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Should the wild horses that roam North America be considered native wildlife? They may have been “introduced” by man, but scientific evidence suggests that they are genetically the same as the horses that became extinct on the continent between 11,000 and 13,000 years. In fact, the genus Equus could have been wiped out entirely had it not crossed the Bering Stait land bridge into Eurasia. Jay F. Kirkpatrick, Ph.D. and Patricia M. Fazio, Ph.D.* look at the evidence.

Are wild horses truly “wild,” as an indigenous species in North America, or are they “feral” weeds – barnyard escapees, far removed genetically from their prehistoric ancestors? The question at hand is, therefore, whether or not modern horses, Equus caballus, should be considered native wildlife.

The genus Equus, which includes modern horses, zebras, and asses, is the only surviving genus in a once diverse family of horses that included 27 genera. The precise date of origin for the genus Equus is unknown, but evidence documents the dispersal of Equus from North America to Eurasia approximately 2 to 3 million years ago and a possible origin at about 3.4 to 3.9 million years ago. Following this original emigration, several extinctions occurred in North America, with additional migrations to Asia (presumably across the Bering Land Bridge), and return migrations back to North America, over time. The last North American extinction occurred between 13,000 and 11,000 years ago.¹

Had it not been for previous westward migration, over the land bridge, into northwestern Russia (Siberia) and Asia, the horse would have faced complete extinction. However, Equus survived and spread to all continents of the globe, except Australia and Antarctica.

In 1493, on Columbus’s second voyage to the Americas, Spanish horses, representing E. caballus, were brought back to North America, first in the Virgin Islands, and, in 1519, they were reintroduced on the continent, in modern-day Mexico, from where they radiated throughout the American Great Plains, after escape from their owners.²

Critics of the idea that the North American wild horse is a native animal, using only paleontological data, assert that the species, E. caballus (or the caballoid horse), which was introduced in 1519, was a different species from that which disappeared 13,000 to 11,000 years before.

Herein lies the crux of the debate.

However, the relatively new (27-year-old) field of molecular biology, using mitochondrial-DNA analysis, has recently found that the modern or caballine horse, E. caballus, is genetically equivalent to E. lambei, a horse, according to fossil records, that represented the most recent Equus species in North America prior to extinction. Not only is E. caballus genetically equivalent to E. lambei, but no evidence exists for the origin of E. caballus anywhere except North America.³

According to the work of Uppsala University researcher Ann Forsten, of the Department of Evolutionary Biology, the date of origin, based on mutation rates for mitochondrial-DNA, for E. caballus, is set at about 1.7 million years ago in North America. Now the debate becomes one of whether the older paleontological fossil data or the modern molecular biology data more accurately provide a picture of horse evolution. The older taxonomic methodologies looked at physical form for classifying animals and plants, relying on visual observations of physical characteristics. While earlier taxonomists tried to deal with the subjectivity of choosing characters they felt would adequately describe, and thus group, genera and species, these observations were lacking in precision.

Reclassifications are now taking place, based on the power and objectivity of molecular biology. If one considers primate evolution, for example, the molecular biologists have provided us with a completely different evolutionary pathway for humans, and they have described entirely different relationships with other primates. None of this would have been possible prior to the methodologies now available through mitochondrial-DNA analysis.

Carles Vila, also of the Department of Evolutionary Biology at Uppsala University, has corroborated Forsten’s work. Vila et al have shown that the origin of domestic horse lineages was extremely widespread, over time and geography, and supports the existence of the caballoid horse in North American before its disappearance.4

Finally, the work of Hofreiter et al,5; examining the genetics of the so-called E. lambei from the permafrost of Alaska, found that the variation was within that of modern horses, which translates into E. lambei actually being E. caballus, genetically. The molecular biology evidence is incontrovertible and indisputable.

The fact that horses were domesticated before they were reintroduced matters little from a biological viewpoint. They are the same species that originated here, and whether or not they were domesticated is quite irrelevant. Domestication altered little biology, and we can see that in the phenomenon called “going wild,” where wild horses revert to ancient behavioral patterns. James Dean Feist dubbed this “social conservation” in his paper on behavior patterns and communication in the Pryor Mountain wild horses. The reemergence of primitive behaviors, resembling those of the plains zebra, indicated to him the shallowness of domestication in horses.6

The issue of feralization and the use of the word “feral” is a human construct that has little biological meaning except in transitory behavior, usually forced on the animal in some manner. Consider this parallel. E. Przewalski (Mongolian wild horse) disappeared from Mongolia a hundred years ago. It has survived since then in zoos. That is not domestication in the classic sense, but it is captivity, with keepers providing food and veterinarians providing health care. Then they were released a few years back and now repopulate their native range in Mongolia. Are they a reintroduced native species or not? And what is the difference between them and E. caballus in North America, except for the time frame and degree of captivity?

The key element in describing an animal as a native species is (1) where it originated; and (2) whether or not it co-evolved with its habitat. Clearly, E. caballus did both, here in North American. There might be arguments about “breeds,” but there are no scientific grounds for arguments about “species.”

The non-native, feral, and exotic designations given by agencies are not merely reflections of their failure to understand modern science, but also a reflection of their desire to preserve old ways of thinking to keep alive the conflict between a species (wild horses) with no economic value anymore (by law) and the economic value of commercial livestock.

Native status for wild horses would place these animals, under law, within a new category for management considerations. As a form of wildlife, embedded with wildness, ancient behavioral patterns, and the morphology and biology of a sensitive prey species, they may finally be released from the “livestock-gone-loose” appellation.

3 Ann Forsten, 1992. Mitochondrial-DNA timetable and the evolution of Equus: Comparison of molecular and paleontological evidence. Ann. Zool. Fennici 28: 301-309.

4 Carles Vila, Jennifer A. Leonard, Anders Gotherstrom, Stefan Marklund, Kaj Sandberg, Kerstin Liden, Robert K. Wayne, Hans Ellegren. 2001. Widespread origins of domestic horse lineages. Science 291: 474-477.

5 Hofreiter, M., D. Serre, H.N. Poinar, M. Kuch, S. Päbo, S., 2001. Ancient DNA. Nature 2: 353-359.

6 James Dean Feist and Dale R. McCullough. 1976. Behavior patterns and communication in feral horses. Z. Tierpsychol. 41: 367

* Jay F. Kirkpatrick, Director, The Science and Conservation Center, Billings, Montana, holds a Ph.D. in reproductive physiology from the College of Veterinary Medicine at Cornell University. Patricia M. Fazio is a freelance environmental writer and editor, and holds a B.S. in animal husbandry/biology from Cornell University, an M.S. in environmental history from the University of Wyoming, and a Ph.D. in environmental history from Texas A&M University, College Station.

This document is the intellectual property of Drs. Jay F. Kirkpatrick and Patricia M. Fazio. As such, altering of content in any manner is strictly prohibited. However, this statement may be copied and distributed freely in hardcopy, electronic, or Website form. Please include footnotes.

Article first published on Horsetalk.co.nz in September, 2006.