Horses are nowhere near as efficient as people in staying cool. It's all because of their body shape and their much greater percentage of heat-generating muscle.
The difference is most apparent in hot and humid conditions, when a horse's rate of heat build-up will be about three and a half times greater than that of a human.
"For us as caregivers to horses, it is critically important to recognise these very high rates of heat storage," says Dr Michael Lindinger, from the Department of Human Health and Nutritional Science at the University of Guelph, in Ontario, Canada.
"Even though we may perceive that warm, humid conditions are not producing undue stress or strain in us, they may certainly be resulting in strain in horses," he says.
The higher rates of heat storage in horses result from a greater proportion of contracting muscle in horses - some 40% of body mass - compared to about 20% in running humans.
Also, horses have a lower skin surface in relation to their size than people, meaning their ability to dissipate heat is greatly reduced compared to humans. Horses have only 40% of the surface area to body mass as people do.
"Horses are capable of producing large amounts of heat at high rates but, compared to humans, are a severe physical disadvantage in dissipating that heat."
Lindinger, whose particular interests lie in the regulation of ion transport across cell membranes and the regulation of body fluid balance, was amongst speakers at the fourth European Equine Health and Nutrition Congress in the Netherlands, where he talked about challenges faced by performance horses in staying cool.
Dr Michael Lindinger
"When heat stress periods are short the thermoregulatory abilities of the horse easily cope despite high rates of heat storage," Lindinger says.
"However, when body heat storage is prolonged, such as can occur during prolonged exercise or transport, dehydration ensues due to sweat losses of water and electrolytes.
"If continued, heat stress can develop into heat strain, which severely compromises health and well-being, and may be life-threatening."
Horses, he says, stay cool in the same way we do. While some heat is lost through respiration, the key mechanism is through the evaporation of sweat from the body.
Blood vessels dilate near the skin in an effort to shed heat, in a process controlled by the hypothalamus and the skin, which stimulates the cooling process when a rise in blood temperature is sensed.
The conversion of chemical energy in a horse to the mechanical energy of locomotion is, at best, 20%, he says, with much of the remainder being turned into heat.
When the intensity of exercise is high, the duration is limited by the quick onset of fatigue and the exercise stops. This normally results in a natural prevention of heat strain.
The biggest risk comes in the form of dehydration and heat strain resulting from prolonged periods of low intensity exercise.
Performance horses are regularly placed under heat stress. © Horsetalk
Research has shown that the wetter the skin, the lower the sweating rate - an effect seen in both horses and humans.
The highest sweating rates found in a 1995 study indicated a dehydration rate of 12 litres of body water per hour for horses, although sweating rates of up to 15 litres an hour have been recorded.
The peak sweating rate in horses is put at 20 litres an hour, compared to 3 litres an hour in humans.
With clinical dehydration defined as a dehydration of 5% of total body water - that's 15 litres for a 450kg horse - it is clear riders need to put in place strategies for preventing dehydration from becoming severe during prolonged exercise.
Adding to a horse's problems, its sweat has been shown to contain much higher concentrations of ions (electrolytes) than that of people.
"Therefore in horses," Lindinger says, "the combination of high sweating rates and high sweat ion concentrations contribute to very high rates of ion losses during periods of heat stress.
"These losses need to be directly addressed if we are to sustain a state of high performance in equine athletes."
Strategies for helping thermoregulation
The key strategy for preventing a drop in performance either during or after periods of heat stress is one that prevents dehydration. This, says Lindinger, can only be accomplished by timely and adequate provision of balanced electrolyte solutions to replace both the water and electrolytes lost through sweating.
It's much harder for horses to stay cool than humans. © Dirk Caremans
Further strategies include finding shade, exposing the horse to wind or fans, or repeated application of water.
"Based on the sweat data, it is clear that prevention of dehydration during exercise requires ingestion of about 15 litres of water an hour with an appropriate mixture of electrolytes.
"This is indeed a daunting task," he says, "but we have observed elite endurance horse/rider combinations to be successful in achieving this over 100-mile (160km) distances."
It is crucial, he says, to provide an appropriate balance of water and electrolytes.
Provision of water alone results in a dilution of already-depleted electrolyte solutions within the body fluid. Kidneys read this ingested water as an overload, resulting in renal excretion of more water - taking with it even more electrolytes. In other words, a horse will become even more dehydrated when drinking only water.
Electrolytes in too great a concentration - in the likes of a slurry or paste - are equally troublesome. This causes the horse's body to initially direct water into the upper intestinal tract to dilute the electrolytes, thus dehydrating the animal further.
What, then, constitutes a good electrolyte? Lindinger says the provision of about 6% of dextrose (D-glucose) will help in the uptake of sodium and chloride ions, while around 2% of fructose may help in the intestinal absorption of potassium - as well as improve the solution's palatability to the horse.
All electrolyte salts should easily dissolve in water - those that do not will most likely travel through the digestive system and be passed out the manure.
Calcium and magnesium should be given as calcium or magnesium lactate, acetate or citrate, and not as oxalate or carbonate (limestone and dolomite).
Some sucrose and flavouring agents can be added, he says.
Endurance events challenge horses, riders and grooms.
The amount of D-glucose is negligible in terms of meeting the energy demands of exercise and is present only to help with the intestinal uptake of water and electrolytes.
The highest drinking rates appear to be achieved when the solution is about 20degC.
"Getting a horse to drink a balanced electrolyte solution can be a challenge," he adds.
Most successful riders have spent a lot of time training their horse to drink electrolyte solutions, he says.
"Initially, the horse can be weaned on to a dilute electrolyte solution made by dissolving a small amount in the drinking water, and this provided as the sole drinking choice.
"The concentration of the electrolyte solution can be gradually increased over a period of weeks.
"When riding longer distances, and a rest break for drinking is desirable, similarly prepare an electrolyte solution and remain until the horse has taken a drink. The horse will learn that the activity will continue once a drink has been taken.
"Clearly," he says, "patience and diligence are required on the part of the rider."