Unravelling the rich history of the Sulphur Mustangs

A unique type of North American Mustang exists in Southwestern Utah near the Nevada Border. They derive their name from a spring that flows out of the Mountain Home section of the Needle Mountain Range. The spring is named Sulphur Spring, and the mustangs that live near it are known as Sulphur Mustangs. Dr Mitch Wilkinson, in the first of a four-part series, investigates their origins.

The ecological and environmental conditions in the Needle Mountains are harsh and unforgiving, making it difficult for wild horses to live. Surprisingly, Sulphur Mustangs have adapted over the centuries and thrived. Their origin is hotly debated, but their unique qualities are unquestioned. One of the defining characteristics of Sulphur Mustangs is the high percentage of horses in the population that exhibit dun dilution coat colors, but not all Sulphur Mustangs inherit this genetic trait.

The Sulphur Herd Management Area (HMA) is located in the western extremes of Iron, Beaver, and Millard Counties. The Bureau of Land Management, an agency of the US Federal government, manages this HMA. Some horses gathered in the Sulphur HMA are placed by the agency for public adoption. From this practice, there developed a strain of domestic Sulphur Mustangs.

A short history of horses in North America

Equids evolved in North America, starting about 55 million years ago. From their North American origins, they spread to South America, Asia, Africa, and Europe. Unfortunately, the three genetically distinct equid lineages found in North and South America during the late Pleistocene era became extinct. Horses were lost along with most of the other New World megafauna during the Quaternary Extinction, which occurred during the Pleistocene-Holocene transition at the end of the last glaciation period¹. DNA traces found in Alaskan and Yukon permafrost indicate that living horses were still present in North America as late as 8000 years ago. However, no scientific evidence exists that they persisted to the modern era².

The reintroduction of the horse (Equus Caballus) to North America began on Columbus’ second voyage in 1493. Historical documents and Spanish traditions indicate that horses were brought from the Iberian Peninsula to the island of Hispaniola³.

For the second voyage, Fernando de Zafra was ordered by King Ferdinand II to recruit 20 military horsemen with spare mounts, of which five were designated to be mares. The horsemen were recruited in the newly conquered Moorish city of Granada. Probably the horsemen were Christian soldiers that took part in the re-conquest of the Emirate of Granada, the last Moorish kingdom in Iberia³.

These Granada horsemen were ordered to join Columbus in Seville by June 20 of that year to begin the voyage. It is believed that these Granada horsemen brought 20 to 30 horses. Mounted soldiers like these warriors are sometimes referred to as lancers. Their horses, the majority of which would have been stallions, were the personal property of the horsemen³.

The horses used in the re-conquest of Spain by Christian armies were a mixture of native Iberian types, but most were from central and southern Spain. So the original stallions from Granada were not the mounts of gentlemen but rather war horses chosen for the hard and brutal conditions of the reconquest. However, it is thought that Columbus also purchased additional horses from horse traders in Seville⁴.

In a letter to King Ferdinand dated 1493, Columbus complained that the Seville horse traders sold him good horses but later substituted wild, inferior horses a few days before he set sail. This fraudulent exchange began the tradition of the “Saca De La Yaguas,” in which the villagers of Almonte, Spain, participate each year. Today, participants recreate the gathering of wild horses from the Guadalquivir Marshes every June, just as their ancestors did centuries ago to supply a portion of the horses taken to the New World by Columbus. The marshes have been a place of refuge for stray and unwanted horses for thousands of years. The Guadalquivir wild horses have always been a mix of different horses that live near the Guadalquivir marshes. They were such a mix during the time of Columbus and continue to be a mixture today. Like all wild horses, they are physically resilient and can survive harsh conditions. Today and in the time of Columbus, they are inexpensive.

How many horses Columbus purchased with the Crown’s money is unknown, but numbers between 30 and 50 horses have been speculated. From the beginning of horse re-introduction to the Americas, the stock was a mixture of different types readily available to the expedition organizers. They were not from a single source^5,24.

Soon after the 1493 voyage, the new colony of Isabella on Hispaniola began horse breeding operations. The horse was the main instrument of war for the Spanish of that era. By 1495, most imports of horses to the New World were mares indicating more extensive breeding operations were taking place⁴.

