Curly coats on horses are caused by multiple gene mutations

The author's horse, Blackie, a wild-born curly horse from Nevada.
The author’s horse, Blackie, a wild-born curly horse from Nevada.

Dr. Mitch Wilkinson, in the second of a three-part series, explains that not all curly coats on horses are caused by the KRT25 gene.  He describes other genetic mutations linked to curly-coat formation, beginning with one first identified in northern Nevada which may also be carried in wild herds in the area.

Yet another mutation which causes curly coats in horses not caused by KRT25 was called the “Cook Mutation”. [2] The Cook family were early breeders of domestic curly horses in Northern Nevada.

The Cook horse bloodlines are present in many of today’s domestic curly horses. Because the Cook horses were being bred from the earliest days of curly horse domestication, a great many of the domestic curly horses that tested positive for the KRT25 mutation might also simultaneously be carrying the Cook mutation.

We will see in Part III how it was deduced that an individual horse can carry two or more curly coat producing genes at the same time. Curly horses that carry only the Cook gene are rare with only a few examples. [2]

The ranch where the family of Art Cook lived and bred their horses was near another family of early curly horse breeders, the Damele family. Benny Damele was instrumental in pioneering the domestic curly horse as a breed. [7] The Cook line of horses can be traced back to the famous Damele stallion, Copper D.

The Cook stallion was sired by Copper D, and was named simply Cook Curly Horse. No genetic samples were ever taken of this stallion nor of any first-generation Cook horses.

This story of the Cook curly horses was related in an essay by Pat Johnson who knew both the Damele family and Art Cook. Mrs. Johnson bred the Cook line of curly horses for many years.

She thought highly of this line of curly horses, but never realized that they carried a different gene mutation. [1]

Where and when the Cook mutation entered the domestic curly horse population may never be known.

It has been speculated that Copper D could have been carrying two curly gene mutations at the same time and passed the Cook mutation on though Cook Curly Horse to the Art Cook breeding program while not infusing the KRT25 mutation.

Another possibility is that Copper D did not pass on a curly gene mutation to Cook Curly Horse, and the curly mutation came from the Damele dam. Many of the Damele mares were wild-caught horses from the Austin, Nevada area. It is suspected that the Cook gene mutation may still exist in the wild herds. The Fish Creek Herd Management Area, which is near both the ranches, may have horses that carry the Cook gene mutation.

Art Cook used only one stallion in his breeding program, and that was the sorrel-colored Cook Curly Horse.  Art gelded the colts and bred his stallion to his daughters or wild horses near his ranch.

Cook Curly Horse was kept in a corral at the Cook ranch, but Art couldn’t build the corral strong enough or high enough to keep his stallion in. Cook Curly Horse would periodically break out of the corral, disappear for a few months, and run with the wild herds. Sometimes he would return on his own and other times he was caught in round-ups.

On one of Cook Curly Horse’s excursions, he was missing for several years and both Art and Benny thought he had died. Incredibly, Cook Curly Horse was found by Benny Damele breeding some of his mares and was promptly returned to Art.

Art Cook registered only three of Cook Curly Horse’s progeny with the ABC registry. One of the colts was named Houdini (ABC P–468). Houdini received his name after he escaped from a locked stock trailer. No one could understand how he did it. Houdini was owned by Pat Johnson, and she related that he was extremely intelligent and quite a character. [1]

Houdini’s direct progeny, named Nipper (Ne-Hi Majic ABC 1172),  pictured below. He had hair follicle samples taken that showed that his curly coat was not due to the KRT25 mutation. [2] [3] [4]

Nipper was suspected of carring the Cook gene mutation.
Nipper was suspected of carrying the Cook gene mutation.
Genetic testing showed that Nipper's curly coat was not due to the KRT25 mutation.
Genetic testing showed that Nipper’s curly coat was not due to the KRT25 mutation.

