Last revised April 20, 2017.

Two donkeys demonstrate the flehmen response.1

When we marvel at the ability of our horse to follow a long convoluted trail back to the barn, we should consider whether he is doing this from memory, or whether he is also picking up his own scent and tracking it. In thick fog or on a moonless night, my money is on his nose.

Chemoreception — the sense of smell — allows human and horse to respond to chemical stimuli. It is a capability found in all animals, from amoeba to horse and beyond. (What could be beyond a horse?)

Mammals don’t just sniff to detect chemicals. They smell with a nose, detect pheromones with a vomeronasal system, taste with a gustatory system, and detect irritants through a trigeminal system. Most of these chemicals come from some distant source, and travel through air or water to our nose. Some, however, find their way into our mouth, where we taste them. Taste, we have learned, is affected by sight and smell: close your eyes, and hold your nose, and try to decide if you are eating a raw potato or an apple.

Many mammals seem to have developed lateral preferences in the use of their nostrils, in the same way that we have developed a preference for handedness. If they are not ambidextrous, most horses seem to prefer to graze with their left front leg extended forward. In one study,2 the distribution of preferences was 43 significantly left, 10 significantly right with 53 being non-significant (i.e. ambidextrous). In horses under the age of four, this same study found a preference for use of the right nostril when smelling stallion feces. The right nostril connects primarily to the right hemisphere — the part specialized for quick reactions to novel stimuli. (The optic and auditory nerves cross over, and the right eye and ear connect primarily to the left hemisphere. See “The Mind of Your Horse” for more.)

As you may recall from high school chemistry, describing chemicals is not as easy as looking at them or smelling them. A fragrance that your horse detects might come from one molecule of a million he has breathed in, or from many dozen such molecules. How do we capture that one in a million? How do we know which one it is? And when we have learned about it, what do we say?

Even if words fail us in talking about chemicals, olfactory receptors don’t let us down. In a mammal’s nose, each olfactory neuron possesses a single type of receptor; each receptor responds to several molecules, and each molecule is recognized by several receptors.3 Scents are discriminated by various combinations of 10 or more receptors. With 350 types of receptors, the number of ways you can produce combinations of 10 is ridiculous. As I’ll show you later in this chapter, a horse can detect about 1,816,285,375,084,304,096,155,409,990,400 different scents. That is about 1,699,868,436,439,970,000,000,000,000,000 more scents than your dog. And it is more than the number of chemical compounds possible. The perfect nose could detect stuff that doesn’t even exist! My old nose doesn’t do so well, but the nasal cavities of Mr. Horse are filled with possibilities.

The science of determining what a horse smells is not yet quite launched. I have had to talk about all sorts of species in this section simply because so little is known about horses.

About Scents

Understanding the sense of smell should begin with a bit of understanding about scents. In their exploration of the skills of scat detection dogs — those trained to detect the scat or poop of various species — Wasser et. al. write “[the] ability to detect odor far from its source varies as a function of topography, wind speed and direction, and the age and contents of the sample. Scent particles radiate and disperse from their source over time. Scent particles are heavier than air, causing odor to collect or pool in depressions and be sporadic, thin, or absent in higher or more exposed places. Odor can rise or fall with shifting humidity, temperature, and air currents or hang up in vegetation, under logs, or against objects.4

Horses Kick Butt

Horses have much better sense of smell than humans, and the sense is more important to them. The difference in olfactory ability can be attributed to several factors:

  1. Olfactory receptor density is constant across the olfactory surfaces in humans and horses, and so the total number of receptors is a function of the amount of olfactory surface area. In a human, the olfactory bulb is 10-12 mm long and 3-4 mm wide; in the horse it is 30-35 mm long and 20-25 mm wide. Convolutions in the olfactory area of the horse increase the surface area even more. In the horse, this area is large and convoluted.
  2. A horse nose can move much more air with each breath, and so can trap more volatile particles with each breath.
  3. A horse’s nostrils are separated by a greater distance than a human’s, and point in opposite directions. With this arrangement, a horse can use stereo olfaction to localize olfactory sources.5
  4. Horses have a vomeronasal organ, which is good at detecting pheromones. Humans have one as young embryos, but it disappears during development.

So horses kick butt. Dogs have a vomeronasal organ too, but horses beat them in the first three differences above: more olfactory receptors, more air with each breath, and a greater spread between nostrils. But because scent pools near the ground, both dogs and horses need to keep their noses down to track a scent, and there the dog may be at an advantage. Whether dog, horse, or human has a better sense of smell may be a matter of debate and further research,6 but I wouldn’t bet against horse or dog.

Flehmen response

A horse has a vomeronasal organ (VNO) or Jacobson’s organ — an auxiliary olfactory sense organ located near the vomer and nasal bones of the skull, at the base of the nasal cavity. This organ connects to a small opening on the roof of the mouth, where it receives odor particles mixed with fluids from the mouth. The VNO is the first stage of an accessory olfactory system, completely independent of the main olfactory system. The VNO contains sensory neurons that detect chemical stimuli, sending information to an accessory olfactory bulb, which sends information on to the amygdala and from there to the hypothalamus.