With the increased horse population from successful breeding operations, the governor of Hispaniola wrote to the King in 1507 that further importation of horses was unnecessary due to the abundance of horses on the island. Regardless, local island horse breeding rapidly expanded due to demand from other infant settlements in the West Indies. After the King received the 1507 letter, a royal ordinance prohibited the further exportation of horses from Spain. The ordinance was in effect for the next decade^4,3.

Spanish colonization and horse breeding quickly spread to other Caribbean islands from the island of Hispaniola. Other horses were added to the breeding population from the Old World over the next decades to improve stock on the new island colonies. Eventually, it was from Hispaniola, Puerto Rico, Jamaica, and Cuba, not Spain, that the vast majority of horses were supplied for the military conquest and subsequent colonization of New Spain.

So what was the composition of the horses imported to begin breeding farms in the Spanish West Indies? Apart from the 20 to 30 war horses privately owned by the Granada horsemen of the second voyage and the unknown number of wild horses from the Guadalquivir marshes purchased by Columbus, we need more historical data to determine the content of the breeding stock. The local legends and the traditions of Almonte, Spain, imply that the villagers continued to gather horses for shipment to the Spanish colonies of the New World for at least the following decade. “Saca de La Yeguas” means “To take out the mares.” This corresponds to historical records indicating the shipment of mostly mares to the colonies. Today, the “Saca de la Yeguas” is a religious/historical festival celebrated yearly.

Shipments of horses from 1494 to 1507 give little information about the type or place of origin of the horses imported, only that the majority were mares. It is assumed they all came from Southern Spain, but it is possible that Northern Iberian horses and ponies could have been included. Pinto-colored horses from Southern France may also have been included. The colonization of the New World went on, but military conquest was on the backs of island-bred Spanish horses.

The earliest breed stock sent to the New World probably had the most effect on the conformation of the succeeding generations of North American horses⁷. Even the best horses imported to Hispaniola were not part of a “breed” by the modern definition, but rather, they were horses from a region such as Andalusia or Extremadura. Alternately, they were described by usage categories such as “war horse”, “pack horse”, or “palfrey”. The conquistadores referred to their horses by the region in which they were bred, such as “Extremadurenos” and “Andalucenos”²³. However, at that time, there was a type of horse that had uniformity which could come close to a “breed” in the modern sense. It was the Spanish Jennet. The uniformity was due to its selective breeding and origin in a limited geographical region, the Moorish caliphates. Through years of selective breeding, Andalusia and Extremadura produced a uniform type of horse under Muslim rule. Jennets of the time were known by the name of the particular region, town, or breeding family from which they originated.⁴ The Moors of Granada instituted the concept of “pedigree” for the famous Grenadine Stud breeding farm outside of Granada. The practice of keeping pedigrees was continued after the conquest of Granada by the Carthusian order of monks²³.

Jennets, whose name came from riding “a la jineta”, which used shortened stirrups, were small horses with a low set, long wavy tails, and narrow, deep chests with forelimbs set close together. Skeletons from horse burials dating from the early middle ages give a reliable estimate of the height and conformation of the typical Southern Iberian horse of the time. It stood around 14 to 14½ hands (142-148cm), was short-backed, and had short cannon bones²⁶. The classic Jennet had a shapely, curved neck with a thick wavy mane, and many were gaited²³. This description is almost the exact physical description of today’s Spanish Colonial horse and certainly Sulphur Mustangs.

Jennet horses were again exported to the New World after the ban of 1507 was lifted. At this time and for the next 200 years, Cuba became the center of Jennet breeding with stock from Spain and Hispaniola. Hernan Cortes’ 16 horses for his first expedition to conquer the Aztec empire came from Cuba, but none survived his defeat at the hands of the Aztecs, starting with what is known as the “Noche Triste”. The account of the “first sixteen horses” by Bernal Diaz were descriptions of second or third-generation Cuban Jennets of the time, not horses from Spain^3,4. “Spanish Jennet” is a modern term that was unknown at the time of Cortez. He would have called his horse a “Cuban horse”.

