Classification of the Horse

Scientists determine how animals are related to one another (classified) by comparing the characteristics they have in common (or do not have in common). Horses belong to a group of mammals called ungulates. Ungulates are animals with hooves -- hard coverings that protect their toes and allow ease in running. Other members of this group include goats, sheep, pigs, deer, bison, and rhinos. Not all hooves are alike, however. Some protect an even number of toes (goats, sheep, pigs, deer, and bison), while others protect an odd number (horses and rhinos).

The term "horse" may be applied in a narrower sense, to indicate only the modern horse species, Equus caballus, or it may have broader meaning, encompassing any member of the horse family Equidae (equids). Equids include not only horses, in the more restrictive use of the word, but zebras, asses, the earliest "dawn horse" (Hyracotherium), and all now-extinct horses that have ever lived. Taxonomists identify seven existing species within the Equidae family (all genus Equus sp.), comprised of the modern horse, three species of African zebras (Plains or Burchell's, Mountain, and Grevy's), an African ass, an Asiatic half-ass, and Przewalski's horse of Mongolia. The closest living relatives of the horse are rhinos and tapirs, despite the fact that horses walk on one toe and both rhinos and tapirs walk on three toes. Rhinos now live only in Africa and parts of Asia. Tapirs look similar to early horses and are among the most primitive of the larger animals on earth. They live in Malaysia and in parts of Central and South America.

Both domesticated and wild free-roaming horses are classified as E. caballus. Wild horses of the eleven western states are direct descendants of domesticated horses that escaped or were released by Spanish explorers, Native Americans, settlers, miners, cavalrymen, and ranchers. With 57 million years of evolutionary history in North America, horses gone wild readily adapt to western rangelands and open woodland ecosystems. Within a few generations, natural selection brings out biological and behavioral characteristics that promote survival in the harshest of environmental conditions -- intense cold or heat, drought, lack of adequate forage (or forage of low nutritive quality), internal and external parasites, insect pests, disease, and predation. Horses are sensitive prey species. Over millions of years, they have developed behaviors and physical attributes that have protected them from predators, although those predators that exerted the greatest selective pressures, e.g., dire wolves, saber-toothed tigers, became extinct about the same time horses apparently became extinct at the end of the Ice Age. Their agility and speed in running, ability to see peripherally (and independently, with each eye), swiveling ears, sensitive smell and hearing, and kicking and biting instincts attest to a long evolutionary journey of wariness.

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Biology and Morphology of the Wild Horse

Wild horses range in size from about 14 hands high (4.7 feet) (one hand equals four inches) to a draft size of 16-17 hands (5.3-5.7 feet). They can weigh anywhere from 550 to 1,200 pounds. Coat color, pattern, and texture of adult wild horses varies greatly, depending on the herd area (geographic location) and ancestry of the herd. One might see bays, buckskins, palominos, pintos, roans, duns, sorrels, gruellas, or Appaloosa-patterned colors. In such herds as those within the Pryor Mountains of Montana/Wyoming, and the Kiger horses near Burns, Oregon, one might also see zebra stripes above the knee and hock, a black dorsal band, and a black cross over the withers -- indicative of Spanish Colonial origin. Wild horses sometimes display ancestry indicating Barb (northwestern Africa), Arab (northern Africa), Sorraia (Portugal), Tarpan (Ukraine and Eastern Europe), or Andalusian (Spain) origins.

The skull of the horse is elongated, with teeth well adapted for chewing and grinding. Special characteristics of the horse’s mouth are highly prehensile lips for gathering food, that work in conjunction with sharp front teeth when cropping grass, and the labile tongue which conveys the food to the back teeth. These have table-like surfaces crossed by ridges that form an ideal grinding surface between the upper and lower jaws. Modern horses have 40 teeth, comprised of 12 incisors for cropping grasses; four canines used for grooming, threat displays, and biting; and 24 large cheek teeth (six premolars and six molars on the side of each cheek) for grinding food. Anatomical peculiarities of the horse’s digestive system compared with other mammals are: (i) that the greatest volume of the tract is in the hind end, namely the caecum, and colon, where the major process of digesting fiber occurs by bacterial fermentation (ii) the relatively small stomach (iii) the absence of a gall bladder (probably associated with the need for a continual supply of bile in an animal which is a continuous feeder). The caecal system of digestion and energy absorption allows horses to subsist on a low quality diet, during periods when forage is scarce. They can even consume their own dung piles for survival, when necessary.

