Last revised April 15, 2017.
Horse rolling in a dusty wallow.1
Horses roll. They do so in places where others roll. Eventually, the area where they roll may lose its vegetation and become dusty. What we can’t see is what they smell. Before a horse rolls, he sniffs the wallow. During the roll, he rubs the dust into his fur, and his scent into the remaining dust. He acquires the scent of the herd, and the next horses to roll acquire his scent.2
Rolling by this feral horse mare is a self-grooming behavior associated with pelage hygiene and insect control.3
There are likely several reasons why horses roll in the dust or mud. A peek around the Internet reveals many answers to the question.
- Speed drying of a wet coat.4
- Refluffs coat to restore maximum insulative properties.5
- Covering the coat with dust that reflects the sun6.
- Helps repel biting insects.7
- Aiding in shedding8.
- Helps remove dead skin.9
- Increasing comfort by scratching.10
- Relieving any tight muscles or stress after being ridden.11
- Something about the rolling site stimulates rolling.12
- Seeing another horse roll stimulates rolling.13
- Rolling “corrects vertebral subluxations”.14 It “helps stretch his back and muscles and exercises his joints when he gets up and down.15” He is doing his own chiropractic work, self-adjusting his body by the force of laying, twisting, and rolling.16
- The dirt makes a barrier for the heat.17
- Mud helps keep the horse cool on a hot day.18
- Rolling is a symptom of colic.19
- Repelling or removing parasites.
- Predators love shampoo in their dinner: “When you clean them they smell clean and different… and if they where out in the wild and smelt like that a preditor [sic] would pick em out immediatly [sic] so they got and roll and get all dirty so they can blend in with their environment”20
The variety of answers to the question hints that no one seems to know why horses roll.
When we ask “why do horses roll in the dust or mud?”, we are asking for a cause, not a benefit. So answers that focus on the benefits of a layer of mud for protecting the skin from flies or the sun is not about cause. A horse can hardly be expected to be motivated to roll by the rational expectation that flies will be less likely to bother him. And flies don’t explain why horses roll in the snow or take dust baths in winter, when flies are not a problem.
The Law of Parsimony (sometimes called Occam’s Razor) states that among competing explanations, the one with the fewest assumptions should be selected. The simplest explanation is not necessarily more likely to be right, but it is more easily tested and more easily remembered. Often the simple explanation covers more cases than an explanation with many clauses and caveats.
The rolling we need to account for is both when the horse is wet, and when he is dry; when there is fine dust available in an established site or no such site; when there is dust or mud available; when it is summer, and when it is winter; when others in his herd have just finished rolling and when there are no other horses present.
My proposed explanation is very simple: rolling must feel good in the same way that scratching an itch feels good. Bath water and sweat probably itch, and so rolling around in anything might feel extra good. With this explanation, a horse does not need a theory of fly protection or sunburn, and does not need a college course in how to achieve “maximum insulative properties” with his coat.
- Rolling provides negative reinforcement: the itch is unpleasant; removing the itch is pleasant.
- Rolling provides positive reinforcement: rubbing against the ground stimulates Merkel cells — those nerves that trigger pleasurable feelings. I know that the process feels good, because my horse gives the same groan of pleasure when he begins to roll as he does when I scratch his chest or he defecates.
Where does the itch come from? Our old friend evolution. Horses that roll likely have fewer ectoparasites than those that don’t, so evolution favored development of an itch, which would be reduced with a roll. Horses that roll might have a slightly better defense against flies, so evolution favored development of an itch, which would be reduced with a roll. And so on. The itch is all the horse needs to know about, and the roll might address a variety of problems.
If you are minding your own business, and feel something on your arm, your instinct is to brush it off even before identifying it. This is useful in avoiding spider bites. When a horse is wet, it seems likely that the water or sweat on their skin triggers an itch that can be cured with a dust bath.
If you watch horses roll, you’ll see some that lie down on one side, rub about, then roll to the other side. But other horses go down, rub a side, then stand up and go down again, this time rubbing the other side.
