We all know horses have a high tolerance for cold. They have several different mechanisms for accomplishing this.
Starting from the outside in, in cold weather your horse’s coat acts like a down jacket. The fluffy undercoat stands on end to form an insulating layer trapping heat against the skin. This is how the horse can be covered with snow in a storm and not have it melt. The long outer coat helps protect the horse’s “down” from moisture that robs it of the insulating effect.
Blood is also diverted away from the skin surface and extremities to protect against heat loss. In the highly vascular feet this is facilitated by the presence of arteriovenous shunts. The shunts direct blood away from the tissues and directly back to the venous side for return to the body. Periodic opening and closing of the shunts keeps the tissues viable with as little blood as possible.
Inside the horse, most know that the horse’s cecum and colon (the large intestine) function like an internal furnace. The “heat of digestion” is energy given off as heat during the fermentation of fiber and other complex carbohydrates that cannot be digested in the small intestine. This is why horses should always have generous amounts of hay in cold weather.
Shivering is another automatic response to cold that generates heat. When fuels are burned for muscular activity like shivering there is also some energy lost as heat. This is called the “heat of metabolism”. Horses also have increased levels of thyroid hormone in the winter. Higher thyroid hormone levels increase the heat of metabolism by making metabolism of fuels less efficient with more energy lost in the form of heat.
These mechanisms combine to keep most horses comfortable in cold weather but they are not foolproof. Do your part by making sure horses always have a place to shelter away from wind, snow and rain. Remember too that ill, thin, very old or very young horses have less tolerance for cold and may require extra help like housing or blankets.
Eleanor Kellon, VMD