In Cortez’s second attempt to capture the Aztec capitol starting in April of 1521, he started with new canons and new recruits. In contrast, this time, recruits and 40 horses came from Hispaniola, not Cuba. After capturing the Aztec capital, another 144 Jamaican-bred horses were added for future campaigns³.

Not long after Hernan Cortes conquered the Aztec Empire, he established horse and cattle breeding farms in the beautiful valleys of Mexicalzimgo and Oaxaca north of Mexico City. While it is possible some breeding stock came from Spain, the great expense and high mortality rate of horses during the passage over the Atlantic made such horses very rare. Therefore, most of the horses used in the Mexican breeding farms came from the established breeding operations on Hispaniola, Cuba, Puerto Rico, and especially Jamaica. Pizarro’s mounts for the conquest of the Pacific coast of South America and the Inca Empire came from Jamaica⁴.

By 1550, Central Mexico was known for its large horse herds and excellent riders. The original Spanish horse populations for Alta California, New Mexico, and the American Southwest did not come directly from Spain, but from breeding farms in Central Mexico. Before that, most of their ancestors came from the islands of the Caribbean. It must be remembered that the exact composition of the horses bred on Spanish horse farms in the West Indies cannot be ascertained purely from historical documents. Due to the lack of archeological faunal remains from these early breeding horse operations, the genomic nature of these original horses needs further research. Genetic samples of early California, Central Mexican, New Mexican, and Southwestern horses of the 16th and 17th centuries are also lacking, with less than 10 samples known to exist.

While horse breeding and horse introduction were occurring in Mexico and South America using the descendants of Spanish Jennets mixed with bloodlines from the ancestors of wild Marismeno and other “common” Spanish horse types, the horse in Spain was radically changing. In the early 16th century, which was still in the lifetime of Columbus, the Crown of Castile under Charles I ordered the Carthusian order of monks to take the finest Jennets and mix them with European Warmbloods to make a much larger and elegant horse. The Carthusian order soon established breeding operations in Jerez, Seville, and Cazalla. Despite popular belief, no elegant Cartujeno Andalusian ever took part in the initial Spanish settlement of the New World, for they did not exist during those decades. By 1730, which was 230 years after Columbus’ second voyage, the Carthusian Andalusian was an established breed but had little resemblance to its Jennet founders. Eventually, this new type of Spanish horse was shipped to the Americas in limited numbers in the 17th and 18th centuries. Still, they exhibited little influence on the already established horse populations of New Spain. Even today, it is not uncommon to find genetic markers associated with Northern European Warmbloods in Andalusian and Lusitano horses. Under the reigns of Charles V and Phillip II, this new Spanish type of horse was considered the finest in the world. In Spain, the original Jennet slowly became extinct, but the Jennet lived on, although mixed with other types of horse, in the Americas⁶.

So why was this history of horses in America presented in this paper? Dr Phillip Sponenberg probably said it best regarding Spanish Colonial horses, including Sulphurs and their related cousins. “These horses are a direct remnant of the horses of the Golden Age of Spain, which type is now mostly or wholly extinct in Spain. Therefore, the Colonial Spanish horses are a treasure chest of genetic wealth from a time long gone”⁷.

1995 study of Sulphur Mustangs

In 1995, Dr E. Gus Cothran and colleagues collected 118 Sulphur Mustang blood samples for a University of Kentucky study. The samples were used to detect variations of alloantigens at seven blood group loci or locations. In addition, starch and polyacrylamide gel electrophoresis and isoelectric focusing were used to detect variations in loci found in serum and red blood cell lysate sites in the collected blood samples.

The results of this study show a definite link to Iberian ancestry. Sulphur Mustangs have eight genetic variants which are associated with Iberian horses. Although in low frequency, two were associated with old Spanish bloodlines.

The data shows that the Sulphur herd does indeed have Spanish ancestry. However, this data cannot determine the extent of that ancestry. Sulphurs are also paired with many North American gaited breeds such as Mountain Pleasure horses.

