How horses keep warm.

It takes more than a shaggy coat to keep your horse warm in winter. A thick, blocky body retains heat for long periods, a massive digestive tract processes a mostly fibrous diet to generate large amounts of heat, lightly muscled legs require less blood circulation and thus lose less heat, long nasal passages warm cold outside air before it reaches the lungs.

During a ferocious winter a few years ago, three friends were stranded on a farm in the mountains. For two long weeks, snow fell and drifted nearly 10 feet high, while wind rammed against the doors of the farmhouse and hissed under the window frames.
Fearing frostbite or worse, the friends spent most of each day huddled around the fireplace, subsisting on Spam, cocoa and Monopoly. When they did venture out, icy air seared their lungs and left their fingers, toes, ears and noses throbbing. The cold sank in so quickly that they barely had time to check on the horses in the run-in shed before their teeth began to chatter.
One day, their frigid foray up the hill found the shed empty. Shrinking a little deeper into their coats, the friends slogged off into the hills to find the horses. They crested a rise above a remote, windwhipped pasture and stood amazed at what they saw: Far from the shelter of their shed, the horses were trotting along paths in the snow, bucking and playing as if it were the first day of spring. Even the ancient Shetland mare appeared absolutely blissful, snug in a winter coat that puffed out like a dandelion going to seed. The horses were not simply tolerating the cold - they were reveling in it.
You'd think that, in a test of horse and human against the winter elements, our species - armed with its dazzling array of high-tech fabrics and insulators - must easily prevail. Not so. Any horse owner who's ever pulled on some of that space-age winterwear and trudged out to the pasture on a January day knows who's ahead in the fight to stay warm outdoors. We humans may pride ourselves on being able to survive under all sorts of conditions anywhere on the globe, but we have invented nothing to match the efficient ingenious cold-beating mechanisms of Equus caballus. How does the horse do it? It all boils down to the three A's: adaptation, acclimation and acclimatization. Here's how they work.

Adaptation: crafted for cold.

All warm-blooded creatures can tolerate a broad range of temperatures, but each species has a natural comfort zone that reflects the climate in which it evolved. Biologists call this the 'energy-neutral range', meaning that within those limits - assuming dry, windless weather conditions - the animal need expend no extra energy to maintain normal body temperature.
Because our ancestors arose in central Africa, where natural selection favored those best suited to warm temperatures, our energy-neutral range is about 50 to 85 degrees Fahrenheit. Horses, on the other hand, evolved in elevated, northerly climes, where extreme cold presented an entirely different set of survival challenges. Their energy-neutral range is about 15 to 60 degrees. No wonder your horse relishes weather that seems frigid to you. The human body is built to dissipate heat, while the horse is constructed to produce and maintain it. It's all a matter of physical adaptation.
Several bodily adaptations allow the horse to 'run hot' in cold environments. First, there's his massive digestive tract, which prodesses a mostly fibrous diet and generates a huge amount of heat (far more than the human digestive process produces). In addition, the horse - like moose, elk and other large, cold-adapted ungulates - has a comparatively thick, blocky body that retains heat for a long period. (Think of the hot baked potatoes mothers once slipped into their children's coat pockets to keep them warm. The human frame is slender, more like a quick-cooling french fry than a whole potato).
Even the extremities of the horse are marvelously adapted for cold. Because his legs have proportionally less muscle than ours, the cells in his legs require less blood circulation for maintenance and consequently lose less heat. This lower metabolic need also means that a horse's legs have no problem with the reduced cellular activity brought on by cold. While our toes are among the first appendages to succomb to frostbite, adult horses almost never get frozen feet.
Consider also the blunt equine muzzle, so richly supplied with blood that it can whitstand bitter cold without freezing. (By contrast, the angular human nose is all too vulnerable to frostbite). And then there are the horse's long nasal passages, where equally blood-rich bone spiral called turbinates warm the frigid air before it can reach the lungs and potentially cool the body core. Our own noses are designed more to filter air than to warm it.

Acclimation: on the spot cure for chill

Obviously, your horse is much better equipped to deal with cold than you are. But that doesn't mean he's completely impervious to winter weather. Several heat-robbing factors can act against him when the mercury drops.

