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A origem do cavalo domesticado

Fernando_SilvaFernando_Silva Administrator, Moderator
Neste último encontro do ReV no Rio, estávamos falando sobre o livro "Germes, armas e aço", do Jared Diamond, e o Manuel Dória (ele era um dos dois foristas conhecidos como Azatoth) discordou dele por achar que animais selvagens, como a zebra, podem ser domesticados.

Isto me lembrou um livro que menciona como o cavalo foi domesticado: há uns 7 mil anos, as tribos que viviam às margens das estepes russas (e que dariam origem aos indo-europeus), conseguiram encontrar uns poucos machos que aceitaram viver em cativeiro. Destes poucos, talvez um apenas, descendem todos os cavalos domesticados do mundo. As fêmeas eram mais domesticáveis, mas a análise do DNA mostra que há apenas 77 fêmeas ancestrais.

Os cavalos selvagens, quando crescem, abandonam o grupo para formar seus próprios haréns. Para isto, precisam disputar as fêmeas com outros machos. Aqueles que não são combativos o suficiente ficam sozinhos e não transmitem seus genes para uma próxima geração.

Ao serem capturados pelos humanos, alguns desses machos, que normalmente viveriam sem fêmeas e sem descendentes, acabaram gerando todos os cavalos domesticados do mundo.
Modern horses are genetically schizophrenic, like cattle (chapter 8) but with the genders reversed. The female bloodline of modern domesticated horses shows extreme diversity. Traits inherited through the mitochondrial DNA, which passes unchanged from mother to daughter, show that this part of the bloodline is so diverse that at least seventy-seven ancestral mares, grouped into seventeen phylogenetic branches, are required to account for the genetic variety in modern populations around the globe.

Wild mares must have been taken into domesticated horse herds in many different places at different times. Meanwhile, the male aspect of modern horse DNA, which is passed unchanged on the Y chromosome from sire to colt, shows remarkable homogeneity. It is possible that just a single wild stallion was domesticated. So horse keepers apparently have felt free to capture and breed a variety of wild mares, but, according to these data, they universally rejected wild males and even the male progeny of any wild stallions that mated with domesticated mares. Modern horses are descended from very few original wild males, and many, varied wild females.

Why the Difference?
 
Wildlife biologists have observed the behavior of feral horse bands in several places around the world, notably at Askania Nova, Ukraine, on the barrier islands of Maryland and Virginia (the horses described in the childrens’ classic Misty of Chincoteague), and in northwestern Nevada. The standard feral horse band consists of a stallion with a harem of two to seven mares and their immature offspring. Adolescents leave the band at about two years of age. Stallion-and-harem bands occupy a home range, and stallions fight one another, fiercely, for control of mares and territory.

After the young males are expelled they form loose associations called “bachelor bands,” which lurk at the edges of the home range of an established stallion. Most bachelors are unable to challenge mature stallions or keep mares successfully until they are more than five years old. Within established bands, the mares are arranged in a social hierarchy led by the lead mare, who chooses where the band will go during most of the day and leads it in flight if there is a threat, while the stallion guards the flanks or the rear. Mares are therefore instinctively disposed to accept the dominance of others, whether dominant mares, stallions—or humans.

Stallions are headstrong and violent, and are instinctively disposed to challenge authority by biting and kicking. A relatively docile and controllable mare could be found at the bottom of the pecking order in many wild horse bands, but a relatively docile and controllable stallion was an unusual individual—and one that had little hope of reproducing in the wild.

Horse domestication might have depended on a lucky coincidence: the appearance of a relatively manageable and docile male in a place where humans could use him as the breeder of a domesticated bloodline. From the horse’s perspective, humans were the only way he could get a girl. From the human perspective, he was the only sire they wanted.
"The horse, the wheel and language" - David W. Anthony
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