Many sources will tell you that a VNO is found in ungulates (hoofed mammals) and cats. In fact, a VNO is found in some ancient fish, across amphibians, reptiles and mammals. In amphibians, it is known in caecilians, frogs and toads, newts and salamanders. In reptiles, it is known in snakes and ground-living lizards. Older studies claimed that it is missing or vestigial in turtles, but this proves to be false, and turtles have it and use it too.7 It is known in most mammals. It may not work well under water, and is likely vestigial or absent in modern fish (it is found in the African lungfish and the paddle fish, FWIW), crocodiles,8 and aquatic mammals, such as whales. It is also vestigial or missing in many arboreal (or formerly arboreal) species, including arboreal lizards, birds, apes and humans.9 It may be that pheromones don’t work as well under water or high in the air as they do near the ground: perhaps water and air currents interfere with pheromones as an intraspecies signaling device.

Where the epithelium of the olfactory bulbs responds to small, volatile molecules, the VNO senses large, non-volatile, species-specific molecules such as those found in body secretions.10 These chemicals can work as pheromones — chemicals produced and released into the environment by an animal that affects others of its species — which are released by one horse to produce automatic hormone-like reactions in another. It is believed that our armpits produce such pheromones, which would account for a horse’s interest in rubbing his nose in your armpit.

When a horse shows the flehmen response, he extends his neck, raises his head, curls his upper lip, inhales, draws substances into its VNO through his mouth, then holds his breath. Stallions do this more often than mares or geldings11, but all horses may show the response when presented with secretions connected to the birth of a foal or to an objectionable odor. For males, urine from a mare in estrus is a common trigger. A quick look at Google images resulting from searching for ‘flehmen response’ reveals that it occurs in rhinos, deer, zebras, giraffes, tapir, moose, sheep, mountain sheep, water buffalo, cape buffalo, sheep, cattle, and big and little cats as well as horses. The flehmen response is a very noticeable facial expression, but it only means that a fascinating scent has been detected, and the horse is using his vomeronasal gland to learn more about it.

Horses are “obligate nasal breathers” — they always and only breathe through their nose. (Take a look at a race horse on the track — perhaps using Google images — and you’ll see his mouth is closed, even as he crosses the finish line). But intake for the VNO is in the roof of the mouth. How can it draw air through the mouth? The flehmen response both raises the upper lip to try to seal the nostrils, and in the course of doing this, opens the mouth for the intake of air. Some of the inhaled air then comes through the mouth and can be taken up by the VNO on the palate.

Researchers at Cornell12 have reported on a very interesting set of studies in the field and lab of the flehmen response. In their field studies, their careful observations revealed:

  • The flehmen response is most frequently preceded by nasal, rather than oral, investigation of substances;
  • A stallion’s rate of flehmen response varies with the estrous cycles of a mare;
  • The rate of flehmen response does not vary with time of day;
  • The flehmen response was most frequently followed by marking behaviors rather than courtship behaviors. This suggests that the flehmen response is more involved with monitoring a mare’s estrous cycle than courtship. Perhaps the flehmen response contributes to the chemosensory priming of the stallion for reproduction.

In an experiment in their lab, these researchers at Cornell presented stallions with urine or feces of mares in various stages of the reproductive cycle as well as with their own or other males’ urine or feces. The occurrence of sniffing and flehmen was used to determine the discriminatory ability of the stallions. The experiment showed that stallions can differentiate the sex of a horse on the basis of its feces alone, but cannot differentiate on the basis of urine. This study only examined stallions as sniffers, but I suspect that mares and geldings can also determine the sex of the horse from the feces it has left behind. Mares, stallions, and geldings all show the flehmen response when presented with urine or vaginal secretions, though the response seems strongest in stallions.13

By the way, there is no evidence that “most horses will also display the flehmen response when they are in pain”14 And there is plenty of evidence that the flehmen response occurs in the absence of pheromones. One researcher15 invoked the response with dog food that accidentally remained on her hands after washing them.

Studies have shown that humans do react to pheromones of our own species. We now know that a woman’s menstrual cycle changes when she is exposed to the sweat of other women, that a pheromone causes menstruation to synchronize in females who live together,16 and that the scent of ovulating women raises a man’s testosterone level. But such human research has struggled to identify a single human pheromone17, and the vestigial nature of the VNO in humans leads us to wonder how pheromones are detected.


My sense of smell is terrible. I don’t think it was ever good, but now it is almost non-existent. This sad state of affairs removes me from the world of being bothered by poop or being thrilled by a hot roast beef sandwich. In fact, it has made it much harder for me to write this section, because I don’t personally know what I’m talking about. I’d rather work on the sections on senses I still have: vision, hearing, and touch.

We know a little about horse poop. We know, for instance, that both male and female horses can identify other horses from the smell of their poop.18 And we know that testosterone levels in stallion poop are highest during the breeding season, and that they correlate with harem size.19 Mares may prefer stud muffins, as sensed by sniffing their poop.

Last night I was minding my business reading a story about a biologist named Sam Wasser who was working to stop illegal trade in African ivory.20 His career in poop began when he learned of a cancer researcher who was tracking his patients’ hormone levels by analyzing their stool samples. He learned that a single gram of poop can contain millions of sloughed-off cells, each with a copy of the DNA of its maker. Working in Africa, he collected and analyzed local elephant poop, and asked biologists and game wardens all over Africa to collect poop and send it to him.

DNA has many important jobs to do, but some regions of a DNA strand are “junk” — non-protein parts where, because they are unused, can allow mutations to accumulate. Such mutations occur naturally and are transmitted from mother to offspring; because they are unused, such mutations hurt nothing. Because mutations vary naturally, they can be used as markers, identifying families.

