by Jay F.Kirkpatrick，Ph.D.and Patricia M.Fazio，Ph.D.（Revised January2010）
Are wild horses truly“wild，”asan 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，Equas caballus单击功能区上，should be considered native wildlife。
The question is legitimate，and the answer important.In North America，the wild horse is often labeled as anon-native，or even an exotic species，by most federal or state agencies dealing with wildlife management，such as the National Park Service，US Fish and Wildlife Service，and the Bureau of Land Management.The legal mandate for many of these agencies is to protect native wildlife and prevent non-native species from causing harmful effets on the general ecology of the land.Thus，management is often directed at total eradication，or at least minimal numbers.If the idea that wild horsweed，ed，native wildlife，a great many current management approaches might be compromised.Thus，the rationale for examining this proposition，that the horse is a native or non-native species，is significant。
The genusEquas单击功能区上，which includes modern horses，zebras，and asses，is the only surviving genus in aonce diverse family of horses that included27genera.The precise date of origin for the genusEquasis unknown，but evidence documents the dispersal ofEquasfrom North America to Eurasia approximately2-3million years ago and a possible origin at about3.4-3.9 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 probably occurred between13000and11000years ago（Fazio1995），although more recent extinctions for horses have been suggested.Dr.Ross MacPhee，Curator of Mammalogy at the American Museum of Natural History，and colleagues，have dated the existence of wooly mammoths and horses in North America to as recent as7600years ago.Had it not been for previous westing ge，into northwestern Russia（Siberia）and Asia，the horse would have faced complete extinction.However，Equassurvived and spread to all continents of the globe，except Australia and Antarctica。
In1493，on Columbus’second voyage to the Americas，Spanish horses，representingE.caballus，were brought back to North America，first in the Virgin Islands，and，in1519，they were reintroduced on the continent，in modern-day Mexico，from where they radiated throughout the American Great Plains，after escape from their owners or by pilfering（Fazio1995）。
Critics of the idea that the North American wild horse is a native animal，using only selected paleontological data，assert that the species，E.caballus（or the caballoid horse），which was introduced in1519，was a different species from that which disappeared between13000-11000 years before.Herein lies the crux of the debate.However，neither paleontological opinion nor molecular genetics support the contention that the modern horse in North America notics。
Equas单击功能区上，a monophyletic taxon，is first represented in the North American fossil record about four million years ago byE.simplicidens，and this species is directly ancestral to later Blancan species about three million years ago（Azaroli and Voorhies1990）。Azzaroli（1992）believed，fossil records，thatE.simplicidensgave rise to the late PlioceneE.Idahoensis单击功能区上，and that species，in turn，gave rise to the first caballoid horses two million years ago in North America.Some migrated to Asia about one million years ago，while others，such asE.niobrarensis单击功能区上，remained in North America。
North America，the divergence ofE.caballus内部various ecomorphotypes（breeds）includedE.caballus mexicanus，或American Periglacial Horse（also known as）E.caballus laurentiusHay，ormidlandensisQuinn）（Hibbard1955）。Today，we would recognize these latter two horses as breeds，but in the realm of wildlife，the term used is subspecies.By ecomorphotype，we refer to differing phenotypic or physical characteristics within the same species，caused by genetic isolation in discrete habitats.In North Amcaeri， isolated lower molar teeth and a mandible from sites of the Irvingtonian age appear to beE.caballus，morphologically.Through most of the Pleistocene Epoch in North America，the commonest species ofEquaswere not caballines but other lineages（species）resembling zebras，hemiones，and possibly asses（McGrew1944；Quinn，1957）.3 Initially rare in North America，caballoid horses were associated with stenoid horses。By the late Pleistocene，the North American taxa that can definitely be assigned toE.caballusareE.caballus alaskae（Azzaroli1995）andE.caballus mexicanus（Winans1989—using the namelaurentius）。Both subspecies were thought to have been derived fromE.niobrarensis（Azzaroli1995）。
Thus，based on a great deal of paleontological data，the origin ofE.caballusis thought to be about two million years ago，and it originated in North America.However，the determination of species divergence based on phenotype is at least modestly subjective and often fails to account for the differing ecomorphotypes within a species， described above.Purely taxonomic methodologies looked at physical form 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，spa， these observations were lacking in precision.Nevertheless，the more subjective paleontological data strongly suggests the origin ofE.caballussomewhere between one and two million years ago。
Reclassifications are now taking place，based on the power and objectivity of molecular biology.Ifone 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。