A horse with normal withers will usually lie down on one side, rub against the ground, then roll to the other side and repeat. A horse with high withers may need to stand after the first side, and lie down on the other to complete the job. The fact that horses always lie on both sides during the rolling operation is consistent with the itch theory — the itch must trigger the roll, the roll must stop the itch, and both sides must itch. If a horse were to role for theoretical reasons, such as adding sunblock or fly repellent, I think we’d see more horses just do one side, or sometimes do one side twice.
If you are a hot horse, and spot a cool pond or puddle, you know what you can do to feel good. A splash will bring immediate cooling, and that will feel good.
This is intelligent design. We don’t need an instinct to remove our hand from a hot pot, and another instinct to remove our hand from an open flame, and still another to avoid putting our hand in boiling water. All we need is “pain” whenever we encounter something that will likely injure tissue. And all we need to know about pain is that it is to be avoided. All we need to know about an itch: scratch it. I think simple motivations work just fine.
If a horse in the herd rolls, there is a chance that another nearby horse will also roll. This phenomenon is called social facilitation. In animals, social facilitation occurs when one animal’s performance is made more likely or more intense by the presence of another that is performing that behavior.21 Satiated chickens are likely to eat more when in the midst of hungry chickens that are dining. Horses are more likely to roll after they have seen another horse rolling.
This pony is rolling in the snow. The pony is not wet from a bath or from sweat. Mud and dust are not acquired… So why would he do it? It must feel good.22
If you have just bathed your horse, you will be disappointed to find him rolling in his pasture 10 minutes later. Knowing that the water on his skin is likely triggering an itch that a dust bath will cure, you might try drying him before he attempts to dry himself. I find that my horse’s face is important in triggering rolls, even though he doesn’t get it into the dust: if I squeegee him after a bath and dry his face with a towel, he is much less likely to roll when given a chance.
After a ride, my horse’s face is likely sweaty, even if it is not evident to me through his coat. I take this same filthy towel and hold it horizontally in front of his face. He pushes into it with his head, and as I rub left and right, he bobs his head up and down. The result is that he gets his face dry and his itchy eyes rubbed without me poking him in the eye or otherwise causing trouble. This head rubbing is under his control, and I’m always surprised at how much of it he craves. During a greeting, when my horse doesn’t have any issue with sweat, he is likely to tuck his face into my armpit, and self-administer a good rubbing. I’d rather make him happy than keep my shirt clean. T. Rex may have suffered from itchy face too, and retained his tiny arms to deal with the need. He likely drew many flies to the bits of rotting meat between his teeth and on his jaws. A walking fly makes an itch — another reason for T. Rex to have arms.
Rolling in Mud Must Cool
Rhinos, elephants, hippos, and domesticated pigs all seem to enjoy a roll in the mud. This is likely because while these species have sweat glands, they sweat by passive diffusion, which does little to cool. Further, in these animals the ratio of size to surface area means that they are more often troubled by the heat than the cold. If they can cover themselves with water, then evaporation can cool them, much the way sweat would. I believe that these four species seek out water, turn the neighborhood into a muddy mess, and find themselves covered in mud, where a fresh spray of clean water might have done just fine. The mud happens to make life more difficult for flies, but this is not why they enter the water.
Horses can sweat copiously, as can humans. These two species are the only mammals known to sweat copiously to keep cool.23 (Other primates only sweat through their armpits, palms of their hands, and soles of their feet, and don’t gain as much cooling from this limited sweating as humans do with full-body sweating.)
Horses are professionals at sweating, likely because their ratio of mass to surface area makes them net heat generators, their bodies are covered in hair that unavoidably keeps them warm, and exertion generates additional heat that must be removed. So horse sweat contains a special ingredient — latherin — which acts as a surfactant to get horse’s hair wetter, providing more surface area for sweat to evaporate from. Latherin accounts for the thick foamy stuff on a sweating horse. The lather means that your sweating horse is actually wetter than you are when you sweat. See the chapter on Drinking for more on latherin.
Despite latherin , horses seem to enjoy rolling in the mud on hot days. The mud serves the same purpose for them as it does for rhinos: it cools them as the water in it evaporates.
