Horses and Cold

In “How Horses Cope With Cold,” Heather Smith Thomas says:

Horses handle cold weather better than humans do; equines evolved in the cold climates of northern Europe and Asia. Their natural “comfort zone” (energy-neutral temperature zone, in which they don’t need to expend extra energy to maintain normal body temperature if weather is not wet or windy) is from about 15 to 60 degrees Fahrenheit. The horse’s body is better at creating and conserving heat in cold weather than dissipating it in hot weather. … Horses have a normal body temperature of about 100 degrees Farenheit (38 degrees Celcius). They maintain this temperature in cold weather with the help of several mechanisms which include shivering, changes in hormone levels, increased body metabolism, increased digestion of fiber (adding more fiber or more protein to the diet can help a horse keep warm, since digestion of these nutrients produce heat), growing longer and thicker hair which can stand up on the skin to make a layer of insulating air pockets, increased feed consumption, and increased activity. Cold horses on a frosty morning often run and buck to warm up. … If the horse gets cold, the blood vessels in his skin constrict to minimize heat loss, and the hair shafts stand on end for better insulating. If he continues to be cold, he starts to shiver, with his muscles rapidly contracting and relaxing–which quickly raises his metabolism rate and amount of fuel burned in the muscles. With his large blocks of muscle, the horse can shiver much more readily and more comfortably than a human. Since most of this muscle action is being converted to heat, this is a very effective way to warm himself. It takes a great deal of energy, however, to shiver for a prolonged period; this can use up his energy stores.

Read the rest. Good stuff. I’d have like the essay better if there were references to actual scientific research and articles at the end. The last sentence and paragraph is fascinating; it’s all physics, chemistry, and their application in biology: the theory of insulation; the theory of thermodynamics; the law of conservation of energy; the conversion of chemical energy to mechanical energy to heat. This is a topic I’ll need to look into more, to see how this is all consistent with science and evolution. Update (9:30 PM):  Note that it is the air trapped in the horse’s hair that provides most insulation (air is a poor thermal conductor when no convection takes place). We use the same principle in double-paned windows, and in dressing in layers. Update (10:43 PM):  In “Cold Weather Horse Care Review” (TheHorse.com, January 05 2010, Article # 15582), Liz Brown writes:

Water is essential for regulating body temperature, said Greg Meyer, extension educator for large animals at Ohio State University. “We typically think of keeping horses cool with water, but water is required for energy for keeping them warm as well.” Copyright © 2010 BLOOD-HORSE PUBLICATIONS. All rights reserved.

And in “Cold Weather Nutrition” (TheHorse.com, February 01 2008, Article # 11409), Kimberly Peterson, DVM, writes:

Generally, horses at rest in ambient temperatures of 70°F consume 2% of their body weight in roughage (hay) per day. A 1,100-pound horse will eat approximately 22 pounds of hay per day. Assuming an energy density of 1.0 Mcal/lb, which is typical of many hays, this equates to approximately 22 Mcal or 22,000 Kcal. Roughage in the diet is the main source of heat for the horse. The bacterial fermentation of fiber in roughage, occurring in the large intestine, results in the majority of heat produced during digestion. … Added fat in the form of vegetable oil is an efficient calorie source at 4 Mcal/pound (four times the energy of hay). Up to 12% of the day’s total calories in the form of fats and oils is well-tolerated. While fat digestion releases almost no heat, it provides calories for energy and the maintenance of body condition (1 cup vegetable oil=2 Mcal). Copyright © 2010 BLOOD-HORSE PUBLICATIONS. All rights reserved.