Both blood and hair samples were taken of Nipper’s foal, Karma. They also showed that the direct progeny of Nipper, Karma, did not carry the KRT25 mutation, but had a very curly winter coat. This line of curly coated horses is currently being sequenced to find the gene mutation responsible. [2]

Karma, owned by Krista Harvey, is the foal of nipper. Karma is suspected of carrying the Cook gene mutation.
Karma, owned by Krista Harvey, is the foal of nipper. Karma is suspected of carrying the Cook gene mutation.

Cook gene in wild Nevada horses

Domestic curly horses from Northern Nevada are all partly descended from horses caught from the wild herds of the area.

The genetic mutations found in Damele horses and Cook horses all came from the wild herds.

Today, many wild caught horses with curly coats in the Northern Nevada region test positive for the KRT25 mutation, but some do not.

Below is a photograph of a curly coated horse captured near Elko, Nevada who has a curly coat that is not due to the KRT25 mutation.

It is theorized that this horse may be carrying the same mutation as the Cook line of domestic curly horses. Genetic samples of several horses of this type have been collected.

Whether this horse, Elko Nevada, is carrying the Cook gene or another unknown gene mutation, curly coat producing genes in addition to KRT25 are incorporated in the wild herds found near Elko, Eureka and Austin, Nevada. Bureau of Land Management Herd Management Areas near these locations are still known to contain curly coated horses. [2]

Elko is a wild-born Nevada mustang,owned by Angie Gaines. Elko is suspected of having the Cook gene mutation.
Elko is a wild-born Nevada mustang,owned by Angie Gaines. Elko is suspected of having the Cook gene mutation.

Sulphur mustang curly coated horses

There is another type of North American curly coated horse which can be found in the Utah wild horse herds, most notably the Sulphur herd in southwestern Utah. These curly coated horses do not carry the KRT25 mutation. Curly coated horses are extremely rare in the Sulphur mustang herds, but they do exist.

The Sulphur Herd roams a vast, remote region centered around the Needle Mountain Range. This herd is named after one of the few springs found in the arid high desert, the Sulphur Spring. These horses are descended from some of the first horses brought to the southwest by Spanish colonists and explorers in the late 1500’s. To see them in the wild requires a journey of several hours by paved and dirt road from Milford, Utah. [17] [24] [9]

A Sulphur mustang.
A Sulphur mustang.

The most striking characteristic of the Sulphur Herd is the very high incidence of Dun Dilution Factor in the population. [9]

Columbus’s second voyage to the New World included semi-feral horses from the Guadalquivir River marshlands located near the Spanish village of Almonte in the province of Huelva, Spain. [14] [15]

In subsequent voyages, an estimated 500 or more semi-feral horses from the Guadalquivir marshes were brought to breeding farms on the Caribbean islands of Hispaniola, Puerto Rico, Cuba, and Jamaica. These horses brought with them the dominant gene for the Dun Factor. The Spanish called these semi-feral, peasant horses “Jacas”.

Although Columbus complained about the quality of the striped horses, he did not know that these horses were some of the hardiest horses in the world at that time. [12]

Their genes and bloodlines went into other “better quality” Spanish colonial horses that were imported later in the colonization of New Spain. Along with their resilient, adaptable physiology, these “Jacas” also passed on the high incidence of Dun dilution factor to their decedents in Spanish America. [6] [8]

Spanish colonial horses arrived for the first time in numbers to establish a population in 1589. In the decades before this, the southwest was being explored by Spanish expeditions and treasure seekers who were mounted on stallions and geldings, for mares were forbidden to be taken from the Caribbean breeding farms. Coronado’s expedition included only two older mares. [8] [11] [16]

In 1589, from central Mexico’s newly established horse breeding farms whose parent stocks came from the Caribbean islands, stallions and mares were obtained in large numbers to establish Spain’s first southwestern colony in present-day New Mexico. Juan de Onate led seven hundred colonists and over 1,500 horses which included 237 mares from central Mexico across the Rio Grande to establish the colony of New Mexico. He was the colony’s first governor.

