet dogs practised either nosework or heelwork for two weeks. The dogs doing nosework subsequently showed a more positive response to an ambiguous situation, which researchers interpreted as an increase in optimism. A sniffing walk is not identical to a structured nosework exercise, so the result cannot tell us exactly what happens during an ordinary trip around the park. It does support the broader value of allowing dogs to use a sense that is central to how they engage with their surroundings.

Giving a dog time to sniff does not require every walk to take all afternoon. There will be roads to cross, places where stopping is unsafe and days when everybody needs to get home. A walk can include stretches where you keep moving and periods when the dog is allowed to choose the pace and investigate.

That pause beside a lamppost may appear uneventful because most of what is happening cannot be seen. Your dog is sampling a world laid over the top of ours, full of arrivals, departures and changes we will never notice. The route may be familiar to you. The information waiting along it is never quite the same twice.