Hey
everyone. I’ve hit a bit of a writer’s block with the European series, so I’ve
decided to start the next one and come back to it later. The next five articles
are going to be explorations of the rewilding potential for five taxonomic
families that have been completely (or partially in one case) extirpated from
temperate North America. These are, in order of discussion, the Equidae,
Camelidae, Tayassuidae, Tapiridae, and Elephantidae. For today’s article we
will be starting with the Equidae, a family which owes its origins and much of
its evolutionary history to North America, and yet is no longer found there in
a truly wild state, having been extirpated sometime during or after the
Pleistocene-Holocene transition. I will discuss the taxonomic identity of the
mid-late Pleistocene equids of North
America, and the reintroduction possibilities for this group.
The
taxonomic history of the North American horses has been long and complex. It
has been repeatedly confused by the presence of feral European animals,
repeated prehistoric migration between North America and Eurasia, and the
tendency of palaeontologists to radically overclassify. In the past few
decades. We’ve gone from having dozens of recognized native North American
equids, with taxonomic affinities variously ascribed to horses, asses, and
zebras, to having only two legitimate species, one of which is still very much
with us. Basically all fossil equids from the mid-late Pleistocene of North
America can be placed into two groups. The first is the caballine group, which
is now known to be of the same species as the living Equus caballus, which
includes many extant and extinct groups of animals from Eurasia, including the
modern domestic horse (which descends from the extinct European wild horse or
tarpan) and the wild Przewalski’s horse, or takhi, from Asia. The second group
is the stilt-legged equids, which were originally thought to be members of the
living Asinus (sub)genus, which
includes the Eurasian and African wild asses (Asinus hemionus and Asinus
asinus), as well as the domestic African ass, or donkey. This conclusion
was arrived at through morphological analysis, but genetic studies have since
shown that this group is distinct and deserves its own genus. Consequently all
stilt-legged equids from the mid-late Pleistocene of North America are referred
to as variations of the single species Harringtonhippus
francisci. With this information we can now refer any equid remains from
this time period to one of these two groups.
The
caballines have been, in a sense, already reintroduced. Areas of North America
once occupied by various extinct subspecies or varieties of Equus caballus (ex. complicatus, conversidens, niobrarensis, lambei, occidentalis, scotti,
etc) were very quickly recolonized by feral Spanish horses, Equus caballus caballus, after said
horses were brought to North America by the conquistadors. These populations
resulted in the modern mustang, which was quite happily able to recolonize its
former range through a combination of natural migrations and trading by First
Nations peoples. These days however, mustangs are quite rare, and have been
severely reduced in number and frequently crossed with other breeds. Some herds
have purposely had individuals with certain colourations removed in order to
create a more “primitive” appearance in the remaining herds, leading some to
assert that the animals have always looked this way and that their primitive
markings are the result of natural selection, when in reality the breed remains
quite domestic and can easily be brought back into captivity. Knowing this, and
considering their reduced population, I think other options could be explored
when selecting horses for an experimental rewilding site. Considering their
truly wild status (which has been unsuccessfully challenged in recent years),
their truly primitive appearance and behaviour, and their adaptations to (semi-)arid
steppes and semi-deserts, which represent much of the available habitat in
central North America, I believe the Asiatic wild horse (Equus caballus przewalski) would be an excellent choice for
introduction to North American rewilding areas. Having made a considerable
recovery from their near-extinction in the first half of the twentieth century,
there exists a sizeable captive population in North America which could be used
to establish a free-living breeding population on the Great Plains or adjacent
habitats.
This would run
contrary to my suggestions for rewilding domestic horses or crossbreeds in
Europe. This is for two reasons: the first being that domestic horses are more
closely related to the European wild horse (Equus
caballus ferus) than is the Asiatic wild horse, and the second being that
most of the available habitat in Europe is different from what takhis are
adapted to, and more easily occupied by native horses which have developed
appropriate ecotypes. Genetic evidence suggests that the two living horse
subspecies are about equally related to Equus
caballus lambei/lenensis, which occupied the Bering strait, and are
consequently likely to be equally related to the other North American horse
subspecies, since Beringia would have represented the migration corridor
between Eurasia and North America. Knowing this, it seems more appropriate to
select the taxon that is already considered wild, could use the conservation
support, and will not require any significant selection. The fact that takhis
already look like primitive wild animals will also be useful for rewilding
projects funded by ecotourism. Their ability to occupy non-steppic habitats is
established from their introduction to conservation areas in Europe with
varying levels of forest cover. Probably the best course of action is to begin
with the central plains, where most American rewilding projects are
focused/planned anyway, and work outwards from there. Their ability to thrive
in colder climates, such as those that might be experienced in rewilding areas
in the far north, will need to be assessed. However, considering we now have
the whole genome of an extinct relative adapted to these areas, genetic
hybridization might be an angle. Hybridization with existing domestic breeds
adapted to these areas, such as the Yakut pony, would certainly be easier, but
it would also potentially introduce some less desirable behavioural traits. The
best course of action will be decided when relevant.
As to the
replacement for Harringtonhippus, the
substitute used will be a generic proxy, rather than a subspecific one. Based
on morphological and chromosomal differences, I think it appropriate that the
various Equus subgenera be elevated
to genus level, including Asinus, the
group that includes the wild asses. Although not actually a member of this
group, the North American stilt legged equids shared some remarkable
morphological similarities with Old-World stilt-legged horses surviving today,
and seem to have occupied similar open and arid habitats based on fossil
records. Knowing this, I think the best proxies would be various subspecies of Asinus hemionus, the Eurasian wild ass,
which were once widespread across multiple habitat types in Europe and Asia. While
the kulan and onager subspecies have been proposed for redistribution in Europe
and western Asia, and the khur subspecies
is in the process of being reintroduced to new sites in India, the Mongolian hemionus subspecies and the Tibetan kiang subspecies might potentially be
used in North American rewilding projects, with the former being well suited to
the prairies and deserts of northern Mexico, the central/western United States,
and southern Canada, and the latter being adapted to colder habitats such as
those found in northern Canada and Alaska. These two subspecies are the most
populous of any of the Eurasian wild ass varieties, with almost 70,000 of the kiang subspecies remaining in the wild,
and almost 20,000 of the hemionus subspecies.
Oddly enough however, they are also the least studied taxa, and the rarest in
captivity. Consequently, individuals used in rewilding projects in North
America would probably have to be wild-caught, which would be advantageous for acclimatization
purposes, but expensive due to the capture and transportation costs.
There are many
potential benefits to equid reintroduction in North America. Horses are an
important part of the grazing regime in any ecosystem where they occur. Their
ability to eat tougher and poorer quality forage allows them to open up habitats
for more selective herbivores. Feral mustangs were often followed by herds of bison
for this exact reason, in a dynamic mirroring that of zebras and wildebeest in
Africa. Something worth considering however is that predation by wolves and
pumas would likely be insufficient for population control and human intervention
would possibly be necessary to keep numbers in check. The main predator of the
horse in the Pleistocene was the American lion (Panthera (leo) atrox). North America probably has a better chance
of returning lions to its ecosystems, at least in fenced game reserves, than
does Europe, and I for one would be very excited to see that particular
predator-prey relationship first-hand. A truly rewilded Great Plains could potentially
become one of the greatest ecotourism destinations on Earth.
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