Sulphur Mustangs: Links to North America’s Spanish Colonial horses

A wild Sulphur stallion.
A wild Sulphur stallion. © Harry Furey (2022)

Genetic research has revealed the relationship between the Sulphur Mustangs and other North American Spanish Colonial horses, writes Dr Mitch Wilkinson in the second part of an investigation into the history of the unique southwestern wild horse.

The present study and the 1995 blood variant study established that contemporary Sulphur Mustangs have a historical relationship to other living Iberian and New World Iberian breeds. Although a large portion of their origin was from Iberia, this paper has shown that Sulphur Mustangs have been separated from their ancestral populations for centuries and have followed an independent evolutionary development25.

Further principle coordinate analysis of microsatellite alleles found similarities with other Spanish Colonial horses of North America that indicate a common origin and background. As first defined by Dr Phillip Sponenberg, the Spanish Colonial horse of North America is of undoubted Iberian origin. However, the exact nature and background of these Colonial horses populations are still a mystery in most cases.

Some groups of these horses have partially documented backgrounds, while others have a history only defined by legends and speculation. An overlap of individual horses’ genetic principle coordinate plots implies a close relationship and possible common origin. As shown in the PCoA graph of individual horse genetic coordinates below, population cluster overlaps indicate a relationship between Sulphur, Grand Turk, Baca, and Wilbur-Cruce populations.

A PCoA graph showing a cluster (circled in purple) that includes the genetic coordinates of individual horses from Sulphur Mustangs (yellow), Wilbur-Cruce (purple), Baca ( white), and Grand Turk horses (aqua) Florida Cracker (dark green) and Santa Cruz (blue) is seen below the central cluster outlined in purple.
Fig. 7: A PCoA graph showing a cluster (circled in purple) that includes the genetic coordinates of individual horses from Sulphur Mustangs (yellow), Wilbur-Cruce (purple), Baca ( white), and Grand Turk horses (aqua) Florida Cracker (dark green) and Santa Cruz (blue) is seen below the central cluster outlined in purple.

However, each population is quite distinct because of hundreds of years of geographical separation and the introduction of other horse types. Although a common ancestor for all these groups starting in the Islands of the West Indies is probable, when that common ancestor existed for each group is unknown.

The centroid cluster graph below (Fig. 8), was generated from the same data graphed in Fig. 7. It shows a less complicated representation of the relationship between Sulphur Mustangs and horses of Grand Turk Island in the Caribbean, Baca horses of New Mexico, and Wilbur-Cruce horses of Arizona. With fewer data points, this second graph allows the viewer to discern distances between the centroids. A centroid is the average center of each population cluster.

Fig. 8: The graph shows centroid relationships between Sulphur Mustangs and other groups of Spanish Colonial Horses. Sulphur Mustangs (SU- yellow), Baca horses of New Mexico (BA – white), Wilbur-Cruce Horses of Arizona (WC – purple), Horses from Grand Turk Island in the Caribbean (GT – blue), Santa Cruz Island horses from California (CZ – brown), and Florida Cracker Horses (FC – green).
Fig. 8: The graph shows centroid relationships between Sulphur Mustangs and other groups of Spanish Colonial Horses. Sulphur Mustangs (SU- yellow), Baca horses of New Mexico (BA – white), Wilbur-Cruce Horses of Arizona (WC – purple), Horses from Grand Turk Island in the Caribbean (GT – blue), Santa Cruz Island horses from California (CZ – brown), and Florida Cracker Horses (FC – green).

The distance between Sulphur Mustangs and the other Spanish Colonial horse groups can be estimated by how close each population is to its neighbors. It must be remembered that the graph in Fig. 7 shows four populations with overlapping clusters, which implies a close genetic relationship from the 15 microsatellite markers used. Florida Cracker horses and Santa Cruz Island mustangs show less of a relationship.

Grand Turk Island Horses

Grand Turk and Caicos Islands horses.
Grand Turk and Caicos Islands horses. © Pat Morris

The history of the horses of Grand Turk Island in the Caribbean is equally mysterious and undocumented. Grand Turk Island was thought to be the first land sighted by Columbus in 1492, but the absence of any water source caused Columbus to reject the island for colonization. The lack of fresh water on the island is still a problem. Grand Turk’s feral horses depend on natural rain pools and the charity of the human population for water. Many island residents place troughs in their yards for horses to drink10.

Spanish horses were brought to the island in the 1600s as work animals to harvest sea salt needed for the budding Caribbean fishing industry. It is believed that these horses died when the island was uninhabited just before America’s War of Independence in the late 1700s, but this is only speculation10. Sometime in the late 1800s, local legend tells of the importation of horses from the neighboring island of Jamaica. Although the Grand Turk horses overlap Sulphur Mustangs on the PCoA graph, the lack of documented history gives little insight into the origins of both populations, but both populations have similar conformation.