While elephants once lived in much of Africa, a given elephant and her family lives in only a very small territory, and so the family markers found in one of these territories differ from those found elsewhere. Sam Wasser made a map. After 10 years and thousands of analyses, he reached the point where, presented with a blind sample of elephant dung, he could guess where in Africa it had been collected to within 190 miles.21 To be a brilliant scientist, you have to start with a good idea and work very very hard. Remember Einstein: it is only 1 per cent inspiration.

The DNA that is found in an elephant’s poop is the same that is found in that elephant’s skin, whiskers, heart, lungs, hair and tusks. The DNA is the elephant. So now Sam Wasser can put his maps to good use catching poachers. When a shipment of tusks is seized by Customs, Sam may be invited to analyze its origins. He can look at the DNA of a tusk, and determine exactly where in Africa it came from. It turns out that most ivory is coming from just two places, but that’s not where my version of this story is headed.

What intrigued me most in reading Sam’s story was that during his early research, he realized that scat “contains a huge amount of information, from the DNA of the animal that left it, to the DNA of all the things the animal was eating, to the microbiome in its gut, to its reproductive hormones, to its stresses and nutritional hormones, to toxins.22

So here’s a surprise for me: horse poop is not merely grass in a blender. It is the horse, with grass added. Or grass with the horse added. If your horse puts his DNA, pheromones, hormones, toxins and other things in his poop, a smell of his poop likely reveals the same information as a smell of him would offer. When two horses go nose to nose, these same things must come flying out of the nostrils of one, and into the nostrils of the other. A recent study confirms that “mitochondrial, protein encoding and microsatellite nuclear DNA extracted and amplified from feces of Malayan sun bears and North American black bears is shown to be identical to that extracted and amplified from the same individual’s tissue or blood.23” So we can extend our notion of “you are what you eat” to “thou art what thou fart”.

And that is another surprise for me. I know you can tell if I’ve eaten garlic. But not whether I’ve eaten a tomato or a cucumber. It never occurred to me that my breath contained anything but hot air. As you have guessed, I’m not the brightest bulb. In any case, we need to learn what is coming out in an exhaled breath that might interest a horse. Does it also contain his DNA, pheromones, hormones, toxins and other things?

And that led me to a much more obvious point. Consider your horse on a trail ride. You go out and back on the same trail. On the way you encounter a poop. When are about 15 or 20 feet from the poop, your horse may begin to lower his head. Given a little freedom, he’ll pause and smell it before moving on. But on the return trip, he doesn’t even slow for this poop. I had once attributed this to memory — he could remember that he’d previously smelled this poop, right here after the bridge near the big oak tree. But that would be silly. In fact, your horse knows much about a poop from a distance of 15 or 20 feet. He knows at that distance whether it contains any secrets he has not mastered. His memory of those secrets is profoundly good.

This 15 or 20 foot distance is remarkable. It means that your horse can identify a poop by smell at that distance.

At a distance of 15 or 20 feet your horse recognizes his own poop, and never ever stops to smell it. Even though it might look identical to all the other poop on your shoe, in fact it isn’t, and he knows it. He knows his own poop.

Horses in a pasture will often stop and smell each other’s poops. In each instance, they must know who they are smelling, and the information in the poop will be like the information they’d get from smelling its creator, without the risk. This may be why horses seem especially keen to smell the poop of a dominant horse. They know who made it, and they don’t need to get hurt finding out more about him. But don’t they already know what he smells like? A horse on a trail ride doesn’t waste his energy by smelling a poop twice, so we must think that any poop contains recent information. He walks up to a horse’s poop, and at a distance knows who made it. When he pauses and examines it, he learns details of their recent history — today’s hormone levels, diet, whatever.

Of course, on a trail ride, a horse may encounter a poop from a horse that they’ve never met. The novelty of this poop doesn’t hurt a thing. He can learn about that horse by smelling it, and know that it was here recently. When he actually meets this horse, he’ll know where he has been, and he’ll seem more familiar.

If poop contains the horse’s secrets we could speculate that if only we could get a list of the secrets contained, we would have a list of the things that it is possible for a horse to detect from a smell. We aren’t guaranteed that horses can extract every possible piece of information that is available, but the evolution of smell in mammals has had 225 million years to work out these details. Even the early mammals seemed to find smell important — check out this reconstruction of Juramaia, from 160 million years ago. It seems safe to say that most mammals can detect what is important in the poop of their species. I say “mammals” because no other class of vertebrates seems to have developed a very good sense of smell. I’m guessing that all mammals share a good sense of smell, but that those that dedicate as much of their skulls to olfaction as the horse does must have the best sense of smell.

Jurmaia. Note the extended skull, and space that must have been allotted for olfaction.24

What are the secrets contained in poop?

We don’t know very much about what horses knows about horse poop, but we do know something about what can be detected in the poop of other mammals. I suspect that horses are as good as deer and leopards at identifying what is to be found in their species’ poop.

Sex can be determined from DNA obtained from feces of Sitka black-tailed deer25, red deer26, moose, mountain goat, deer, caribou,27 leopard, Japanese marten, Siberian weasel, and feral cat28. And we know that mares know the sex of the maker of unfamiliar horse urine they are presented.29

Reproductive state of females can be determined by looking at hormones in feces. One study of reproductive states in female sea otters could identify estrus (receptive and fertile), anestrus (not receptive or fertile), luteal (post ovulation), delay (any gap in time prior to implantation), and implanted (fertilized egg attached to wall of uterus — pregnant)30.

Reproductive activity in females can be inferred from progesterone31, estrogen conjugates, and estradiol in urine or feces in a variety of animals including moose, Eld’s deer, Pere David’s deer,32 and white-tailed deer33.

