Tag Archives | Rocky Mountain Horses

Aging Silvers

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This is probably one of the better-known images of a bay silver horse. It was used to illustrate the color in the paper identifying the silver mutation, and has been widely shared on the internet since that time. His name is Unconventional (“Connor”), and he is a Morgan bred by Laura Behning. Laura was instrumental in identifying and documenting the silver dilution in the Morgan breed. Her website documenting those lines is a wonderful resource for anyone interested in silver, and is particularly notable for the number of images of silver foals.

Here is another image of Connor as an adult. In this image, he still has the strong contrast between his flaxen mane and his body. (This is also another good image for seeing the bay countershading on the face and neck that differentiate bay silvers from chestnuts.)

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When silver dilutes are young adults, this contrast is part of what makes the color so striking. It also makes them relatively easy to identify. That is not necessarily true as they age, however. Most horses with lighter manes and tails lose some of that contrast with age, but that seems to be particularly true of silver dilutes. This next image is pretty typical of how the flaxen mane looks on an older silver.

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Most bay silvers have dark roots, but as the horse ages the dark parts of the mane tend to spread until it is just the ends of the mane that are pale. The forelock tends to stay palest the longest, as the images of this pony show.

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It is likely that, had his mane not been cut, he would have somewhat lighter ends. Still it is the forelock that is unusually pale, and that color changes pretty abruptly at the ears, as the image of his forelock parted and tucked behind his ears shows.

Tails darken as well, usually starting with a dark core like the one visible on this Rocky Mountain Horse.

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In some cases, older silver dilutes can appear to have completely dark tails, or at least tails that do not look significantly different from a horse that has sun-faded. This Rocky Mountain horse is a seventeen year-old buckskin silver. Her tail is almost completely dark, while she has retained the paler tips on her mane and the flaxen forelock.

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This mare does not show a lot of flaxen contrast on her lower legs. They were visibly chocolate – not black – in person, but the kind of mottling seen on some bay silver legs was not present. It seems that the more uniformly dark silvers are more likely to lose the contrast on their manes and tails, though it seems all lose it to some extent with age.

This does complicate the search for silver in historical records. Horses like the Groninger stallion Iregon do not look silver in the handful of images available, even though they are known sources of the silver dilution. Silver is not visible on chestnut horses, because they have no black pigment to dilute.

And if that was not complicated enough, it appears there are other factors unrelated to silver that can turn the mane and tail of a bay horse silver or flaxen. The next post on silvers will be about some of the silver mimics.

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Silver dilution and eye defects

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The recent posts on eye defects in dogs reminds me that I meant to share a recent paper on eye defects in silver horses. The issue of eye defects first came to light within the Rocky Mountain Horse breed, where it was initially called Anterior Segment Dysgenesis (ASD). That name was recently discarded in favor of Multiple Congenital Ocular Anomalies (MCOA).

Since its discovery, questions remained about whether or not MCOA was directly linked to the silver dilution, or if it was a more recent mutation tied to one of the Rocky Mountain founders. The recent study, “Multiple congenital ocular anomalies in Icelandic horses“, tied the issue directly to the silver mutation.

In this study we have shown that the MCOA syndrome is segregating with the PMEL17 mutation in the Icelandic Horse population. This makes the hypothesis that the MCOA mutation has recently arisen unlikely.

The Icelandic population is significant because it has been isolated from other domestic horses since 982 AD. If silver Icelandics have the same problem as silver Rocky Mountain Horses, then it is far more likely that the silver mutation is involved.

One of the things that makes this interesting is that the silver dilution – which is linked to eye defects in horses – occurs in the same location as the merling gene in dogs. Both are  PMEL17 (SILV) mutations. Both also dilute black, but not red, pigment. Those are interesting parallels between two colors that have such a visually different appearance.

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