A series of genetic analyses，carried out at the San Diego Zoo’s Center for Reproduction in Endangered Species，and based on chromosome differences（Benirschke et al.1965）and mitochondrial genes（George and Ryder1986）both indicate significant genetic divergence among several forms of wildE.caballusas early as200000-300000 years ago.These studies do not speak to the origins ofE.caballus per se单击功能区上，配电点对总排水量E.caballusby200000 to 300000 years ago.Thus，the origin had to be earlier，but，at the very least，well before the disappearance of the horse in North America between13000–11000 years ago.4The relatively new（30-year-old）field of molecular biology，using mitochondrial-Danalysis，has recently revealed that the modern or caballline horse，E.caballus，总equivalent toE.lambei单击功能区上，a horse，according to fossil records，that represented the most recentEquasspecies in North America prior to extinction.Not only isE.caballusgenetically equivalent toE.lambei单击功能区上，but no evidence exists for the origin ofE.caballusanywhere except North America（Forstén1992）。
According to the work of researchers from Uppsala University of the Department of Evolutionary Biology（Forstén1992），mitochondrial-DNA，forE.caballus，is set at approximately1.7 million years ago in North America.This，of course，is very close，geologically speaking，to the 1-2million-year figure presented by the interpretation of the fossil record。
Carles Vilà，also of the Department of Evolutionary Biology at Uppsala University，has corroborated Forstén’swork.Vilàet al.（2001）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，corroborating the work of Benirschke et al.（1965），George and Ryder（1995），and Hibbard（1955）。
Astudy conducted at the Ancient Biomolecules Centre of Oxford University（Weinstock et al.2005）also corroborates the conclusions of Forstén（1992）。Despite a great deal of variability in the size of the Pleistocene equids from differing locations（mostly ecomorphotypes），the DNA evidence strongly suggests that all of the large and small caballline samples belonged to the same species.The author states，“The presence of a morphologically variable caballine species widely distributed both north and south of the North American ice sheets raises the tantalizing possibility that，in spite of many taxa named on morphological grounds，most or even all North American caballines were members of the same species.”
In another study，Kruger et al.（2005），using microsatellite data，confirms the work of Forstén（1992）but gives a wider range for the emergence of the caballoid horse，of0.86 to 2.3 million years ago.At the latest，however，that still places the caballoid horse in North Amerih 860000years ago。），外接接头，外接接头E.lambeifrom the permafrost of Alaska，found that the variation was within that of modern horses，which translates intoE.lambei活动弯头E.caballus单击功能区上，genetically.The molecular biology evidence is incontrovertible and indisputable，but it is also supported by the interpretation of the fossil record，as well。
Finally，very recent work（Orlando et al.2009）that examined the evolutionary history of a variety of non-caballine equids across four continents，found evidence for taxonomic“overspitting”from species to generic levels。This overspitting was based primarily on late-Pleistocene fossil remains without the benefit of molecular data.A co-author of this study，Dr.Alan Cooper，of the Australian Centre for Ancient DNA，stated，“Overall，new genetic results suggest that we have underestimated how much a single species can vary over time and space，and mistakenly assumed more diversity among extinct species of megafauna.”
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 domesticate is quite irrelevant.Domestication altered little biology，and we can see that in the phenomenon called“gold，gold”，”where wild horses revert to ancient behavioral patterns。Feist and McCullough（1976）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。
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.Przewalskii（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 during the 1990s and now repopulate their native range in Mongolia.Are they areintroduced native species or not？And what is the difference between them andE.caballusin North America，except for the time frame and degree of captivity？
The key element in describing an animal as native species is（1）where it originated；and（2）whether or not it co-evolved with its habitat。Clearly，E。6个caballusdid 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，with no economic value anymore，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-gose”。
Please cite as：Kirkpatrick，J.F.，and P.M.Fazio.Revised January2010.Wild Horses as Native North American Wildlife.The Science and Conservation Center，ZooMontana，Billings.8pages。
LITERATURE CITED
Jay F.Kirkpatrick，Director，The Science and Conservation Center，ZooMontana，Billings，holds a Ph.D.in reproductive physiology from the College of Veterinary Medicine at Cornell University。
Patricia M.Fazio，Research Fellow，The Science and Conservation Center，ZooMontana，Billings，holds a B.S.in agriculture（animal husbandry/biology）from Cornell University，and M.S.and Ph.D.degrees in environmental history from the University of Wyoming and Texas A&M University，College Station，respectively.Her dissertation was a creation history of the Pryor Mountain Wild Horse Range，Montana/Wyoming。
This document is the sole intellectual property of Drs.Jay F.Kirkpatrick and Patricia M.Fazio.As such，管束，管束，管束，管束.However，this article may be copied and预制框架in hardcopy，electronic，or website form，for educational purposes only。