Species is detectable in feces using a sophisticated biochemistry lab and computer software.34 DNA extracts are purified, modified, and its allele sizes and band sizes are assessed with software, and compared to those of various species. Presumably your horse carries such as chemistry lab in his head, because only horse poop is of interest to him. And presumably such chemistry labs are versatile, because dogs can be trained to locate specific species.

Individual identity is detectable in feces by looking at a half dozen microsatellite DNA locations35 (tracts of repetitive DNA in which certain DNA motifs are repeated, typically 5–50 times). By matching such locations, the researcher can tell whether the DNA comes from a particular individual that has been previously identified. Both male and female horses can identify other horses from the smell of their poop.36

Physiological stress can be inferred from feces from cortisol.37 Just a single antibody has been shown to detect stress in the African elephant, black rhinoceros, Roosevelt elk, gerenuk, scimitar-horned oryx, Alaskan sea otter, Malayan sun bear, cheetah, clouded leopard, longtailed macaque, yellow baboons and the northern spotted owl.38  Other studies show that alpha animals in packs or herds have higher levels of stress, as indicated by higher levels of cortisol.39

Scent Marking

Stallions seem to behave much like other mammals in marking territory. Stallions appear to use their poop to mark territory, making small deposits where they encounter the poop of other stallions or his own poop. Stallions show great interest in these deposits, but mares appear uninterested in such manure piles, and geldings may lose interest with age.40 So scent marking is a guy thing, between stallion and stallion. Ladies: if you are concerned about certain behaviors of your husband, please remember that he is a stallion.

King and Gurnell (2007)41 and Boyd and Kasman (1986)42 provide details of two solid observational studies of wild Przewalski horses in Mongolia. They identified two forms of scent marking: defecation on stud piles formed from repeated pooping in the same place, and the overmarking of mare feces and urine. Stud piles were marked with dung by the harem holder and sniffed before and after dung was deposited. The stud piles were not placed around the edges of a territory, but were more likely along routes used by the herd and common in the core parts of harem ranges or where harem ranges overlapped. Thus, rather than being used to defend range boundaries, stud piles were placed predominantly where they would be encountered by male intruders. Stallions may face mares when the mares urinate or defecate, approach the elimination, sniff it with an arched neck, then cover the elimination — usually by urinating on poop, and always by urinating on her urine.

At a minimum, fecal deposits of males signal the presence of males, thus serving as a “message board” to other males with overlapping territories.43 When two stallions are close to each other, the alternation of sniffing and defecation may be part of the effort of establishing or maintaining dominance.44 These stallions can see and interact with each other, learning about the other’s capabilities, and sniff the other’s dung, to memorize the other’s identity.

But does it smell? DNA doesn’t likely have much of a smell. What do we know of the smells in horse poop? Rikako Kimura comes to the rescue with his research on volatile substances in the feces, urine, and urine-marked feces of feral horses.45 He examined the identity and amount of volatile substances through acid/steam distillation and gas chromatography–mass spectrometry. Kimura found that the frequency of excretion and scent marking differed between breeding and non-breeding seasons. The concentration of each substance (fatty acids, alcohols, aldehydes, phenols, amines and alkanes) in the feces differed according to maturity, sex and stage in the reproductive process. The feces of foals, a stallion in the breeding season, a stallion in the nonbreeding season, an estrous mare, a non-estrous mare, a mare at the late-pregnancy stage, and a non-pregnant mare in the non-breeding season each had a characteristic chemical fingerprint.

Kimura suggests “volatile substances in feces may act either singly or as a combination of several chemical species as a releaser pheromone that provides information on individual horses.46

Scent Unmarking

Scent marking may announce that the harem holder is the mare’s consort, but a very interesting study has found evidence of scent unmarking: after a stallion urinates on a mare’s feces,  the levels of tetradecanoic and hexadecanoic acids in an estrus mare’s feces drops to that of a non-estrous mare, perhaps because of the high concentration of cresols in the urine of stallions during the breeding season.47 Mr. Horse appears to be covering her tracks while leaving his own.

Cresols in the stallion’s urine appear to play a major role in masking a mare’s fecal scent, concealing critical information about her condition. No, there does not seem to be an application here for deodorizers: cresol has a strong, persistent odor, and is the main offender if you are standing down-wind from a pig farm.48


I don’t know about other “essential oils”, but lavender essential oil has been shown to deliver short-term anxiety reduction in humans,49 mice,50 dogs,51 sheep,52 and horses.53 In such experiments, an animal is often stressed initially, with something like an airhorn. Lavender essential oil is mixed with air and pumped into the area where the animal is. Heart rate typically drops faster after the stress in the lavender condition. I have only found evidence of short-term benefits, and think it likely that your horse might not like scented shampoo, no matter what the ingredients.

But there is a vast credible literature that now exists on the benefits of lavender essence.

  • Those receiving aromatherapy have experienced a significant improvement in sleep quality after intervention.54
  • Lavender aromatherapy and hand massage improves emotions and reduces aggressive behavior of elderly with Alzheimer’55s.
  • A lavender scent appears to reduce anxiety in dental patients.56

How does lavender do its good work? There is evidence that it decreases cortisol secretion from the adrenal gland and increases serotonin secretion from the GI tract.57

If you want to do some experiments yourself, you’ll find many sources of lavender essential oil on Amazon. Try it on yourself, first.