The ears of a horse are controlled by 13 sets of muscles. They can swivel independently of each other, picking up sounds both ahead and behind at the same time, allowing horses to evaluate their surroundings or to signal intentions or state of mind to others within the group. Horses' necks and manes are long -- well suited for grazing -- while long legs and strong hooves, adapted for running on hard or rocky ground, support a muscular body. The foot as a whole absorbs concussion, and by its continuous growth, it is able to replace the surface lost by everyday wear and tear. For swatting insect pests, a long tail, covered with coarse, wispy hairs, functions well.

Sexual maturity for mares normally occurs after one year, with the period of reproductive capacity lasting up to 22 years of age -- essentially for as long as most wild mares live. Wild horses are seasonal breeders, with female estrus (heat) beginning in late March and lasting through July. Mares in heat are receptive for six days, followed by 15 days of non-receptivity before returning to heat. This cycle is repeated until she is impregnated or when the breeding season ends. With a 340-day (11.5-month) gestation period, the time of breeding assures that most wild foals will be born during February through June, when environmental conditions will better support survival. One foal is normal. Within the first half-hour of life, foals will be on their feet, nursing, and capable of flight. Sex ratios for foals at birth have been reported to be 1.3 males per 1.0 females. Young are generally weaned at 9-11 months. However, as early as six weeks, foals will begin to sample grass but will continue nursing, as well, for months. Stallions, while they may reach sexual maturity after one year, are normally found in bachelor groups and do not breed until they have attained their own harem of mares, foals, and yearlings, at about 4-6 years of age.

While it is not unusual for pampered domesticated horses to reach age 30 and beyond, it appears that few wild horses live this long. However, longevity of wild horses is purely speculative, as it is difficult to accurately determine the age of a horse beyond 20 years of age. No empirical research has been carried out. Those who have spent years observing wild horses in the field, and have identified individual horses, feel that many do live beyond the age of 20 and, possibly, even longer. In dealing with the rigors of the wild, free-ranging wild horses encounter dangers, diseases, and, sometimes, predators from which a domesticated horse would be afforded some protection, and, therefore, undoubtedly do possess a shorter lifespan.

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TAXONOMIC CLASSIFICATION OF THE MODERN HORSE, EQUUS CABALLUS - back to top

§ Taxonomy is the systematic method of classifying plants and animals or the classification of organisms based on degrees of similarity, purportedly representing evolutionary (phylogenetic) relatedness.

Kingdom Animalia (animals)

Phylum Chordata (chordates)

Chordata is the phylum of animals possessing a notochord, or dorsal stiffening rod, as the chief internal skeletal support at some stage of their development. Most chordates are vertebrates (animals with backbones), but the phylum also includes some small marine invertebrate animals. The three features unique to chordates, and found in all of them, at least during early development, are the 1) notochord, composed of gelatinous tissue and bound by a tough membrane; 2) tubular nerve cord (or spinal cord), located above the notochord; and 3) gill slits, leading into the pharynx, or anterior part of the digestive tract (the throat, in higher vertebrates). In addition, all have blood contained in vessels, and the tunicates and vertebrates have a ventrally located heart. All have a post-anal tail, that is, an extension beyond the anus of the notochord or backbone and of the body-wall musculature, containing no internal organs. In vertebrates -- animals of the subphylum Vertebrata -- a backbone of bone or cartilage segments, called "vertebrae," develops around the notochord. Its upward projections partially surround the nerve cord.

Subphylum Vertebrata (vertebrates)

Vertebrates constitute the vast majority of living chordates, and they have evolved an enormous variety of forms. The backbone of vertebrates protects the nerve cord and serves as the axis of the internal skeleton. The skeleton provides strength and rigidity to the body and is an attachment site for muscles. The vertebrae in the middle region of the trunk give rise to pairs of ribs, which surround and protect the internal organs. A cartilaginous or bony case encloses the brain. Bone is a substance unique to vertebrates.

