When it comes to understanding dappling as a pattern of light and dark, the key is temperature.
Darkly dappled greys like the Eriskay Pony above are helpful in understanding why the various dappling patterns all have a consistent look. That's because grey horses first begin to lose pigment where they are warmest.
That is probably why some horses grey out rapidly along the area where their mane normally covers their neck. As anyone with long hair knows, the area underneath a lot of hair can get pretty warm on a summer day!
In the case of dapples, what is causing the heat are clusters of capillaries under the surface of the skin. In fact, thermal images of horses make it clear why dappled greys look the way they do.
Infrared thermography is used in equine medicine to detect injuries and inflammation. The cameras use sensors to pick up the surface temperature of an object, which is then translated into a color map. Purples and reds indicate the hottest areas. As the colors approach blue and black, the temperature is cooler.
A thermal image of a horse has a familiar look to most equestrians. When inverted and switched to grayscale, it looks just like a dapple grey pattern.
Other forms of dappling do not seem to be caused by differences in surface temperature in the same way as grey. However, the temperature map does correspond with the proximity of blood vessels, and those are thought to be related to dappling of all kinds. The more extensive forms of dappling often follow the outline of blood vessels along with the clusters of capillaries. I'll save that for a future post since I have a lot of photo examples.
I mentioned in yesterday's post that this is my husband's field of science. Knowing some of the common misconceptions about the equipment, I should share one important caveat when looking at thermal images. Infrared cameras only measure the surface temperature of an object. In the thermal image above, the horse's halter shows up primarily as black. That means that the surface of the tack is registering as cool. The area could still be warm, but the camera cannot "see" the temperature of the skin underneath the halter. In the same way, the mane will likely register as cool, but the horse might be quite warm and sweaty underneath. (That's why thermal images do not prove that leaving hair long keeps an animal cooler, by the way.)