However, the exploitation of Natives by the Spanish colonists and the cruelty of the colonial government led by Onate caused a growing resentment among the Pueblo tribes. Juan de Onate was replaced by the Spanish Crown in 1609 for ineptitude, but Onate’s successors did little to ease the plight of the Pueblo Tribes.  During the next seven decades, Native Americans were being trained in the Spanish art of horsemanship to herd the livestock kept at the Catholic Missions.

Finally, the resentment of Spanish domination led to a revolt of the Pueblo Indians in 1680.  The revolt was led by a shaman from San Juan Pueblo named Pope.

The Pueblo Revolt expelled the Spanish from the area for over a decade. The fleeing Spanish colonists left the majority of their horses behind. Estimates range from 7,000 to 10,000 horses fell into Native American hands. The horses the Spanish left became the mounts for Native Americans and the seeds of the early mustang herds. [11] [6] [8]

A century and a half later in the early 1830s, the Old Spanish Trail passed near the Sulphur Herd Management Area. There was a brisk trade in horses from Los Angles to Santa Fe. At the time, the southwest was still under Spanish control. Some of the Spanish Colonial horses from Alta California may have escaped during this time and contributed their bloodlines to the Sulphurs, also. [9]

In 2015, Swedish researchers at Uppsala University found that 1,617 base pairs were missing from the 8th chromosome of non-dun horses. In genetic terms, this is called a deletion.

Horse populations that carry the dun factor, like the Spanish marsh horse ancestors of the Sulphur horses, are usually found around the world in semi-feral, wild horse populations.

The researchers wondered why this observation was so consistent. By comparing today’s modern horses to ancient, frozen horse specimens from Alaska and Siberia, they came to the startling conclusion that at one time all horses had the dun factor and solid colored horses began by a genetic accident sometime after domestication around 5000 years ago.

It was the preference of human breeding practices which transformed horse populations from being 100% dun dilution to the majority of today’s horses having solid colors. [23]

A graphic showing the deletion of 1617 base pairs from the 8th chromosome.
A graphic showing the deletion of 1617 base pairs from the 8th chromosome.

The research also identified the gene which is responsible for the asymmetric distribution of pigment cells called melanocytes which give dun horses their characteristic appearance. The darker markings and stripes on dun horses are called Primitive Markings. These markings are used for camouflage. The gene which produces dun characteristics is called the TBX3 gene. It is a dominant gene and needs to be active on only one of the two chromosomes which make the 8th chromosome pair to produce dun markings. [23]

However, if a chromosome is missing the 1617 base pairs, the TBX3 gene is inactivated in that chromosome. If both of the 8th chromosomes are missing the 1,617 base pairs, the TBX3 gene is completely inactivated causing a solid colored horse to be created. [20]

Horses with primitive dun characteristics are found to this day in pockets around the world. The Sorraia River basin of Portugal has the Sorraia horse; Przewalski’s horse is found in Mongolia; the Yakutian horse lives in Siberia; Norwegian Fjord horses have been bred for centuries; and Utah has Sulphur mustangs. [24]


Leg bars on a Sulphur Mustang.
Leg bars on a Sulphur mustang.
Shoulder and neck stripes on a grullo Sulphur mustang.
Shoulder and neck stripes on a grullo Sulphur mustang.
A wild Sulphur stallion, in the Sulphur Herd Management Area.
A wild Sulphur stallion, in the Sulphur Herd Management Area.

The continued survival of Sulphur mustangs is in question, but Sulphur mustangs that carry the, as yet, unknown curly coat-producing gene are definitely on the verge of extinction. At the present time, there is only one known curly breeding stallion in the Sulphur HMA. There are three curly Sulphur mustangs known to be in captivity.