Class Mammalia (mammals)

Mammals arose from reptiles in the Jurassic period and are now the dominant form of terrestrial vertebrate life. Like the birds, they have a four-chambered heart and a double-circuit circulatory system and are able to regulate body temperature. In the case of mammals, the insulating covering is provided by hair, a feature unique to the class, although in a few forms (particularly in marine species) nearly all the hair is lost, and insulation is provided by fat. A second distinguishing characteristic of mammals is the production of milk by females for the nourishment of the young. All mammals have internal fertilization, and all but the most primitive (the egg-laying monotremes of Australia) bear live young. The mammalian egg contains little yolk. In marsupials, the young are born at an extremely undeveloped stage and continue to develop in a milk-supplied pouch. In the vastly more numerous placental mammals, nourishment is passed from the circulatory system of the mother to that of the embryo by means of a placenta, and the young are born well-developed. Most mammals have highly evolved sense organs and larger brains than other vertebrates. As a group they display great adaptability to a variety of conditions and have spread to all regions.

Ungulates (hoofed mammals)

Ungulate is a broad term used for those groups of mammals that have substituted hooves for claws during their evolution. Ungulate is not an order unto itself, but a group of mammals encompassing two orders: Perissodactyla and Artiodactyla. Hooves are a character that appear to arise from a terrestrial, herbivorous lifestyle, with rapid locomotion. Ungulates are relatively large animals, none weigh less than 1 kg (2.2 lbs.), and they comprise the majority of terrestrial mammals over 50 kg. Modern ungulates belong to two different orders which diverged from a common hoofed ancestor 60 million years ago. Even though there is a superficial similarity between pigs and tapirs, rhinos and hippos, horses and cows, the two mammals of each pair do not belong to the same order.

Many hoofed mammals live in grasslands and savannahs. Ungulates have evolved features that are adaptive for life on open grasslands, such as long legs to increase running speed. To lengthen the legs, ungulates evolved digitigrade locomotion, that is, they walk on their toes. The hoof of a horse or cow is anatomically an enlarged toe. Artiodactyls such as deer, sheep, and goats walk on two toes; perissodactyls either walk on three toes (rhinos, tapirs, some extinct horses) or on one toe (living horses). The remaining toes not used for walking are reduced, as in pigs and tapirs, or completely lost, as in rhinos and most ruminants. Many ungulates have also evolved large, complexly grooved molar teeth to grind grasses and other food plants.

Order Perissodactyla

Horses, zebras, tapirs, and rhinos belong to the order Perissodactyla, an order that contains only 17 living species, in three living sub-groups, while camels, pigs, pecaries, hippos, deer, giraffe, cattle, antelope sheep, and goats all belong to the order Artiodactyla. An underlying common theme flows through the undulate body plan. Undulates usually have a long muzzled head, held horizontally on the neck, a barrel-shaped body, supported by both forelegs and hindlegs of roughly equal length, and small tails. The skin is quite thick, usually with a coat of bristly hairs instead of soft fur. All ungulates have thickened hard-edged keratinous hooves. There is a reduction in the number of digits, and a lengthening of the metapodials (bones comparable to those in the back of the human hand). The result is that the animal is balancing on the tips of its toes. With this limb construction, termed unguligrade, both speed and endurance are achieved.

Ungulates have a good, but not exceptional, sense of hearing, with small to medium-sized ears that can rotated to detect the direction of sound. They possess a very good sense of smell and excellent vision. Eyes function in both daylight and night conditions and are fairly binocular to judge speed and distance accurately. Communication depends mainly on sight and sound, with some use of olfactory cues and scent markings. The evolution of the ungulate limb illustrates adaptation to a mobile and open area existence. Evolution of teeth, skulls and digestive anatomy parallel changes in their locomotion. All ungulates are herbivores, feeding on leaves, flowers, fruits and seeds (pigs may be more scavenger-like). Food has to be taken directly from the tree or ground into the mouth because of limb evolution. Lips, teeth and tongue are adapted for this purpose. Plants they consume tend to be high in carbohydrates and sugars but lacking in proteins and fats. This does not seem to constitute a problem for ungulates, and many have even lost the gall bladder, which supplies bile salts that emulsify and break down fats.