A typical Grand Turk horse.
A typical Grand Turk horse. © Pat Morris and Fred Neehuis
Fig. 9: A Grand Turk horse, left, compared to a Sulphur Mustang at right. 
Fig. 9: A Grand Turk horse, left, compared to a Sulphur Mustang at right.

Baca Spanish Colonial Horses

A pure Baca Colonial horse.
A pure Baca Colonial horse. © Carol Fuller Powell

Another population that overlaps the Sulphur Mustang population is the Baca horses of New Mexico. These Spanish Colonial horses were preserved at the Baca Farm by generations of the Baca family. The Baca Farm was established in the early 1800s, and the horses bred there through the years were believed to be descendants of horses from early Spanish Colonial times. After his return from the Korean War in 1953, Doreteo “Joty” Baca began a serious breeding program to preserve what he believed was the original strain of Spanish Colonial horse found in New Mexico. He took horses from neighboring ranches, Native American Pueblos, and wild horses from the mountains near Tijeras. Although modern descendants of surviving horses from the Coronado expedition are unlikely, the colonization of New Mexico by Juan de Onate in 1598 brought 1000 Spanish Colonial horses from Central Mexican breeding farms to the new colony. These horses were bred and multiplied until the Pueblo Revolt of 1680. At that point, thousands of Spanish Colonial horses fell into Native American hands. Many abandoned horses became feral11.

In the early 1800s, thousands of horses were trailed from California to Santa Fe on the Old Spanish Trail and then on to Missouri for sale via the Santa Fe Trail. There is every reason to believe there was some mixing of indigenous New Mexican horse stock with the new Alto California stock arriving by the Old Spanish Trail during the early 19th century. With the establishment of the Baca Farm in the 1930s, Joty Baca speculated that remnants of Spanish Colonial horses were still present in the area, according to Baca family oral history. As previously shown, the ancestors of both groups of horses, either in California or New Mexico, had their origins in breeding farms in central Mexico. Before that, Mexican horses began on the Spanish Islands of the Caribbean. Although there are documents showing some horses were directly imported from Spain in Colonial times, these horses had little influence on the extensive existing Mexican horse populations.

So the question of whether the horses preserved by Joty Baca were close to the horses brought to New Mexico on the Old Spanish Trail or the original New Mexican horse stock from the Onate colonization, or a mixture of both makes little difference. Originally, they were all Mexican-bred Colonial Spanish horses.

Wilbur-Cruce Horses

Diego, a full Wilbur-Cruce horse.
Diego, a full Wilbur-Cruce horse. © Duv Cardenas

The best documented Colonial horse population comes from ranch stock located in Southern Arizona, the Wilbur-Cruce strain. In the 1870s, a horse trader named Juan Sepulveda sold 25 mares and one stallion to Dr Wilbur, who had established a ranch near the Arizona-Mexican border.

Juan Sepulveda told Dr Wilbur that the horses came from Sonora’s Rio San Miguel and were the direct descendants of stock kept at Mission Dolores. Mission Dolores was the first mission in Sonora, Mexico, and was started in 1687 by a Jesuit missionary named Father Eusebio Francisco Kino12.

Father Kino became a talented producer of cattle, sheep, goats, and above all, horses. It is said that some of his horses came directly from Spain, but most, if not all, of his horses, probably came from horse breeding farms near Mexico City, where he was posted after arriving from Spain in the spring of 1681. The Dolores Mission and Ranch supplied horses and other stock to missions in Arizona and California. It also supplied military expeditions. Father Kino was known for his explorations of western Spanish lands and took an active part in the establishment of 24 missions in Alto California and Arizona. Father Kino died in 1711, but Rancho Dolores continued and was renowned for its fine horses12.

So what happened over the 160 years from Father Kino’s death to the sale of horses to Dr Wilbur in Arizona Territory in 1870? Was new stock added to Father Kino’s strain of horses? Did Juan Sepulveda really obtain his horses from Rancho Dolores? Did the horses of Father Kino play a significant role in producing the future equine stock of Alto, California? Unfortunately, we do not know the answers to these questions, but we do know the horses kept by the Wilbur-Cruce family on their ranch remained relatively unchanged from 1870 till 1989.

The close relationship between Baca, Wilbur-Cruce, Grand Turk, and Sulphur Mustangs has to be more than a coincidence. Their mutual history follows the sequence of Spanish colonization of the 15th through the 17th centuries across the New World. Principle coordinate analysis shows they have something in common; every indication points to a similar origin. From Spanish horse history, we know why microsatellite genomic signatures do not overlap modern Iberian horse breeds. PCoA graphic inferences show that these horses from Utah, the Caribbean Islands, Arizona, and New Mexico could have had common ancestors, but when those ancestors existed could not be determined. The relationship of all these horses might be traced to the Spanish Jennet of the early 15th century, but the Spanish Jennet no longer exists. However, Jennet samples may exist today in archeological museums in Spain. All that is needed is a researcher interested in making the comparison.