Curly Sulphur mustangs have thick manes and tails which remain full during the summer months, much like the Curly Jim phenotype. They also produce body curls that have a distinctive wave-like pattern. Spanish head, ear, and eye shape is also noticeable and reminiscent of the Andalusian. [16] [18] [19] [17] [13] [14]

The tapered head shape and distinctive eye morphology of Sulphur Mustangs are reminiscent of the Andalusian.
The tapered head shape and distinctive eye morphology of Sulphur mustangs are reminiscent of the Andalusian.

The wild caught Sulphur mustang pictured below has an unknown gene for a curly coat and is heterozygous for the dominate dun gene giving primitive markings.

Mezcal is a wild-born Sulphur mare, shown in her winter coat. She is owned by Dan and Duv Cardenas.
Mezcal is a wild-born Sulphur mare, shown in her winter coat. She is owned by Dan and Duv Cardenas.

The Sulphur mare in the picture above was bred to a non-curly coated Wilbur-Cruce mustang. The resulting foal named, Renegade, had a curly coat. The Wilbur-Cruce sire did not show any curly traits and is unlikely to carry any mutations that could explain the curly coat of the foal. Therefore, the mutation that produced the curly coat in the dam was inherited by the foal and probably has a dominant mode of transmission. [21] [22] 20]

Renegade was the offspring of the culy mare Mezcal.
Renegade was the offspring of the curly-coated mare Mezcal.
Renegade as a young adult, showing a winter coat.
Renegade as a young adult, showing a
winter coat.

Spanish mustang curly coated horses

Some horses in the Spanish Mustang Registry also have curly coats. These horses almost always have some type of Sulphur Horse connection in their background. Whether the same mutation is common in both populations remains unknown.

Ivy, a Spanish mustang owned by George Kurek, of Dizzy Horse Farms, shows her winter curly coat.
Ivy, a Spanish mustang owned by George Kurek, of Dizzy Horse Farms, shows her winter curly coat.

Patagonian curly coated horses

There are curly coated horses in Argentina. In the wild horse herds of Patagonia, a percentage of the horses have curly coats. Hair samples have been obtained from several of these horses and they do not have the KRT25 mutation. Whether they have one of the undiscovered mutations mentioned in North American curly coated horses will be determined after isolation of other curly genes.

The Variation of Animals and Plants under Domestication is a book by Charles Darwin that was first published in January 1868. In the book, Darwin cites the work of another naturalist from a generation before concerning his observations of curly coated horses in Paraguay. The horses are given as an example of adaption to a harsh, cold environment.

Felix de Azara
Felix de Azara

The naturalist who penned his observations in 1802 was a Spanish military officer and engineer named Felix de Azara. Azara stated in his book, Quadrupeds de Paraguay, that the curly coated horses were observed near the Rio de la Plata that forms the border of Argentina and Paraguay. Many thought these horses never existed or became extinct. Darwin never actually saw the horses that Azara described. [26]

In 2013, Andrea and Gerardo Rodriquez contacted Angie Gaines through the Curly Mustang Association Facebook page. The couple stated that they were raising curly coated horses that had been captured from the wild horse herds of Patagonia, Argentina. Their ranch was close to a remote village named Maquinchao, but they had internet service.

Hair follicle samples were sent from Argentina to Gus Cothran’s lab in Texas. The samples proved not to have the KRT25 mutation. In order to run tests to isolate the curly coat causing mutation these horses have; blood samples must be obtained. It was impossible to ship blood samples from such a remote location without spoilage.

Mario Poli is a geneticist who works for the Argentine government at the Instituto de Genética “Ewald Favret” in Hurlingham, Argentina. He was contacted in the hope that blood samples from the horses could be processed in Argentina for shipment to the United States. It was a five-hour drive, one way, to Maquinchao to obtain the samples. Mario sent his sister-in-law to gather the blood samples. It took four years to overcome the logistics needed to gather these samples.

By an odd coincidence, the very day the scientific paper about the isolation of KRT25 was published, Mario sent the processed samples to the genetics lab at Texas A&M University for genetic analysis.