This study showed that although Iberian bloodlines could be detected in the Sulphur population, the amount of influence, the date of introduction of Iberian horses to the parent population, or the percentage of Spanish blood could not be determined⁸.

A phylogenic tree or dendrogram shows the Sulphur herd clustering within other Iberian breeds but does not show an exact ancestry⁸.

Genetic Study Method

In 2020, adopted owners of Sulphur Mustangs were contacted and asked to send follicle samples of their horses to the Animal Genetics Department of Texas A&M University under the direction of Dr Rytis Juras. About 65 samples were collected for the study, and 15 standard microsatellite markers determined the genotype of each horse. The resulting data was examined using a novel application of principle coordinate analysis. Principle coordinate analysis is a statistical method that converts complex information into graphical data points representing relationships between points by distance. The results of these studies revealed the singular genetics of Sulphur Mustangs and their relationship to other Spanish Colonial Mustangs. Although their origins remain a mystery, this study showed they have a unique genetic heritage that gives them an irreplaceable place in American history.

There will be outliers with any feral horse population, even one as isolated as the Sulphur Mustangs. In this case, the Sulphur Herd Management Area constitutes a vast area. On the periphery of this area, some horses do not genetically fit into the population but are adopted as Sulphur Mustangs due to their residence within the borders of the Sulphur HMA. Occasionally, horses have filtered into the central HMA and genetically changed the original Sulphur Mustang core population. Therefore, the study’s first goal was to determine whether the Sulphur Mustangs sampled represent a single breeding population. A single breeding population would be represented by the presence of a central data cluster using the genotypes of individual horses and applying the resulting data points to a principle coordinate analysis statistical test.

Using principle coordinate analysis (PCoA) and graphing the results, the following graph shows a central population cluster with outliers. In the below image, the central population cluster is circled in red, and blue arrows show the outliers. The graph confirms that Sulphur Mustangs are a discrete population. However, some outliers should be considered something other than Sulphur Mustangs, although gathered in the same HMA. For example, the two individual horses marked with blue arrows did not fit the Sulphur genetic profile.

Outliers were removed from the study population, and what remained was compared to known Iberian and North and South American breeds of known Iberian descent.

Over the past 200 years, during which Sulphur Mustangs are believed to have been present in the Needle Range of Utah, there has been mixing with other breeds. This contributed to Sulphur Mustangs undergoing their own course of evolution. As previously discussed, the Sulphur Mustang’s Jennet forbears from Spain were changed and mixed with other Spanish horses in the breeding farms of the West Indies, then changed again in Central Mexico. Later, Sulphur Mustangs were altered by the harsh environment of the Needle Mountain range and occasional mixing with other stray horses. Blood markers analyzed in the 1995 study show a robust Iberian connection, as does mtDNA, but they have changed from their ancestral Spanish horses of 500 years ago. Over that vast time period, Iberian horses also changed²⁴.

As shown by the history of Spanish horses in the Americas, horses mounted by the Spaniards of the 1493 expedition would have little resemblance to the 21st-century horses of Andalusia and Extremadura provinces of Spain. These horses have undergone their own evolution over the past 500 years. Even the wild Marismeno horses of the Guadalquivir marshes have changed profoundly from the time of Columbus. To improve the wild Marismeno stock, modern Andalusian horses have been bred into the wild herds, and inferior horses are culled.

For a second PCoA graph, central coordinates or centroids of each population were plotted in the PCA graph of Iberian breeds to allow the viewer to discern the relationships and genetic distances between populations. The PCoA graph below shows that the Sulphur population centroid (SU) holds a singular position on the upper left, which is distant to the modern Iberian and New World breeds of known Iberian descent. It should be noted that many South American breeds were founded by a mixture of horses from the Caribbean and select horses imported directly from 15th-century Spain. This is unlike North American Spanish horses, which were founded almost exclusively from Caribbean breeding farms⁷.

