Biology

What causes dappling in horses?

Quick answer: depends on the horse (also nobody knows). Yeah, not very useful, I know, but true: the reason for a horse’s dappling may be in its nutrition, coat quality, color genetics, and many other factors, not all of which are not quite well understood, and all of which may act in tandem. I was curious about it and went to try and figure out what makes some horses dappled while others aren’t.

First, we should define what dapples are. Dapples are irregular spots that may appear in a horse’s coat in a different shade than the surrounding hair. These aren’t like the leopard complex spots (as found in the Appaloosa) and may appear and disappear throughout the horse’s life. They give horses a very beautiful look, but it’s not all horses that have them — some will only have them for a brief period of time, during summer, some will never show them.

Highland
Dapples. (c) Christine Sutcliff, 2010.

The mechanics of the dappling effect are a mystery. They’re more common in grays, for good reason: as the horse grays out, some hairs might appear lighter or darker than the others around it, as they grow and whiten.

They do appear in other horses, however, although not as frequently or permanently. Some horses will only show dapples in summer, others only in winter; some will show them regardless of nutrition, others only when in excellent condition. Some will never dapple, ever, which suggests a genetic predisposition there, although it’s not yet know what, exactly.

There are some hints, though, so let’s check them out.

The silver gene

The “silver” gene, often known as silver dapple, is a relatively rare coat color dilution. It acts exclusively on the black hair pigment (eumelanin), so chestnut horses may be carriers without ever expressing it, or expressing so minimally it’s unnoticeable.

In bays and blacks, however, it dilutes hair color to a chocolate brown (sometimes referred to as sepia), which often shows dapples (hence the name). It affects mane, tail and lower leg hairs in special, with the mane and tail often becoming white or a golden-brown (flaxen).

In spite of being known as “silver dapple”, not all silver horses do show dapples or dappling all the time (or ever), either. They seem to be separated dynamics, as with all other colors, but do happen frequently in horses with this coat color.

Intimate Moment
A silver dapple pony and rider. Notice the clear dappling on the hindquarters. (c) Martina Vanelli, 2008.

Silver dapple is an uncommon color, but some breeds do show it frequently. One example is the Rocky Mountain Horse, an American breed that’s popular for its silver dapple coat (although it can be other colors as well). Unfortunately, as it often happens with genetic mutations, the gene is also associated with multiple congenital ocular anomalies (MCOA) in Rocky Mountain horses, although some breeds with this color do not seem to show.

Rocky Mountain Horse
Rocky Mountain Horse, (c) Olav Cresp, 2017.

Gray horses

Gray horses do dapple quite often, though some may skip the dappled phase altogether, and some might never leave the dappled phase. The exact mechanics of it remain a mystery, but the reason behind the dappling is uneven lightening of the coat colors, with some hairs keeping a darker color than others, causing the dappled effect.

20020721 Raycito, Arabian stallion at a national show in Munkedal, Sweden
Dappled gray. (c) Josefine S., 2012.

The sooty effect

Sooty is when a horse has darker hairs, usually black, mixed in with the usual coat color. These usually grow along the topline of the horse (back, shoulders, neck) and give a countershading effect to the coat, which can be quite attractive.

While sooty is almost certainly genetic, its mechanics is not fully understood yet. This can be very minimal or so extensive the horse almost looks another color altogether. Some believe very dark bays and chestnuts may have a sort of sooty variation, although that’s not proven and may not be the true cause of the coat’s darker color.

Horse Dappled Sooty Buckskin Stallion
Sooty on buckskin. Notice the dapples. (c) LouB213, 2015.

As of today, there is no test for the genetics of sooty, so it’s only visual inspection that will tell whether a horse does or does not have it.

In any case, the sooty effect does have a tendency to bring out the dapples. It’s not a guarantee; some horses may have sooty and no dapples. But they will more likely to have them, possibly due to the countershading effect already present with the darker hairs.

Other dapples

The real mystery is when dapples appear in horses that are neither gray nor silver dapples. There is some evidence linking dapples to capillary arteries near the surface of the horse’s skin, as shown through thermal imaging. As the hairs heat differently, dapples appear — a link that might also extend to exposure to sunlight, as some horses only dapple during the summer.

Napoleon - Palomino part Saddlebred Stallion
A dappled palomino in summer. (c) Paula van der Riet, 2012.

Other possible causes are hormonal changes throughout the horse’s life. Some might show dapples only in their youth, while others might show them only later in life (in fact, sooty also tends to become stronger with age). Some mares will dapple when pregnant, and not at any other time.

While not all horses will dapple, one seemingly clear link is fitness. Horses with a predisposition for dapples will show them while in excellent health, if they have a good balance of trace minerals and omega-3 fatty acids. This provides a good coat quality, and thus, makes the dapples clearer. Though there seems to be a clear connection between good nutrition and dapples, this is not always true, as some horses will show them even when malnourished or sick.

Curiously, palominos seem to be especially prone to them, although not all palominos will have them — so that isn’t a singular factor, either.

In essence

There are many possible ways a horse can develop dapples (genes such as silver and sooty, nutrition, grooming, just ’cause), and the mechanics are not truly well understood, so we cannot predict whether a horse will or will not have dapples. What is certain is that they look very beautiful, and can be a striking visual effect in those lucky ones who have them.

Unfortunately, I wasn’t able to find more papers on the matter (did find many interesting subjects for future posts, though) and had to prowl through less than scholarly sources.

If you know more about this subject, if I’ve missed something or got anything wrong, please let me know in the comments below!

References

E Sild, S Värv, H Viinalass. The occurrence of silver dapple in horses. Veterinarija ir zootechnika, 2012.

S Kaps, BM Spiess. Multiple congenital ocular abnormalities (MCOA) in Rocky Mountain horses and Kentucky Mountain Saddle horses in Europe. Pferdeheilkunde, 2010.

S Rieder. Molecular tests for coat colours in horses. Journal of Animal Breeding and Genetics, 2009.

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