Any of these factors can rob your horse of body heat, leaving him in danger of hypothermia (subnormal temperature of the body). Of course, he's got his own solutions for cold: using his rump as a wind block, huddling with other horses to conserve heat, seeking shelter or running to boost his metabolism.
But if these behavioural responses fail, he is also equipped with emergency heat-generating mechanisms, called acclimation responses, to cope with the threat.
Acclimation responses work at two levels: at the surface of the body and at the core. Imagine, for example, that an icy wind begins to blow across the pasture. Your horse hunkers down with his back to the blast, but before long he is losing heat faster than his body can generate it. That's when the acclimation responses kick in.
Blood vessels in the skin constrict, and hair shafts stand on end (piloerection). After a while, the wind grows stronger, and he begins to shiver. All of these mechanisms serve to raise your horse's body temperature. The extent of the response is dependent on the intensity, duration and location of the chilly stimulus. A brief blast of wind may result in only a quick surface heating response, but if the cold persists, acclimation mechanisms in the core of his body swing into action, boosting your horse's metabolic rate and thus his body temperature.
Both levels of acclimation response operate through a complex, interactive system of feedback loops that connect the brain, the central nervous system and the adrenal glands. When body temperature falls, cold-sensing nerve cells throughout the horse's body fire warnings to one or more central heat-regulating hubs in the spinal cord and brain. (Colder than normal passing blood flow may also trigger nerve-cell 'thermostats' within the thermoregulatory centers themselves).
The chief command center for thermoregulation is thought to be the hypothalamus, a small but life-critical structure deep in the base of the brain, although the spinal cord or another region of the brain geared to react to falling temperatures may also be involved.
After comparing incoming temperature information to the body's energy-neutral range, the command centers issue order, by either electrical or chemical means, to begin emergency heating efforts:

- Shivering.

When the body perceives a serious drop in temperature, the central nervous system commands motor neurons in each major muscle group to set off a single, vigorous contraction. But almost instantaneously, tension-sensing proprioceptive nerves perceive the muscle as too tense and fire a command to halt the contraction. As the muscle relaxes, the proprioceptive nerves stop firing, allowing the muscle to contract again.
This rapid-fire tensing and relaxing of heavy muscle groups - the phenomenon we call shivering - quickly sends metabolism soaring. The cycle occurs incredibly fast; a shivering muscle may contract 10 to 20 times per second.
With their enormous blocks of muscle, horses are superb shiverers; they appear to shiver more comfortably and readily than do humans. And since nearly all the muscle action is converted to heat, shivering is a highly effective heating device (in man, it is believe to increase metabolic rate eightfold). But the warming comes at a huge cost to energy stores, so it is only a short-term remedy.

- Countercurrent heat exchange.

The uppermost layers of a horse's skin are suffused with veins that normally circulate the blood close to the outer air before returning it to the lungs. In hot weather, the resulting heat radiation is desirable, but under frigid conditions, the heat loss could be dangerous. To minimize surface radiation in the cold, the horse's venous blood takes a detour. Orders from the thermoregulatory centers blood block flow into veins close to the cool skin surface and reroute it into vessels called venae comitantes, which run deep under the skin, right next to arteries. The result: blood returning to the heart and lungs is warmed by the outgoing (countercurrent) arteries, preventing cold blood from penetrating the body core.

- Piloerection.

Triggered by contraction of the smooth muscle attached to the lowest point of each hair follicle, the haircoat straightens up so it 'stands on end'. This creates a larger insulating pocket of air between the skin surface and the cold cruel world.

- Circulation shunts.

Protection of extremities is another strategy by which the horse fends off cold temperatures. For example, although the horse's body lacks a mechanism for increasing blood flow to the feet, it has developed a system to prevent them from freezing.
What little warming blood flow reaches the foot is normally diffused (and thereby cooled) in the capillaries that serve the foot cell's limited metabolic needs. When the body's thermoregulators get a message that the feet are too cold, direct shunts open up in the feet so that blood flows from the smallest arteries directly into larger veins, called venules, without passing through the capillaries. After the feet have warmed sufficiently, the shunts close again to restore nutritive capillary flow. Shunts are also used in the tail and ears. Another means of warming critical exposed areas, such as the muzzle, is to open more subsurface blood vessels to compensate for surface losses.
Stallions have an additional vulnerable 'extremity', the scrotum. Normally exposed so it can maintain a slightly lower operating temperature for optimum fertility, this nearly hairless organ is protected against winter weather by a muscle called the dartos, which 'puckers' the scrotum up against the body under cold conditions.

- Heightened metabolism.

As cold continues to stress the body, the thermoregulatory centers turn their attention to generating more internal heat, sending out messages to the adrenal glands to boost core metabolism.
Nerve impulses signal the adrenal medulla to release epinephrine and norepinephrine, neurotransmitters that raise blood pressure and heart rate and increase metabolism by stimulating the release of free fatty acids and the breakdown of glycogen. At the same time, the hypothalamus spurs its adjudant, the pituitary gland, into action, ordering the release of large amounts of adrenocorticotropic hormone (ACTH) into the bloodstream.
Arriving in the adrenal cortex, ACTH triggers the production of cortisol, a steroid that increases the body's heatgenerating metabolism of fat, carbohydrates and protein. The result: a warmer, happier horse.