Curly coated horses found in wild horse herds in Patagonia, Argentina.
Curly coated horses found in wild horse herds in Patagonia, Argentina.

Some special thanks are in order to Duv Cardenas and her in-laws who helped with translations on messages to Argentina.

Siberian curly coated horses

A type of curly coated horses that test negative for the KRT25 mutation is found in Siberia.  These horses live near Lake Baikal in southern Siberia near the Mongolian border. The Zabaikalski horses are the traditional horses of the Zabaikalski Cossacks and are being preserved as a national treasure at state-run breeding facilities. [2]

Hair follicle samples were shipped to Texas A&M lab from Siberia. Little else is known about these horses.

Lake Baikal is the largest fresh water lake on Earth.
Lake Baikal is the largest freshwater lake on Earth.
The semi-wild Zabaikalski curly coated horses live in Siberia. About a quarter of these horses have curly coats.
The semi-wild Zabaikalski curly coated horses live in Siberia. About a quarter of these horses have curly coats.
A Zabaikalski curly coated foal showing an unknown gene for curly coat, plus the Lp gene mutation for Appaloosa coloring.
A Zabaikalski curly coated foal showing an unknown gene for curly coat, plus the Lp gene mutation for Appaloosa coloring.

Mongolian curly coated horses

These horses exist in Northern Mongolia. Because of the close geographical location to the Zabaikalski horses of Siberia, they might be related.  There are only a couple of pictures in the ICHO database of Mongolian curly horses. The pictures are shown below.

A friend of a well-known Canadian curly horse breeder, Shelly White, traveled to Mongolia as a part of a non-profit organization called Mongolian Women International. Shelly’s friend, Julie Veloo, found five Mongolian curly horses and obtained hair samples that were sent to Gus Cothran’s lab at A&M University. All five samples were negative for the presence of KRT25.

These curly horses have some other mutation that causes their curly coats. Whether it is the same as the Siberian horses, only time and genetic research will tell. [2]

Mongolian curly coated horses.
Mongolian curly coated horses.

The future

Obtaining specimens from remote locations around the world is difficult at best. As we obtain more information about the existence of the different mutations which cause curly coats in horses around the world, we may finally piece together the mystery of curly horses.

As a curly horse enthusiast, keep in mind friends and acquaintances that might be going to some of the locations mentioned in this article who might be able to obtain samples to aid in our search.

In Part 3, curly coated horses suspected of carrying multiple curly producing genes will be discussed.  Future curly research and the conservation of endangered curly horses will also be covered.

Part 1 can be read here.

Part 3 can be read here.


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  2. Reveglia, Earlene. Genetic Data Baes of the International Curly Horse Organization. 2017.
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  4. Curly Horse Pedigree Database.
  5. ICHO registry site and archives. Curly Traits: including inheritance and curly hair types. also, History of the Curly Horse.
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  17. Roubidoux, Ron. Wild Horses of Utah’s Mountain Home Range. Sulphur Springs                   Horse Registry Inc. archives, 1994
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  19. Cothran, Gus. Genetic Analysis of Two Feral Horse Herds in the Cedar City Management Area of Utah. Univ. of Kentucky, 1997.
  20. Pierce, Benjamin. Genetics: A Conceptual Approach, 5th Edition. Macmillan Pub.,2013.
  21. Bowling, Ann, Horse Genetics. CAB International, 1996.
  22. Bailey, E. and Brooks, S. Horse Genetics – 2nd CAB International, 2013.
  23. Imsland, Freyja; Andersson, Leif; et al. Regulatory Mutations in TBX3 Disrupt Asymmetric Hair Pigmentation That Underlies Dun Camouflage Color in Horses. Nature Genetics, vol. 48, Issue 2, Pages 152 – 158.
  24. Oelke, Hardy. Born Survivors on the Eve of Extinction. Premier Publishing Equine,1997.
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  27. Azara, Felix de. Quadrupeds de Paraguay. The Spectator Archive.1802.


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