Weird But True Horse Facts: Hot & Cold

Today we learn weird facts about thermoregulation and the effects of global warming.


Global warming shrinks horses.

During the Paleocene-Eocene Thermal Maximum about 56 million years ago there was a massive carbon release into the atmosphere with about 1/3 of all mammals shrinking in response, because smaller size helps animals regulate their body temperatures when the environment is hotter. The earliest-known horse, Sifrhippus, during this time period averaged in at 12 pounds and was about the size of a miniature schnauzer. [source]

During another 5 to 10°C bump in global temperatures around 130,000 years ago, ancient horses’ body mass shrunk by about 30 percent. When the climate cooled back down, horses grew by about 76 percent, eventually reaching their current size. [source]


An illustration of the earliest-known horse, Sifrhippus, dwarfed next to a modern horse. Credit: Danielle Byerley, Florida Museum of Natural History.

An illustration of the earliest-known horse, Sifrhippus, dwarfed next to a modern horse. Credit: Danielle Byerley, Florida Museum of Natural History.

Horses have something called a guttural pouch to keep them cool-headed.

The guttural pouch is a strange, fist-sized cavity in the horse’s skull, slightly behind the ears. Officially, there are two guttural pouches, one for each eustachian tube, though it is generally viewed as one structure. For a long time no one understood its purpose, but Dr. Keith Baptiste, while a graduate student at the Department of Veterinary Internal Medicine at the Western College of Veterinary Medicine, came up with a solution. He theorized the guttural pouch might play a significant role in cooling a horse’s arterial blood and protecting the brain from hyperthermia.

After extensive testing, Dr. Baptiste concluded, “At rest the air temperature in the guttural pouch was even a little warmer than the horse’s core temperature. But during exercise, it maintained a nice, even temperature of about 37.5° Celsius (99.5° Fahrenheit). In exercising horses, the blood definitely cooled as it ran through the air space of the guttural pouch–from about 39.5°C (103.1°F) at the place where the internal carotid artery enters the pouch to about 37.8°C (100.04°F) at the exit point when the horses were cantering.” [source]

Horses increase blood flow in their hooves to keep warm.

Arteriovenous anastomosis shunts inside the hoof capsule change the direction of blood flow to aid in cooling or warming the hooves. In cold weather, the shunts inside a horse’s hoof will allow increased blood flow and therefore maintain heat.

If these structure become damaged – especially in horses prone to insulin resistance/equine metabolic syndrome – hooves can become sensitive during extreme weather. Eleanor Kellon VMD had this to say, “Evidence supporting this theory was obtained by an owner of a horse with a history of repeated bouts of winter laminitis. She took her horse to her veterinarian’s clinic for thermographic examination. Thermography measures the surface temperature of the body. The horse was placed in a room with an air temperature of  40° Fahrenheit. After removing leg wraps and lined hoof boots, the temperature of the front feet, which had been repeatedly affected by laminitis in the past, dropped considerably lower than the temperature of the hind feet.”

Dr Kellon VMD teaches on-line courses on horse nutrition and health. A link to her website can be found here.

Got any “Weird But True” horse facts you want me to investigate? Be sure and give me a shout in the comments! Special thanks to Haley Macdonald Johnson on this week’s tip about guttural pouches!

Go Riding.

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