Rolling in Dust Might Support Scent Exchange
Rolling in dust might be favored by a wet skin, and rolling in water (mud) might be favored by a hot day. But how do we account for rolling in mud or dust when a horse is cold, or dry? One good theory is that such a horse is picking up the scent of the herd when it does this. (We know that horses always choose the same spot to roll, and sniff the dust bath area prior to a roll. They are quite likely to stir the ground with one foot as they sniff and before they roll. This pawing of the ground will likely expose scents that come from the disturbed soil where they were protected from being carried away by wind. We also know that dogs will sometimes roll on a dead animal, presumably to acquire the delicious scent thereof.)
Tonka carefully sniffs the ground prior to a roll.24
A horse that smells like the herd belongs to the herd. A horse that has just been bathed has likely lost the scent of the herd. Rolling in the dust might replace that lost scent, and might help protect the horse from attack by pasture mates that didn’t recognize him. Scent is important to anyone who can recognize all of their friends from their poop. Your horse’s identity is bound to his own scent more than it is to his visual image.
If a dust bath contains the scent of other horses who have used it, then it stands to reason that a roll in the dust will leave a scent behind, effectively marking this territory. So the dust bath is suspect as both a possible source of the scent of the herd, and as a means of adding one’s scent to this common repository. Of course, picking up or leaving behind a scent won’t motivate a horse. But if this scent exchange is important to the species, then evolution might have simply made rolling feel good, and our explanation would be “Because it feels good.”
1 Image source: DeBold, Don. https://www.flickr.com/photos/ddebold/7147283759
2 Keiper, Ronald R. The Assateague Ponies. Tidewater Pub, 1985.
3 image source: Ransom, Jason I., and Brian S. Cade. “Quantifying Equid Behavior–A Research Ethogram for Free-Roaming Feral Horses.” U.S. Geological Survey Techniques and Methods 2-A9, 23 p. (2009) http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1018&context=usgspubs
4 “Why Horses Roll. Why Your Horse Rolls — When It’s a Good Roll, and When It’s Not”. December 11, 2016; McDonnell, Dr. “Rolling Along”. http://research.vet.upenn.edu/HavemeyerEquineBehaviorLabHomePage/HavemeyerEquineBehaviorClinic/FrequentlyAskedBehaviorQuestions/WhydoHorsesRoll/tabid/2988/Default.aspx; “Why do Horses Roll in Mud?” http://animals.mom.me/horses-roll-mud-2603.html
5 McDonnell, Dr. “Rolling Along”. http://research.vet.upenn.edu/HavemeyerEquineBehaviorLabHomePage/HavemeyerEquineBehaviorClinic/FrequentlyAskedBehaviorQuestions/WhydoHorsesRoll/tabid/2988/Default.aspx; “Why do Horses Roll in Mud?” http://animals.mom.me/horses-roll-mud-2603.html
6 Gore, Rick “Allowing a Horse to roll after a bath – Rick Gore Horsemanship – www.thinklikeahorse.org” Mar 20, 2010 https://www.youtube.com/watch?v=_cdPAWczTgc
7 McDonnell, Dr. “Rolling Along”. http://research.vet.upenn.edu/HavemeyerEquineBehaviorLabHomePage/HavemeyerEquineBehaviorClinic/FrequentlyAskedBehaviorQuestions/WhydoHorsesRoll/tabid/2988/Default.aspx; Gore, Rick “Allowing a Horse to roll after a bath – Rick Gore Horsemanship – www.thinklikeahorse.org” Mar 20, 2010 https://www.youtube.com/watch?v=_cdPAWczTgc; “Why do horses roll – and it isn’t to cool them off or act as a repellent.?” https://answers.yahoo.com/question/index?qid=20071202094144AAe31h5; http://www.schleichhorseclub.co.uk/uploads/ckfinder/files/015_PONY_JUNE_2015%20(web)(1).pdf
