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
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 (-10 to 15 degrees celsius). 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.
Radiation - the transfer of heat from the body to surrounding cold air.
Conduction - direct contact with cold substances (such as snow, ice, mud
or water) that causes heat to be wicked away from the skin. The impact
of conduction on body temperature is significant, as researcher Paul
Siple found back in 1939, when he showed that, at 0 degrees Fahrenheit
(-17c), a 40 mph wind produced a cooling effect on the humane body
equivalent to a temperature of minus 55 (-48c). (Humans shed heat much
more easily than horses, however, so the effect on our four-legged
friends may not be as great).
Convection - the action of wind or cold drafts to hasten radiation cooling.
Normal heat shedding - heat loss incurred through bodily functions, such
as urination, defecation or respiration.
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:
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.
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.
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.
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
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
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.