Other Uses of His Nose

Horses use their noses for many social tasks.58

Social investigation: when two horses meet, they normally will breathe nose to nose, exchanging respiratory breath and learning about both identity and what each other has been eating.

Marking: making and checking dung piles, and smelling the scents left behind at dust wallows where the herd rolls.

Fighting: when stallions fight, they will sometimes pause, sniff the nearby dung pile, and add to it.

Reproduction: Stallions sniff shoulder, flank, rump, and perineal area of a mare, and check her urine on the ground, as part of a more complex process of determining her condition.

Parturition: Mama horse sniffs the fluids and membranes of birth, sniffs her new baby, and when she begins to nurse, sniffs the base of his tail.

Identifying mom’s poop… Baby, in turn, dines on mama’s dung that he recognizes from her scent. This coprophagia is likely important in transferring important bacteria from her gut to his. Elephants, giant pandas, koalas, hippos, cockroaches and many more species59 engage in coprophagia, which helps them invest a sterile gut with key bacteria critical to successful digestion. If your mare gives birth in a stall, remove only the oldest poop from the stall, leaving the fresh stuff for the foal to dine on. A constantly clean stall will produce a foal with serious digestive problems! The expression “eat shit and die” would be better as “eat shit or die”.

Exploring the world: Horses young and old follow their noses, exploring their world, investigating things.60

Detecting predators: Horses can identify predators from their scent, but contrary to widespread belief, they are not frightened by the smell. Instead, they simply become more vigilant.61 Riding in the mountains near Yellowstone, the wrangler warned others in our pack trip: “there is recent bear scat on the trail here. Hang on.” No hanging on was needed. Our horses carried on, with raised heads and more attentiveness with ears, eyes, and nose.

How good is a horse’s nose?

A sense of smell is common throughout mammals, but capabilities differ considerably. The champion with its nose is also the mammal with the largest nose: the elephant. Elephants are beyond belief in capability. They use scent in foraging, social communication, and reproduction.62 They can distinguish between two Kenyan ethnic groups based on scent.63 They can recognize 30 individual family members from olfactory cues in mixtures of urine and earth.64 And they can discriminant between odors that have a one-carbon atom difference between pair members.65

More work has been done on the sense of smell in the dog than in the horse. The dog can detect a scent which is as low as 3 parts per million (ppm), allowing the dog to detect 18 or more species at a distance of a quarter mile or more.66 Is a horse this good?

Researchers in Japan67 and elsewhere have explored the “olfactory receptor gene repertoire” in a number of mammals. Such genes come and go as evolution proceeds, so while some genes are shared by most mammals, some are only found in some mammals. These genes give olfactory receptors (ORs) their discrimination power; the more such genes an animal has, the greater its ability to discriminate between one scent and another. Their study determined the number of intact olfactory receptor genes in each species. Here are their findings:

  • Elephant: 1,948
  • Rat: 1,207
  • Cow: 1,186
  • Mouse: 1,130
  • Horse: 1,066
  • Dog: 811
  • Guinea Pig: 796
  • Rabbit: 768
  • Human: 396
  • Chimpanzee: 380
  • Marmoset: 366
  • Macaque: 309
  • Orangutan: 296

You may find your favorite species on the list above, and draw your own conclusions. But remember that while one receptor is controlled by one gene, several receptors — around 10 — are involved in detecting a particular odor. To learn how many scents a horse can discriminate, the math is something like this:

How to determine the number of scents a horse can discriminate, based on 1,066 intact olfactory genes and the use of 10 receptors per scent.68

How many different scents can an elephant, horse, and human detect?

  • Elephant: 768,848,385,653,681,544,158,014,018,828,800
  • Horse: 1,816,285,375,084,304,096,155,409,990,400
  • Dog: 116,416,938,644,335,988,290,420,588,800
  • Human: 84,566,971,335,511,359,875,980,800

A small increase in the number of olfactory genes produces a huge increase in olfactory capability. And so your horse is about 15 times better than your dog at discriminating scents, and can detect 1,699,868,436,439,970,000,000,000,000,000 more scents than your dog. Your dog, of course, also kicks butt, and is about 1,385 times better than you.

Horses in a pasture seem to choose to roll in the same spots as other horses have rolled in, beginning by sniffing the spot. After the roll, the spot likely smells a bit more like the horse that just rolled, and the horse that just rolled smells a bit more like others in the herd. Such shared scents may help bond a herd, and help distinguish members from newcomers. Bathing your horse likely changes his scent, removing some of the dust from the dustbaths he has taken, and adding the scent of any shampoo or conditioner you have used. Your horse may appreciate this less than you’d think. When a horse likely rolls after the bath you gave him, it restores the scent of the herd to his hide.

Why should we care about a horse’s sense of smell?

There are several good reasons.

An olfactory encounter with poop or the sweat of a male horse may briefly raise the aggressiveness of a male horse. As two geldings march along on the trail and begin to sweat, one or both may show heightened aggressiveness toward the other. Aggressiveness in male mice has been shown to be reduced with chemical or surgical masking of their olfactory capabilities. If your horse becomes aggressive toward other geldings or stallions on trail rides, you might consider daubing some strong-smelling stuff in his nose before your ride, to impair his ability to smell other males. Or let him wear a bandana drenched in lavender essence. Or move to the front of the line of riders, so that the offending horse is behind you.

There is a body of literature that indicates that familiarity breeds comfort.69 If there is a group scent that derives from everyone rolling in the same dust bath, then you might consider collecting some of that dust from the dust bath, and applying it to the back of a horse before you first introduce him to the herd. If such dust carries a herd-defining scent, the new horse becomes less identifiable as a newcomer, and more identifiable as a bona fide herd member.

