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A grullo, like all duns, exhibits a lighter body coat than mane and tail color, clear primitive markings: a distinctive dorsal stripe, horizontal striping on the back of the forelegs, often a transverse stripe over the withers), and the dark "dun mask" on the face. Zebra stripes are visible on the left back leg. The dun gene also produces light guard hairs in the mane and the tail.

Grullo is a color of horses in the dun family, characterized by tan-gray or mouse-colored hairs on the body, often with shoulder and dorsal stripes and black barring on the lower legs. In this coloration each individual hair is mouse-colored, unlike a roan which is composed of a mixture of dark and light hairs. There are several shades of grullo, informally referred to with a variety of terms including black dun, blue dun, slate grullo, silver grullo, silver dun, or lobo dun. In the Icelandic horse, the grullo color is called "grey dun," and in the Fjord horse, Norwegian,"grå" or "gråblakk" (literally, "gray dun").

The word grullo originates from the Spanish word "grulla", which refers to a slate-gray crane. Because of the Spanish origin of the name, some people will refer to a mare as a grulla and a stallion or gelding as a grullo, pronounced "grew-ya" and "grew-yo" respectively.

In terms of equine coat color genetics, all of these shades are based on the dun gene acting as a dilution gene over the black gene. Because the grullo color is not due to the gray gene, a grullo horse remains the same basic color from birth, though some minor shade variation may occur from summer to winter coats. If a grullo also carries the gray gene it will be born a mouse tan-gray shade, but then lighten and eventually develop a white hair coat with age. Because black is less common in general than bay or chestnut, grullo is likewise less common than red duns or bay (classic or zebra) duns. For example, only 0.7% of quarter horses registered each year with the AQHA are grullo.

The most obvious way to tell a grullo is not only the existence of gray or tan-gray body color, but also its primitive markings, which include some or all of the following: dark face; cobwebbing around the eyes and forehead; dark mottling on the body; leg barring (sometimes called tiger striping); dark ear tips and edging; dark ear barring; dark shadowing of the neck; dark dorsal and transverse striping; dark mane and tail guard hairs.

Primitive roots

The appearance of grullo color among domestic horse breeds raises interesting questions about the Tarpan, (Equus ferus ferus) a relative of the domestic horse that became extinct in the 19th Century, which appears to have had dun or grullo coloration. The Tarpan has been considered a true wild horse, an undomesticated relative or ancestor of the domestic horse. However, some authorities in the early 20th century held the opinion that most equines called tarpans in fact were domestic or feral horses, not a separate species.[1]

Several breeds with the grullo color have been developed in efforts to recreate ("breed back") the Tarpan. These breeds include the Heck horse and Konik. One of the first experiments in this regard was published in 1906 by James Cossar Ewart, who obtained a Tarpan-like horse by crossing a Shetland mare and a black Welsh pony.[1]


  1. 1.0 1.1 J. C. Ewart (1906). "The tarpan and its relationship with wild and domestic horses". Nature 74: 113–115. doi:10.1038/074113a0. http://books.google.com/books?id=G9URAAAAYAAJ&pg=PA113. "[F]or more than a century all the horses living in a wild state in Europe, which happened to be of a mouse-dun color, seem to have been regarded as Tarpans.". 

Sources and External Links

  • Dun Zygosity test from Veterinary Genetics Laboratory, School of Veterinary Medicine, University of California, Davis. Web site accessed January 13, 2008.


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