Large, flat, square molars are used to disrupt the cell wall of the plant material so the digestible contents inside the cell can be utilized. The lower jaw can be moved crossways to the upper for grinding of food in contrast to the more latitudinal motion of other mammals. There is typically a gap between the milling molars and the plucking incisors. The incisors are retained in some species as weapons and absent in others. Most ungulates have a fibrous diet so they must utilize the cellulose content of food. The ingested food is fermented by bacteria somewhere along the digestive tract; products of fermentation can then be absorbed and utilized by the ungulate. In perissodactyls, the hindgut areas of the cecum and colon are the site of fermentation, and they are possess low efficiency in the utilization of cellulose. Most ungulate species have opted for becoming either predominantly browsers or predominantly grazers. The most abundant of the two are the grazers, evolving mouth parts and digestive systems to cope with the high cellulose and high silica content of grasses, and agility and speed to escape from predators that arise in open grassland areas.

Family Equidae (horse family)

The Equidae family consists of horses, asses and zebras. The Equidae family have a mane, 40-42 teeth, and skulls with long nasal bones. These are herd animals and fast runners, preferring to flee from danger rather than face it. The Equidae family are herbivores.

Genus Equus (Latin for horse)

Nine species fall within this genus. The domestic horse and pony are classified as Equus caballus.

Species caballus (Latin for pack-horse, nag)

(Author: Linnaeus, 1758. Citation: Syst. Nat., 10th ed., 1:73.)

= Equus caballus

{"Mammal Species of the World," National Museum of Natural History, Smithsonian Institution, Washington, DC, http://www.nmnh.si.edu/cgi-bin/wdb/msw/names/query, April 5, 2001; Dr. Timothy L. Lewis' Mammalian Ecology Classes, Wittenberg University, 200 West Ward Street, Springfield, Ohio 45501-0720 http://www5.wittenberg.edu/ academics/biol/courses/mammals/perart.htm; George Gaylord Simpson, Life: An Introduction to Biology (New York: Harcourt, Brace and Company, 1957); "Phylogeny of Equines," and "Family Equidae," Cavalry.org Headquarters Web Site, http://www.cavalry.org/phylogeny.htm}

ANCESTRY OF WILD HORSES - back to top

Blood and the Question of Origin

Legend has it that the Spanish wanted a war horse with the speed and endurance and toughness of the African Barb but also of sufficient size to carry armored knights. Thus, they began an experimental breeding program in Andalusia, in southwestern Spain, crossing barbs with European draft horses. They were able to produce the type of horse they wanted and, much later, these horses became known as Andalusians, owing to the region of Spain from which they originated. According to this conventional wisdom, Andalusians were the first horses to come to the New World during the latter 1400s and early 1500s. As Spanish exploration of North America increased, the Spanish homeland could not produce Andalusians fast enough, so immigrants to the New World began importing pure barbs from the Maghreb region of northern Africa. The Andalusian and the African Barb are therefore thought to be, according to this hypothesis, the earliest horses brought into the Americas. 1

However, no documentation exists to confirm or dispute this. Marye Ann Thompson, Secretary of the Spanish Mustang Registry, advises:

I personally would not attempt to try to determine what "breeds" were in early horses. There is no way to be possibly accurate nor even come close to accuracy. There is no way to second guess what happened four or five hundred years ago. ... Suffice to say, the Spanish brought them over so they were "Spanish Horses." Just because we have breeds today, such as the Pasos, Andalusians, etc. that doesn't mean that they existed 500 years ago. Their ancestors did, but were not called that and possibly didn't even look that way. ... Concerning the old Spanish horses, there is no truth to be ascertained as there is no way to turn back time and even if we could, I doubt if we would find out much as each small area in Spain and probably all other places in the world, were fairly ignorant of what went on just a few miles from them and probably didn't care so I imagine that they chose horses for breeding and all other matters simply on the basis of what they preferred in size, color, usefulness, etc. and had little thought about the background as long as they were what they wanted. 2