When we make physical contact with our horses, their ability to smell us improves. When your horse slides his nose into your armpit, you have probably maximized your olfactory contact. If your horse seems to seek out such contact, he is not being “pushy”. He is seeking this sensory input, and relishing it. Your shirt may need washing when you are done, but you will have bonded much better than if you insist that he keep away from you.


1 Image source:

2 McGreevy, P. D., and L. J. Rogers. “Motor and sensory laterality in thoroughbred horses.” Applied Animal Behaviour Science 92, no. 4 (2005): 337-352.

3 Leblanc, Michel-Antoine. The Mind of the Horse. Harvard University Press, 2013. P 337.

4 Wasser, Samuel K., Barbara Davenport, Elizabeth R. Ramage, Kathleen E. Hunt, Margaret Parker, Christine Clarke, and Gordon Stenhouse. “Scat detection dogs in wildlife research and management: application to grizzly and black bears in the Yellowhead Ecosystem, Alberta, Canada.” Canadian Journal of Zoology 82, no. 3 (2004): 475-492.

5 Stoddart, D.M. 1980. The Ecology of Vertebrate Olfaction. Chapman & Hall, London. p. 25.

6 Summarized in Leblanc, Michel-Antoine. The Mind of the Horse. Harvard University Press, 2013. P 341.

7 Okamoto, Kiyoko, Yukiko Tokumitsu, and Makoto Kashiwayanagi. “Adenylyl cyclase activity in turtle vomeronasal and olfactory epithelium.” Biochemical and biophysical research communications 220, no. 1 (1996): 98-101.; Murphy, F. A., K. Tucker, and D. A. Fadool. “Sexual dimorphism and developmental expression of signal‐transduction machinery in the vomeronasal organ.” Journal of Comparative Neurology 432, no. 1 (2001): 61-74.; Fadool, D. A., M. Wachowiak, and J. H. Brann. “Patch-clamp analysis of voltage-activated and chemically activated currents in the vomeronasal organ of Sternotherus odoratus (stinkpot/musk turtle).” Journal of experimental biology 204, no. 24 (2001): 4199-4212.; Hatanaka, T., and O. Matsuzaki. “Odor responses of the vomeronasal system in Reeve’s turtle, Geoclemys reevesii.” Brain, behavior and evolution 41, no. 3-5 (1993): 183-186.

8 Weldon, P. J., and M. W. J. Ferguson. “Chemoreception in crocodilians: anatomy, natural history, and empirical results.” Brain, behavior and evolution 41, no. 3-5 (1993): 239-245.

9 This is reviewed in several sources, including Bertmar, Gunnar. “Evolution of vomeronasal organs in vertebrates.” Evolution (1981): 359-366; Takami, Shigeru. “Recent progress in the neurobiology of the vomeronasal organ.” Microscopy research and technique 58, no. 3 (2002): 228-250.

10 Coren, S., Ward, L.M., Enns, J.T. 1999. Sensation and Perception. Harcourt Brace, NY. P. 226.

11 Stoddart, D.M. 1980. The Ecology of Vertebrate Olfaction. Chapman & Hall, London. p. 25.

12 Stahlbaum, Cathi C., and Katherine A. Houpt. “The role of the flehmen response in the behavioral repertoire of the stallion.” Physiology & behavior 45.6 (1989): 1207-1214.

13 Marinier, S. L., A. J. Alexander, and G. H. Waring. “Flehmen behaviour in the domestic horse: discrimination of conspecific odours.” Applied Animal Behaviour Science 19, no. 3-4 (1988): 227-237.

14 Pro Equine Grooms. “Question!”

15 Saslow, Carol A. “Understanding the perceptual world of horses.” Applied Animal Behaviour Science 78, no. 2 (2002): 209-224.

16 McClintock, M.K. 1971. Menstrual synchrony and suppression. Nature 229, 244-245.

17 Yuhas, D. “Are Human Pheromones Real?” Scientific American May 1, 2014.

18 Krueger, Konstanze, and Birgit Flauger. “Olfactory recognition of individual competitors by means of faeces in horse (Equus caballus).” Animal cognition 14, no. 2 (2011): 245-257.

19 Khalil, Ashraf M., Keiko Nakahara, Mikihiko Tokuriki, Yujiro Kaseda, and Noboru Murakami. “Variation in fecal testosterone hormone concentration with season and harem size in Misaki feral horses.” Journal of Veterinary Medical Science 71, no. 8 (2009): 1075-1078.

20 Kolbert, Elizabeth “The Elephant Detective. An American biologist wields an innovative new weapon against the illegal trade in African ivory” Smithsonian Jan-Feb 2017 p 29-34. This is online at

21 Wasser, Samuel K., Andrew M. Shedlock, Kenine Comstock, Elaine A. Ostrander, Benezeth Mutayoba, and Matthew Stephens. “Assigning African elephant DNA to geographic region of origin: applications to the ivory trade.” Proceedings of the National Academy of Sciences of the United States of America 101, no. 41 (2004): 14847-14852.

22 Kolbert, Elizabeth “The Elephant Detective. An American biologist wields an innovative new weapon against the illegal trade in African ivory” Smithsonian Jan-Feb 2017 p 29-34.

23 Wasser, S. K., C. S. Houston, G. Me Koehler, G. G. Cadd, and S. R. Fain. “Techniques for application of faecal DNA methods to field studies of Ursids.” Molecular Ecology 6, no. 11 (1997): 1091-1097.