8 McDonnell, Dr. “Rolling Along”. http://research.vet.upenn.edu/HavemeyerEquineBehaviorLabHomePage/HavemeyerEquineBehaviorClinic/FrequentlyAskedBehaviorQuestions/WhydoHorsesRoll/tabid/2988/Default.aspx; “Why Horses Roll. Why Your Horse Rolls — When It’s a Good Roll, and When It’s Not”. December 11, 2016
9 “Why do Horses Roll in Mud?” http://animals.mom.me/horses-roll-mud-2603.html
10 McDonnell, Dr. “Rolling Along”. http://research.vet.upenn.edu/HavemeyerEquineBehaviorLabHomePage/HavemeyerEquineBehaviorClinic/FrequentlyAskedBehaviorQuestions/WhydoHorsesRoll/tabid/2988/Default.aspx
11 “Why Horses Roll. Why Your Horse Rolls — When It’s a Good Roll, and When It’s Not”. December 11, 2016 http://horses.about.com/od/understandinghorses/fl/Why-Horses-Roll.htm
12 McDonnell, Dr. “Rolling Along”. http://research.vet.upenn.edu/HavemeyerEquineBehaviorLabHomePage/HavemeyerEquineBehaviorClinic/FrequentlyAskedBehaviorQuestions/WhydoHorsesRoll/tabid/2988/Default.aspx
13 McDonnell, Dr. “Rolling Along”. http://research.vet.upenn.edu/HavemeyerEquineBehaviorLabHomePage/HavemeyerEquineBehaviorClinic/FrequentlyAskedBehaviorQuestions/WhydoHorsesRoll/tabid/2988/Default.aspx
14 Sutor, Cheryl. “Why Does My Horse Roll?” 1997. http://www.equusite.com/articles/health/healthRolling.shtml
15 “Why do Horses Roll in Mud?” http://animals.mom.me/horses-roll-mud-2603.html
16 “How your horse rolls tells you things you need to know” http://www.myhorseguru.com/2010/05/21/how-your-horse-rolls-tells-you-things-you-need-to-know/
17 Gore, Rick “Allowing a Horse to roll after a bath – Rick Gore Horsemanship – www.thinklikeahorse.org” Mar 20, 2010 https://www.youtube.com/watch?v=_cdPAWczTgc
18 “Why do Horses Roll in Mud?” http://animals.mom.me/horses-roll-mud-2603.html
19 “Rolling is a symptom of colic. You can identify a normal, healthy roll by noticing how the horse rolls, and how it acts when it gets on its feet. When your horse is rolling for the enjoyment of it, it may circle around a few times, get down, roll, perhaps get up again and roll on the other side, regain its feet and then have a good shake to get the dust off. And healthy horses rarely try to roll in their stall. When a horse is rolling because of colic, it will drop down, often suddenly, roll violently, and then stand listlessly afterward, with no invigorating shake to rid itself of dust. It may try to roll in its stall, and it may try to roll multiple times. A horse that has been rolling in its stall may appear disheveled, sweaty and covered in manure and shavings. There’s some debate whether rolling can cause twists in the gut that cause torsion colic. Some people feel that a twist has occurred before the horse starts to show colic symptoms, such as rolling. But, whether or not rolling causes twists, a colicking horse should be prevented from rolling because it expends a lot of energy, and as it violently thrashes, could hurt itself or the people handling it.” — “Why Horses Roll. Why Your Horse Rolls — When It’s a Good Roll, and When It’s Not”. December 11, 2016 http://horses.about.com/od/understandinghorses/fl/Why-Horses-Roll.htm
20 “Why do horses roll – and it isn’t to cool them off or act as a repellent.?” https://answers.yahoo.com/question/index?qid=20071202094144AAe31h5; http://www.schleichhorseclub.co.uk/uploads/ckfinder/files/015_PONY_JUNE_2015%20(web)(1).pdf
21 Zajonc, Robert Boleslaw. Social facilitation. Research Center for Group Dynamics, Institute for Social Research, University of Michigan, 1965.
22 Image source: Pinterest. https://www.pinterest.com/pin/468726273700330031/
23 Vance, S. J., McDonald, R. E., Cooper, A., Smith, B. O., & Kennedy, M. W. (2013). The structure of latherin, a surfactant allergen protein from horse sweat and saliva. Journal of The Royal Society Interface, 10(85), 20130453.
24 Image source: “Horse Dust Bath” July 24, 2015 https://hencam.com/henblog/2015/07/horse-dust-bath/