Key: Su – Sulphur, GL – Galiceno, GR – Garrano, PO – Pottoka, AN – Andalusian, AC – Argentine Criollo, Au – Asturcon, BZ – Brazilian Criollo, CF Colombian Paso, CP – Campolino, CC – Chilean Criollo, PN – Pantaneiro, CI – Chilote, LO – Losino, MQ – Mallorquina, ME – Menorquina, MR – Marismeno, RE – Retuerta, LU – Lusitano, MM – Mangalarga Marchadore, MA – Mangalarga, RP – Puerto Rican Paso Fino, PP – Peruvian Paso, AZ – Terciera, VC – Venezuelan Criollo.

Also, observe the Retuerta horse centroid on the lower left. The Retuerta (RE) centroid holds a singular position on the PCoA graph. The primitive Retuerta horses of the Guadalquivir Valley in Southern Spain are considered unrelated to other Iberian horses. The PCoA graph confirms the Retuerta’s unique genetic position and genetic distance from Sulphur Mustangs and other Iberian and Iberian-derived horse groups. Could the ancestors of Retuerta horses have been included in the initial shipments to New World Spanish colonies? We don’t know.

Queen Isabella was from the royal dynasty of Castille, which was the most influential kingdom in Northern Spain. The Northern Spanish kingdoms consisted of Galicia, Leon, Castille, Navarra, and Aragon. The marriage of Ferdinand II of Aragon to Isabella of Castile began the reign that would unite Spain’s dominions and elevate the nation to a dominant world power. Surely, the horses of these northern kingdoms were being used during the final conquest of the last Moorish kingdoms in southern Spain. Although recorded history does not state this, horses from northern Spain could have been ridden by warriors to Granada in 1492.

As a point of interest, two groups of Northern Spanish ponies of Celtic origin, the Asturcon and the Losino, are shown separated from the other breeds. This grouping also includes the Mallorquina and the Menorquina, two breeds of Spanish horses from Mediterranean islands in the Balearic Archipelago.

Being in geographic isolation, these Balearic horses were found not to be genetically close to other Iberian breeds based on these genetic markers. Even though at least some of their ancestors came from the mainland of Iberia, population divergence and the introduction of other horses over the centuries have caused these populations to become unique breeds. The exact process applies to Sulphur Mustangs. The graph shows that Sulphur Mustangs are a distinctive group of North American horses with Iberian ancestry but have become a separate population. Population divergence and the introduction of other horse bloodlines in Sulphur Mustangs are very similar to the processes which created other Iberian horses such as the Mallorquina, which have followed a particular course of development because of geographical isolation and human breeding preferences.

The Old Spanish Trail

The Sulphur HMA is close to the Old Spanish Trail. This trade route was extensively used from the early 1830s until the advent of the railroad in the mid-1850s. It became famous as the main land route to move goods between Santa Fe, New Mexico, and Los Angles, California.

Woolen goods and enslaved Native Americans moved west from Santa Fe, and horses returned east along the trail to New Mexico in great numbers. After arriving in Santa Fe, many horses would eventually be sold in Missouri via the Santa Fe Trail⁹. The genetic link to other American breeds like the Rocky Mountain horse and Mountain Pleasure horse could have begun during the decades this horse trade route was in existence. It was assumed that Sulphur Mustangs are descended from stray horses brought from Colonial California (Alta California), but there is no scientific or historical proof. The origins of Sulphur Mustangs are as murky and mysterious as any group of North American feral horses. In the future, DNA from remains of early 18th-century California horses could be used to establish a link if such a link exists. Nevertheless, for now, this origin story is only speculation.

Conclusion

The unique position of Sulphur Mustangs in Fig.5 could be affected by the considerable variation in the population sizes of horse breeds included in the graphed data. The graph also reflects centuries of accumulated divergence of an isolated population under the influence of extreme habitat conditions. Even in their isolated location, it should also be noted that occasionally other horse types were introduced into the Sulphur population over the centuries. Furthermore, we have yet to find the exact genotype of horses from Alta California, New Mexico, and the Northern Territories of New Spain in the 17th and 18th centuries, where it is believed the ancestors of Sulphur Mustangs were derived. The procurement of archeological faunal horse samples from the Spanish Colonial era of the Southwest would allow a comparison of Sulphur Mustangs to the horses long believed to be their ancestors.

» Next: The Sulphur Mustang’s Relationship to Other North American Spanish Colonial Horses

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