Using both phenotypic evidence and DNA analysis of the blood of wild horses, Phil Sponenberg, a veterinarian/Ph.D. from Virginia Polytechnic Institute believes the Pryor Mountain wild horse herd, in northern Wyoming/southern Montana to be "...the single most Spanish of the feral horse herds in the USA at this time. The history, phenotype, and bloodtypes of the herd all point to an origin from Spanish horses." He also believes the Pryor horses are a "...unique North American genetic resource... ." Of wild horse herds studied thus far, only four are now considered to be Spanish: 1) Pryor Mountain (Wyoming/Montana); 2) Sulphur (Utah); 3) Marble Canyon (Arizona); and 4) Kiger (Oregon). Other wild horse herds will, in time, be blood-typed, however, and may be added to this list of wild horses with a concentration of Spanish blood. {E. Gus Cothran, "Genetic Analysis of Horses from the Pryor Mountain Wild Horse Reserve [sic]," (typewritten), Equine Blood Typing Research Laboratory, Department of Veterinary Science, University of Kentucky, Lexington, February 1992, p. 6.; D. Phillip Sponenberg, "Evaluation of Pryor Mountain Herd Area BLM Horses - August 1993" (typewritten), Veterinary College, Virginia Polytechnic Institute, Blacksburg, Virginia, August 1993, p. 1.; D. Phillip Sponenberg, "Sponenberg's Comments on the Pryor Mountain Feral Horses" (typewritten), n.d.}

Thus, it is clear that the entire concept of breeds, in the sense that we use the term today, is rather recent. Perhaps just within the past 100 years have most standardized breeds come into being. Therefore, to reiterate, it would be, technically, erroneous to say that the original Spanish horses brought into North America, about 500 years ago, were representative of an intact breed, such as the Andalusian. They were not. It would be more correct to say that specific types of horses were imported at that time, and that, in modern times, breed names for those types were subsequently attached. Each Spanish province, area, even monastery, bred horses of the conformation, size, color, temperament, and ability they liked and that were popular within the area. 2

Through human selection or natural selection (owing to the environment in which they were developed), breeds of animals have come into being. They resemble one another and pass on common traits to their offspring. However, the following excerpt from The Genetics of Populations by Jay L. Lush helps explains why a good definition of "breed" is elusive:

A breed is a group of domestic animals, termed such by common consent of the breeders, ... a term which arose among breeders of livestock, created one might say, for their own use, and no one is warranted in assigning to this word a scientific definition and in calling the breeders wrong when they deviate from the formulated definition. It is their word and the breeder's common usage is what we must accept as the correct definition. 3

The Oxford English Dictionary gives the following definition for breed:

2. Race, lineage, stock, family; strain; a line of descendants from a particular parentage, and distinguished by particular hereditary qualities. (Abstract and concrete.) a. of animals. {Oxford English Dictionary. 2nd ed. 1989 (ed. J. A. Simpson and E. S. C. Weiner), Additions 1993-7 (ed. John Simpson and Edmund Weiner; Michael Proffitt), and 3rd ed. (in progress) March 2000 - (ed. John Simpson). OED Online. Oxford University Press. http://oed.com }

What is now seen in modern wild horse herds, therefore, is a blend of the old Spanish horses and newer English breeds, such as the Quarter Horse, Tennessee Walker, Standardbred, Morgan, Thoroughbred, and two draft breeds. Occasionally, as in a few herd management areas of the northern Red Desert in Wyoming, Shetland Pony and Appaloosa blood are conspicuous. However, before Western Expansionism (movement west by Euro-American settlers), the old Spanish breeds ruled their own wild kingdom, for a time, in the vast, lonely stretches of America's western landscape, prior to white settlement.

F o o t n o t e s

1: E-mail to Patricia M. Fazio, Buffalo Bill Historical Center (BBHC), Cody, Wyoming, March 28, 2001, Jay Kirkpatrick, Ph.D., Reproductive Physiologist, ZooMontana, Billings

2: Consecutive e-mails to Patricia M. Fazio, Buffalo Bill Historical Center, Cody, Wyoming (BBHC), March 27, 28, 29, 2001, Marye Ann Thompson, Secretary, Spanish Mustang Registry, HCR 3, Box 7670 Willcox, AZ 85643

3: "Breeds of Livestock - What is a Breed?" Dept. of Animal Science, Oklahoma State University, Stillwater, March 13, 2001 (Web site opened February 22, 1995)

Courtesy of Unbroken Spirit: The Wild Horse in the American Landscape (@BBHC 2001 All Rights Reserved)