This post first appeared on May 26, 2012
Images of leopard appaloosas with Dalmatian dogs are always eye-catching. Certainly they can look quite closely matched, like this Polish Malopolski and his buddy. Even so, the patterns in the two species are very different in terms of what is really happening to the pigment on the animal. That’s probably off in the weeds for most owners and breeders, but for artists the distinction is actually pretty important.

This touches on one of the reasons why artists who develop an interest in horse color often have such a different perspective. Usually the kind of information a breeder needs is predictive. That is, they need to know what might likely result from crossing this to that, or what they might need to cross if this particular end result is what was wanted. What artists need to know isn’t about prediction nearly so much as it is about possibilities. Not so much what might happen, but what could happen – even far-out-there, not-very-likely, could happen. That’s because artists often want to add something for interest or for composition. For those producing realistic art, that has to be done within the constraints of what is possible. It doesn’t necessarily have to be likely, but it does have to be possible. This unique perspective became apparent to me a number of years ago when I gave my first presentation on horse color. In the question and answer period afterwards, someone in the audience asked if a horse could be both dappled and fleabitten at the same time. It was clear that was not the sort of question my fellow presenter, Dr. Sponenberg, often heard. But it is precisely the kind of question that equine artists ask all the time. Scientists might not notice this kind of detail on an individual horse, but for someone who paints horses, this kind of information – does this happen with this? – has a lot of practical value.

So why do artists need to understand the process behind appaloosa patterns? Spotting is a useful tool, because it breaks up positive and negative space. It makes the horse more visually interesting. If you are particularly clever, it can be used to draw the eye in a way that works with the composition, or to hide flaws. But spotting doesn’t just happen anywhere. It follows rules, and those rules depend with what is happening with the pigment. Understanding the underlying mechanism makes it far less likely that you’ll add some interesting detail that isn’t realistic. When dealing with rare combinations of colors and patterns, it might be difficult to find a reference image to consult. Knowing the process can tell you if there is a reason to bother looking in the first place, because it tells you what is possible. (And when you wing it without a reference, the knowledge will make for more reliable guesses.)

The image above is a ceramic collectible with the kind of spotting (often called cat tracking) seen in homozygous tobianos. It’s a really popular effect. In my normal job (the one I have when I am not trying to get a horse color book to press), I have produced quite a number of these. The problem comes when this gets confused with leopard spotting, and most especially what happens when leopard patterns are combined with the tobiano pattern. That brings us back to the image at the beginning of the post. These three images – the leopard, the Dalmatian, and my ceramic foal –  represent three very different scenarios in terms of the underlying process. I want to take each, one at a time, and explain how they are different despite looking so similar.

This is the typical nose-to-toes kind of leopard. Most people would think of this as a white horse with black spots that have been superimposed on top. That’s not really accurate. From a genetic standpoint, this kind of horse is a two-step process. First she has inherited a gene that progressively adds white hairs to the coat. Those hairs, over time, are going to produce the fairly distinctive pattern known as varnish roan. If the pony in my illustration just had that first gene, she would look like a black version of this pony.

That first gene, known as Leopard Complex, sets things up for leopard but it doesn’t make leopard patterns itself.

That happens when the horse inherits a separate patterning gene in addition to Leopard Complex. In this case, that patterning gene is called Pattern1. What Pattern1 does is take the white from Leopard Complex and amplifies and organizes it. So while our horse looks like she is white with spots, it is perhaps more helpful to think of her as a horse that was roan, but Pattern1 has now taken that mixture of white and dark hair  and reorganized it. Underneath the white hair, what that horse may look like is closer to this.

This is what the underlying skin looks like. She probably does have some truly white skin in the area where a blanket pattern would go. Pattern1 does amplify the white, after all. But under it all she isn’t really a white horse, at least not in the sense that most people would think of as true white. She is more like a roan horse that has been modified a bit. That’s why even nose-to-toes leopards have faces that are shaded much more like a grey than a cremello, because for the most part the face has dark skin, not pink skin. And that is why a pintaloosa looks like this:

The true white areas of the tobiano pattern cover over the appaloosa pattern. The spots from the leopard pattern don’t spread over onto the tobiano pattern because the process with Pattern1 isn’t “add dark spots to the white”, it is “organize the roan into spots.” So the spots don’t happen where the tobiano pattern already took all the roan away. Without the color there in the first place, Pattern1 has nothing to work with.

Of course, if we moved our tobiano pattern out a bit, encompassing more of the dark skin and butting it up close to the “blanket” skin, we could probably get something that looked a bit like the leopard spots migrated over some of the tobiano.

Even so, the spotting is still concentrated in such a way that shows it is an appaloosa pattern with a tobiano pattern layered over the top of it. The spots on the flanks might look like they are in the tobiano white, but really they are just in an area that was already white from the action of the Pattern1 gene. The action is still the same. The tobiano is there adding true, pink-skinned white on the horse, and underneath it Leopard Complex and Pattern1 are just doing their thing.

Even with the tobiano bumping up to the pink-skinned areas of the leopard pattern, it still looks different from the kind of spotting that comes from a horse having two copies of the tobiano gene.

This is a (presumably) homozygous tobiano with cat tracks. Whereas Leopard Complex is a roaning process that Pattern1 takes and organizes into the leopard pattern, this type of spotting is more like a not-entirely-successful attempt to add some more color to a horse that already has large patches of color. Unlike the existing spots, which are large and opaque, these new spots are small and vary in opacity. Some just come through in specks.

Cat tracking tends to cluster around the existing spots to some extent, almost as if these new spots want to occupy the same general area as the existing spots. This is quite different from the spotting on a leopard, which tends to be dispersed across the body.

The exception is the hooves. Tobianos with cat tracks often have a concentration of spots around the coronary band, often turning the hoof completely dark or nearly so.

There are spots on the legs, but typically they are not as numerous as the ones around the feet. The same is true for the face. This is the face that goes with these feet. He does have a few spots in his blaze, but they are not extensive.

So how is this different from the Dalmatian? Well he really is a white animal with colored spots added on top. In dogs, the gene for this is usually called Ticking, but since ticking means something different in horses, I am going to use the older English term for the pattern, which is belton. This post has run really long, so I’ll split that over into a second part. And why delve into the belton pattern in dogs? Because lately there has been a handful of horses that have turned up that just might have that kind of spotting. At the very least there are horses with dark spots inside their white markings that are not tobiano cat tracking and not leopard patterning. More on those will appear in the next post!