24 Image source: That from Nobu Tamura

25 Yamauchi, Kiyoshi, Shin-ichiroHamasaki, Koji Miyazaki, Takefumi Kikusui, and Yukari Takeuchi. “Sex determination based on fecal DNA analysis of the amelogenin gene in sika (sic) deer (Cervus nippon).” Journal of Veterinary Medical Science 62, no. 6 (2000): 669-671.

26 Huber, Susanne, Ute Bruns, and Walter Arnold. “Sex determination of red deer using polymerase chain reaction of DNA from feces.” Wildlife Society Bulletin (2002): 208-212.

27 Brinkman, Todd J., and Kris J. Hundertmark. “Sex identification of northern ungulates using low quality and quantity DNA.” Conservation Genetics 10, no. 4 (2009): 1189-1193.

28 Kurose, N., R. Masuda, and M. Tatara. “Fecal DNA analysis for identifying species and sex of sympatric carnivores: a noninvasive method for conservation on the Tsushima Islands, Japan.” Journal of Heredity 96, no. 6 (2005): 688-697.

29 Hothersall, Becky, Patricia Harris, Lotta Sörtoft, and Christine J. Nicol. “Discrimination between conspecific odour samples in the horse (Equus caballus).” Applied Animal Behaviour Science 126, no. 1 (2010): 37-44.

30 Larson, Shawn, C. J. Casson, and Sam Wasser. “Noninvasive reproductive steroid hormone estimates from fecal samples of captive female sea otters (Enhydra lutris).” General and comparative endocrinology 134, no. 1 (2003): 18-25.

31 Wasser, Samuel K., Kathleen E. Hunt, Janine L. Brown, Kathy Cooper, Carolyn M. Crockett, Ursula Bechert, Joshua J. Millspaugh, Shawn Larson, and Steven L. Monfort. “A generalized fecal glucocorticoid assay for use in a diverse array of nondomestic mammalian and avian species.” General and comparative endocrinology 120, no. 3 (2000): 260-275.

32 Monfort, Steven L., Charles C. Schwartz, and Samuel K. Wasser. “Monitoring reproduction in captive moose using urinary and fecal steroid metabolites.” The Journal of wildlife management (1993): 400-407.

33 Plotka, E. D., U. S. Seal, L. J. Verme, and J. J. Ozoga. “The adrenal gland in white-tailed deer: a significant source of progesterone.” The Journal of Wildlife Management (1983): 38-44.

34 Wasser, Samuel K., Barbara Davenport, Elizabeth R. Ramage, Kathleen E. Hunt, Margaret Parker, Christine Clarke, and Gordon Stenhouse. “Scat detection dogs in wildlife research and management: application to grizzly and black bears in the Yellowhead Ecosystem, Alberta, Canada.” Canadian Journal of Zoology 82, no. 3 (2004): 475-492.

35 Wasser, Samuel K., Barbara Davenport, Elizabeth R. Ramage, Kathleen E. Hunt, Margaret Parker, Christine Clarke, and Gordon Stenhouse. “Scat detection dogs in wildlife research and management: application to grizzly and black bears in the Yellowhead Ecosystem, Alberta, Canada.” Canadian Journal of Zoology 82, no. 3 (2004): 475-492.

36 Krueger, Konstanze, and Birgit Flauger. “Olfactory recognition of individual competitors by means of faeces in horse (Equus caballus).” Animal cognition 14, no. 2 (2011): 245-257.

37 Wasser, S.K., Hunt, K.E., Brown, J.L., Cooper, K., Crockett, C.M., Bechert, U., Millspaugh, J.L., Larson, S., and Monfort, S.L. 2000. A generalized fecal glucocorticoid assay for use in a diverse array of nondomestic mammalian and avian species. Gen.

Comp. Endocrinol. 120: 260–275.

38 Wasser, Samuel K., Kathleen E. Hunt, Janine L. Brown, Kathy Cooper, Carolyn M. Crockett, Ursula Bechert, Joshua J. Millspaugh, Shawn Larson, and Steven L. Monfort. “A generalized fecal glucocorticoid assay for use in a diverse array of nondomestic mammalian and avian species.” General and comparative endocrinology 120, no. 3 (2000): 260-275.

39 Morell, Virginia. “Life at the top: animals pay the high price of dominance.” Science, vol. 271, no. 5247, 1996, p. 292. Academic OneFile, Accessed 4 Feb. 2017.

40 Saslow, C.A. “Understanding the perceptual world of horses”. Applied Animal Behaviour Science 78 (2002) 209-224.

41 King, S. R. B., and J. Gurnell. “Scent-marking behaviour by stallions: an assessment of function in a reintroduced population of Przewalski horses (Equus ferus przewalskii).” Journal of Zoology 272, no. 1 (2007): 30-36.

42 Boyd, Lee, and Lonnie Kasman. “The marking behavior of male Przewalski’s horses.” In Chemical Signals in Vertebrates 4, pp. 623-626. Springer US, 1986.

43 Miller, Richard. “Interband dominance in feral horses.” Ethology 51, no. 1 (1979): 41-47.

44 Feist, James D., and Dale R. McCullough. “Behavior patterns and communication in feral horses.” Ethology 41, no. 4 (1976): 337-371.

45 Kimura, Rikako. “Volatile substances in feces, urine and urine-marked feces of feral horses.” Canadian Journal of Animal Science 81, no. 3 (2001): 411-420.

