I've been told by several ride vets that it's the ratio of body mass to

surface area. The determining factor tends not to be the amount of energy

produced by the animal but, it's ability to keep the body cool. A horse

being

15 hands tall and weighing 1200 pounds will probably have the same surface

area of a horse 14.2 at 850 pounds.

Nope. The 1200 lb horse has a body surface area of 6.817 square meters, or

.00568 square meters of cooling area per pound of body mass. The 850 pound

horse has a body surface area of 5.411 square meters, or .00636 square

meters per pound of body mass. All other things being equal, the smaller

horse is 12% more efficient at cooling himself, and in all likelihood, has

less heat to dissipate to begin with.

Another factor in the equation is the

type of muscle that predominately composes the horses anatomy.

True, but if you're referring to the Tevis data, 95% of the horses had 50%

or more Arabian blood, generally of very similar muscle fiber type

populations, so that was effectively cancelled out as a variability.

Susan G