Curly-Coated Sulphur Mustangs

A Mezcal wild caught curly coated Sulphur Mustang in his winter coat.
A Mezcal wild caught curly-coated Sulphur Mustang in his winter coat. © Duv Cardenas

Understanding a fundamental concept of biology, mutations are critical to preserving endangered horses like Sulphur Mustangs. During the formation of eggs and sperm in a process called meiosis, the genetic material in the DNA is not only rearranged in a process called “recombination”, but it can be randomly changed through the misspelling or rearrangement of DNA nucleotides causing what is known as a “mutation”. Mutations allow animal species to change over time and adapt to changing environments. Therefore, the process is critical for any species to continue to exist.

In the case of Sulphur Mustangs with a curly winter coat, it is essential to realize that the mutation arose within the Sulphur horse population and did not come from other horses with curly coats. How do we know this?

Five known mutations in North America cause curly winter coats in equines. The Sulphur curly-coat mutation is one of the five. Two of the five have been isolated, and tests were developed to find their presence in any particular horse. It happens that the two isolated mutations, SP6 and KRT25, are the most numerous. The other three mutations are rare, but each has a distinctive look or phenotype (appearance)13.

Genetic tests were conducted on the rare Sulphur Mustangs that have curly coats. Neither SP6 nor KRT25 mutation was found to be present. The other two possible mutations have a distinctive appearance and can be ruled out. The conclusion is that the mutation occurred within the Sulphur population and did not come from an outside source. A spontaneous mutation occurred within the Sulphur herd itself sometime in the last 200 years.

The Dun Factor

There was a time in history when all horses had the dun dilation factors and were dun. Sometime after domestication, about 5000 years ago, there was a spontaneous mutation within a gene known as TBX3. This mutation altered the subsequent protein coded by the TBX3 gene. The altered protein could not fulfill its biological mission to direct the migration of pigment cells known as melanocytes during fetal development. In Dun horses, melanocytes migrate to some locations and away from others causing the characteristic stripes and other primitive markings which are found in some Sulphurs. Dun factor can affect horses of any color. There are red, grullo (black), bay, and palomino duns. The non-mutated active dun factor is abbreviated “D” in genetic short-hand. The dun factor has a second partial mutation given the designation “nd2”. The spontaneous mutation within the TBX3 gene that occurred after domestication allowed solid-colored horses to be born without dun stripes14. It is thought that human selection has led to the prevalence of solid-colored horses, and the original dun colors of horses became relegated to a rare horse color.

So the question arises, why does the Sulphur population have such a high percentage of dun factor? Not all Sulphurs are born with the dun factor, and not all horses with a dun factor are Sulphurs.

The answer may be a combination of “founder effect” and “geographic isolation”. Human preference also played a role in Sulphur color. Sometime between the 1680s, when Father Kino was exploring the area, and the early 1800s, someone liked dun-colored horses. By chance, a small group of these horses found themselves at an isolated water source called Sulphur Springs in the Needle Mountains of Utah. How they arrived there is anyone’s guess. They could have been strays from the Old Spanish Trail or horses left or lost by Native Americans. We may never know. Most horses are very reluctant in a desert biome to leave their water source and strike out across the desert landscape unless they are compelled to do so by humans. There have been too many recent reports of wild horses found dead around a dried-up water source or one that has been fenced off. Even when faced with dehydration and death, horses will not voluntarily leave their desert water source. Simply put, “horses are not adventurous even when faced with life-threatening circumstances”.

Sometime in the past, a small group of primarily dun-colored Spanish Colonial horses found themselves abandoned in an isolated location. Luckily they had a water source, just enough food, and were physiologically tough enough to survive. Because most or all of the founding horses had dun factor present in their genetic make-up, they passed this on to future generations. For the same reasons these horses were afraid to leave the Sulphur Springs area, other horses were reluctant to cross miles of arid desert and mix with them. So those first few horses were the “founders” of future Sulphur mustangs, which remained relatively pure until recently.

Because the dun factor produced the colors of horses before domestication, the dun markings are termed “primitive”. The presence of primitive markings has led to the speculation that Sulphur Mustangs may be the descendants of North American Pleistocene horses. Nevertheless, at this point in time, there is absolutely no scientific evidence to support that conclusion. For most of the development of equines, they have had dun factor present. Therefore, a survival advantage could be associated with a dun coloring in pre-domesticated equines that Sulphurs exploited.

Bagheera, a wild-caught Sulphur Mustang.
Bagheera, a wild-caught Sulphur Mustang. © Hannah Catalano

» Next: The Sulphur Mustang’s link to Northern Iberian Horses


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One thought on “Sulphur Mustangs: Links to North America’s Spanish Colonial horses

  • May 27, 2023 at 5:54 pm

    Thanks so much for this well researched description of the plausible history and genetics of the Sulphur mustangs. This elucidates many fascinating characteristics of this unique herd.


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