Acclimatization: gearing up for winter

Short-term measures can warm a horse through a cold snap, but because many acclimation responses - especially the rapid increases in metabolism - tend to drain energy stores, they can't be sustained for a whole season. To avoid exhausting himself in an effort to keep warm, the horse needs an energy-efficient means of generating and retaining heat over long periods of time. At the same time, whatever process prepares him to whitstand colds has to be reversible when the warm weather returns. Fortunately, there is such a mechanism of seasonal adjustment to temperature change: it's called acclimatization.

The horse's acclimatization for cold actually begins long before winter. Just after the summer solstice (around June 22), receptor's in the horse's eyes - and possibly elsewhere in the body - detect the incremental shortening of daylight and relay the information to the pineal body, a primordial organ in the brain. (Even blind horses experience acclimatization changes, suggesting that other receptor points may be modulated through the pineal body).
These subtle hints of coming winter trigger the release of hormones that shift the haircoat from its resting phase into a growing phase. Inside the follicles that house the horse's thin, short summer hairs, thick, long winter hairs begin to grow, pushing the summer hairs ahead of them. If you look closely at your horse, you can see them peeking out in late August. By late September or early October, the winter hairs begin to evict the copious summer hairs from the follicles. The result is shedding.
During the fall, ambient (surrounding air) temperature determines how long and thick the horse's winter haircoat grows. If he is exposed only to warm air - as occurs in southern climes, or when he is blanketed or kept continuously in a warm barn - his winter coat will grow in only slightly heavier than his summer coat. On the other hand, if he's exposed to extreme cold during this time, his coat will be correspondingly thick and long.

Ambient temperature continues to influence the weight of the coat until the winter solstice (around December 22), after which date the lengthening daylight hours trigger the first summer hairs to begin growing in the follicle, and the winter pelt can no longer adjust to climate changes.
Your horse's winter coat puts your best winterwear to shame. Its longe, dense, fine 'pile' is interspersed with longer, bristle-like 'guard' hairs that prop up his fur, creating loft within a thick layer of body-warmed, still air next to his skin and greatly reducing cooling from radiation, convection and conduction. The downward tilt of his hairs deflects falling raindrops and snowflakes before they reach the skin - where they would otherwise conduct huge amounts of heat from the body - and directs them to the hair tips, from which they fall harmlessly to the ground. That's why your horse's skin often remains dry even in moderate rain or heavy snowfall.
And, finally, the thick haircoat makes an excellent windbreaker.

As the temperature drops, the horse's appetite (and hence his caloric consumption) increases, boosting heat-generating digestion and metabolism.
Mother Nature helps the progress along by ensuring that the grazing horse puts on a few pounds in the fall. Among feral horses, this weight gain comes primarily from increased consumption of dry matter as grass dries out, but it may be boosted by the serendipitous discovery of such fattening goodies as wild rye and wild oats, which go to seed as winter approaches. The extra fat layer requires little energy to sustain, has few heat-radiating cappilaries within it and insulates well.
On the cellular level, heat-generating metabolism is also nudged up for the winter, though in a far less dramatic and taxing manner than occurs with the short-term metabolic changes of acclimation. As cold sets in for the long term, the hypothalamus signals the pituitary to release thyrotropin or thyroid-stimulating hormone (TSH). Reaching the thyroid gland in the neck, TSH triggers the release of thyroid hormones that slowly boost metabolism for the long haul.
The impact of all these changes is a marvel of thermoregulation. Efficiently generating more heat while increasing his layers of insulation, the acclimatized horse has greatly improved his ability to tolerate cold. It will take a really severe cold spell to force him to resort to fuelburning, emergency warming responses like shivering. The acclimatized horse is so ideally suited to the frozen tundra that - given food, unfrozen water and minimal shelter - he can thrive in temperatures as low as minus 40.

Brave the outdoors on a frigid day, and, as you flap your arms and stomp your feet in a silly-looking effort to keep warm, you may question your decision to leave your horse in the elements. You needn't worry. Your horse is a cold-weather marvel, whose aptitude for staying cozy in breath-stopping temperatures far outstrips your own. Thanks to a collection of heatgenerating and insulating mechanisms, he'll be just fine.
You, on the other hand, should probably get back inside.