46 Kimura, Rikako. “Volatile substances in feces, urine and urine-marked feces of feral horses.” Canadian Journal of Animal Science 81, no. 3 (2001): p. 419.

47 Kimura, Rikako. “Volatile substances in feces, urine and urine-marked feces of feral horses.” Canadian Journal of Animal Science 81, no. 3 (2001): 411-420.

48 Borrell, Brendan. “Why study pig odor?” Scientific American March 5, 2009.

49 Cooke, Brian, and Edzard Ernst. “Aromatherapy: a systematic review.” Br J Gen Pract 50, no. 455 (2000): 493-496.

50 Buchbauer, Gerhard, Leopold Jirovetz, and Walter Jäger. “Aromatherapy: evidence for sedative effects of the essential oil of lavender after inhalation.” Zeitschrift für Naturforschung C 46, no. 11-12 (1991): 1067-1072.

51 Komiya, Migiwa, Akihiko Sugiyama, Kazuko Tanabe, Tomiya Uchino, and Takashi Takeuchi. “Evaluation of the effect of topical application of lavender oil on autonomic nerve activity in dogs.” American journal of veterinary research 70, no. 6 (2009): 764-769.

52 Hawken, P., C. Fiol, and D.B. Blache 2001. Behavioral reactivity to psychosocial stress determines the effects of lavender oil on anxiety in sheep. Journal of Animal Science. 89 (E Suppl 1): 10.

53 Ferguson, Clarence E., Harry F. Kleinman, and Justin Browning. “Effect of Lavender Aromatherapy on Acute-Stressed Horses.” Journal of Equine Veterinary Science 33, no. 1 (2013): 67-69.

54 Chien, Li-Wei, Su Li Cheng, and Chi Feng Liu. “The effect of lavender aromatherapy on autonomic nervous system in midlife women with insomnia.” Evidence-Based Complementary and Alternative Medicine 2012 (2011).

55 Lee, Sun-Young. “The effect of lavender aromatherapy on cognitive function, emotion, and aggressive behavior of elderly with dementia.” Taehan Kanho Hakhoe Chi 35, no. 2 (2005): 303-312.

56 Kritsidima, Metaxia, Tim Newton, and Koula Asimakopoulou. “The effects of lavender scent on dental patient anxiety levels: a cluster randomised‐controlled trial.” Community dentistry and oral epidemiology 38, no. 1 (2010): 83-87.

57 Mirzaei, Firoozeh, Sara Keshtgar, Masoumeh Kaviani, and A. R. Rajaeifard. “The effect of lavender essence smelling during labor on cortisol and serotonin plasma levels and anxiety reduction in nulliparous women.” Journal of Kerman University of Medical Sciences (2015).

58 This section is adapted from Leblanc, Michel-Antoine. The Mind of the Horse. Harvard University Press, 2013. This is a fabulous book, and one which I’ve now read twice.

59 “Dung eater”. Nature WIldlife. BBC

60 Waring, George H. Horse behavior. The behavioral traits and adaptations of domestic and wild horses, including ponies. Noyes Publications, 1983.

61 Christensen, Janne Winther, and Margareta Rundgren. “Predator odour per se does not frighten domestic horses.” Applied Animal Behaviour Science 112, no. 1 (2008): 136-145.; Christensen, Janne Winther, Linda Jane Keeling, and Birte Lindstrøm Nielsen. “Responses of horses to novel visual, olfactory and auditory stimuli.” Applied Animal Behaviour Science 93, no. 1 (2005): 53-65.

62 Langbauer WR. 2000. Elephant communication. Zoo Biol 19: 425–445.; Rasmussen LEL, Krishnamurthy V. 2000. How chemical signals integrate Asian elephant society: the known and the unknown. Zoo Biol 19: 405–423.

63 Bates LA, Sayialel KN, Njiraini NW, Moss CJ, Poole JH, Byrne RW. 2007. Elephants classify human ethnic groups by odor and garment color. Curr Biol 17: 1938–1942.

64 Bates LA, Sayialel KN, Njiraini NW, Poole JH, Moss CJ, Byrne RW. 2008. African elephants have expectations about the locations of out-of-sight family members. Biol Lett 4: 34–36.

65 Rizvanovic A, Amundin M, Laska M. 2013. Olfactory discrimination ability of Asian elephants (Elephas maximus) for structurally related odorants. Chem Senses 38: 107–118.

66 Bryson, Sandy. Search dog training. Boxwood Press, 1984.; Syrotuck, William G. “Scent and the scenting dog.” (1972).

67 Niimura, Yoshihito, Atsushi Matsui, and Kazushige Touhara. “Extreme expansion of the olfactory receptor gene repertoire in African elephants and evolutionary dynamics of orthologous gene groups in 13 placental mammals.” Genome research 24, no. 9 (2014): 1485-1496.

68 Image source, and a good place to solve other such problems in permutations:

69 For example, see Stang, D. J. (1975). Effects of” mere exposure” on learning and affect.Journal of Personality and Social psychology31(1), 7.; Stang, D. J. (1974). Methodological factors in Mere Exposure research.Psychological Bulletin81(12), 1014.; Stang, D. J. (1973). Six theories of repeated exposure and affect. JSAS Catalog of Selected Documents in Psychology3, 126.; Stang, D. J. (1975). When familiarity breeds contempt, absence makes the heart grow fonder: Effects of exposure and delay on taste pleasantness ratings. Bulletin of the Psychonomic Society6(3), 273-275.


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