Manchado is an extremely rare pinto pattern that has – to date at least – been found only in Argentina. Ever since I first began talking about the manchado pattern, that fact has caused many people to ask if I think there is essentially “something in the water” down there in Argentina that makes that strange color. Some assume that since the pattern is constrained to just that one country, it must be environmental. I am skeptical that there is something unique to any one country that can create such a dramatic pattern. Countries are, after all, human constructs. Why in Argentina, and not Brazil? Or Chile?
To me, it seems more likely that geographic constraints on a color are about founder effect, and not something strange in the Argentinian atmosphere. To put the founder effect in layman’s terms, it simply means that when you start a new group with a small set of animals, the quirky aspects of that specific set of individuals skew what happens later when the population gets bigger. The Argentinian love for odd coloring, for instance, meant that what was brought there was louder on average than the founders used in other countries. (Late nineteenth century literature is full of references to colored horses being bred for the Argentinian market.)
And the breeds where it has occurred aren’t as disparate as one might think. Breeds are more distinct from one another now, but they really were not as recently as 100 years ago. With the possible exception of the Arabian (and even that is open for debate), almost every Argentinian breed on record as having a manchado cannot rule out the use of local mares. At the turn of the last century, top-crossing – that is, the use of males to determine the “breed” designation – was the way things were done almost everywhere, and most certainly where horses of a specific type had to be imported. If a color was in the local mare population, and was not intentionally bred out, then it could spread pretty wide. That would be doubly so if the pattern was recessive.
And the individual horses we know of are not all unrelated. The Hackney stallion that the pattern in the above illustration is based on is the grandson of one of the known manchado mares. Both have multiple lines to uniquely Argentinian horses. It would not be impossible to have a recessive pattern there in the native, pre-studbook population (perhaps already of Hackney descent) which then spread.
I think that is a more plausible explanation than something is making the horses odd-colored after conception. Here is the section on the color from Dr. Sponenberg’s Equine Color Genetics (Third Edition):
“The repeatability of the manchado pattern suggests a genetic cause, though the range of breeds in which it occurs is awkward because they are not related nor are they commonly crossed one with the other to produce breeding stock. Paintings of Hackney horses from the 1800s suggest that the pattern has been around at least since then, if only rarely. The sporadic occurrence of manchado suggests that it might be due to a recessive mechanism, and moreover that the allele is rare.”
That pretty much sums up my suspicions on the color as well.
(I should clarify that when I say the horses can be traced to Argentinian horses, that is not to imply that the bloodlines used were somehow questionable in nature. Many of the breeds have a long history in the country and have separate founder lines from other countries.)