Wildfires have burned more than 400 square miles in Texas since Saturday evening, and the editor of a weekly paper in Jeff Davis County told the Associated Press that he'd seen "horses on fire, buildings on fire, houses on fire." Wait, can a horse really catch fire?
Yes, under the right conditions. Like humans, a horse's body contains significant amounts of highly combustible fat. Yet, despite the fact that fat accounts for between 5 and 10 percent of a horse's body weight, you can't take a match to an appaloosa and expect it to catch fire. First of all, the skin acts as a barrier, insulating the horse's fat from the heat of the flame. Skin is difficult to ignite, and usually chars in the presence of heat. It takes up to 10 minutes of direct flame for skin to crack open and start leaking out rendered fat.
Even if a horse's body started to leak fat, it would be unlikely to go up in flames on its own. That's because animal fat is a lot like candle wax: It needs a wick to burn. (Fat and wax happen to have nearly identical heats of combustion. In fact, medieval candles were usually made of rendered fat.) You can't ignite candle wax without the aid of some other, porous material. The wick soaks up the wax at a steady rate and exposes it to the heat source. This parceling out of the fuel allows it to vaporize and burn.
A wick doesn't have to be a strip of cotton. It can be any substance that maintains a porous structure in the presence of massive heat. Just as the wick exposes the right amount of fuel to the heat, the fuel protects the wick from incinerating. (A candle wick wouldn't last very long without the wax.) In the case of a burning animal or human, the wick might be a carpet (PDF), a wool blanket, or a pile of leaves—anything that can soak up rendered fat and help the body burn.
That's not to say that, without a wick, animal flesh is flame retardant. There's a difference between charring—burning a substance by applying external heat—and ignition. As any crematorium demonstrates, exposing a body to a jet of flames for two or three hours will reduce it to ash. But if the flame is turned off at any time, the body will smolder, and the flames will quickly extinguish.
Nor is it impossible for a live animal to catch fire momentarily. Hair or fur can ignite, but they provide so little fuel that the fire would last only a few seconds—not long enough to compromise the integrity of the skin.
All of this helps explain why fire investigators usually find victims' bodies charred on the outside, but otherwise relatively intact. Even if flames managed to penetrate a victim's skin and start combusting his subcutaneous fat, his body probably wouldn't be incinerated. The flames would come up against several more layers of tissue that aren't likely to catch fire—like muscle and ligaments. A few cases of fully-cremated accident victims have given rise to the widely discredited mythology of spontaneous human combustion.
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Explainer thanks John D. DeHaan of Fire-Ex Forensics and author of Kirk's Fire Investigation.