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The Nature of Horsesby Stephen Budiansky
THE IMPROBABILITY OF THE HORSE
Of the more than 4,000 species of mammals that have occupied the earth during the last 10,000 years, the horse is one of fewer than a dozen that have achieved widespread success as domesticated animals.
That low success rate was certainly not for want of trying on our part. The ancient Egyptians attempted to domesticate hyenas, antelope, ibex, and gazelles (figure 1.1). The American Indians kept pet raccoons, bears, and even moose. The Australian aborigines even kept wallabies and kangaroos. Yet none survive as domesticated animals today.
If it were simply a matter of human will, it would be hard to explain why we should have domestic dogs, sheep, goats, cows, pigs, horses, asses, camels, rabbits, and cats — but not deer, squirrels, foxes, antelope, or even hippos and zebras.
The answer is that it was not a matter of human will. The successful domesticated species were largely "preadapted" to their role through quirks of adaptation and evolution that had nothing whatever to do with human intentions or needs, but that turned out to be vital to their future success in our homes and fields. The horse was no exception. Among the myriad ways of making a living that evolution has cast up, a few — a very few — turned out to be compatible with human ways.
The horse, like the other animals that were to enter into domestication, was a generalist, able to survive on a variety of widely available foods. (An animal such as the giant panda, which eats nothing but bamboo leaves, would surely have been a nonstarter.) These generalists were able to exploit their new domesticated niche with a high potential reproduction rate. They had relatively simple courtship patterns, typically harems in which one male readily mates with multiple females. They were social animals, instinctively given to understanding signals of dominance and submission. They were relatively nonterritorial, not given to disruptive intraspecies combat over fixed bits of ground. In other words, the first step toward domestication was one that nature took millions of years before we even arrived on the scene.
The second step also seems to have been more the doing of the animals than of us. It was animals who discovered the mutual compatibility of our species, and it was they who chose to act upon this discovery. Recent archaeological and animal behavior studies strongly support the idea that domestication was not the human invention it was long supposed to have been, but rather a long, slow process of mutual adaptation, of "coevolution," in which those animals that began to hang around the first permanent human settlements gained more than they lost. Some were killed and eaten, but for every cow or sheep or horse killed, many more flourished on the crops they robbed from our fields and the incidental protection they gained from other predators in the proximity of human habitations. Like the starlings, mice and rats, and chimney swifts that invade our homes today for the food and shelter that are a by-product of our domestic habits, those forebears of our domestic stock took the initiative. We followed.
In the process, these semidomesticated but still free-living animals acquired still more of the characteristics that would make full domestication possible. Those individuals that were more curious, less territorial, less aggressive, more dependent, better able to deflect human aggression through submission, were the individuals that had the edge in this new niche.
It was only in the third stage of domestication, when humans began breeding animals in captivity, that human "invention" began to play a predominant role. But in consciously selecting and emphasizing those traits that appealed to our fancy or our needs, we could still only draw upon what nature provided. If the horse had not existed, we most definitely could not have invented it. The species upon which agriculture and indeed civilization have been built were a remarkable gift of evolutionary chance and opportunism.
The Disadvantage of Being a Horse
A number of species that expanded their range throughout the world during the respites that punctuated the Ice Age glaciations of the Pleistocene epoch (15,000 to 2 million years ago) may have acquired "domesticated" traits as a package deal. The twin pressures of climatic upheaval and massive hunting by humans placed specialist feeders at a decided disadvantage; it was the generalists, which could adapt to a wide variety of climates and circumstances, that flourished. Moreover, it was those species that were migratory, curious, adaptable — as opposed to territorial, suspicious, and conservative — that thrived on upheaval. Sheep, wolves, cattle, goats, camels, and horses all fit this bill. Thus in at least one sense the coincidence of domestication seems a bit more comprehensible: there were sound evolutionary reasons that made horses ripe for domestication. Even tameability may have been part of this package. The pressures that placed a premium on adaptability favored the retention into adulthood of juvenile characteristics, an evolutionary process known as neoteny. Juveniles are curious and adaptable, traits that were in demand in the Pleistocene; they are also playful, submissive, and dependent, traits that proved valuable to man in domestication.
Yet horses possessed a number of other remarkably convenient characteristics — convenient from our point of view — that make the existence of the modern horse seem all the more astonishing. To begin with, its very survival to modern times was practically a fluke. Many things worked against the horse ever making it. The speed, size, and weight-bearing capacity of the modern horse, all vital to its utility to humans, are extraordinarily unusual among mammals. An animal the size of the horse is in fact a prime candidate for extinction, a fact borne out repeatedly in the fossil record of life on earth. Large animals are long-lived as a rule, but they are also slow to reach sexual maturity, require long gestation periods, and rarely bear more than one young at a time. A drought, an insect outbreak that strips vegetation bare, or any other climatic or ecological disturbance can deliver a blow to a population of large-bodied animals that it takes years to recover from — if it recovers at all. A population of small animals that reproduce quickly and in large litters, on the other hand, can bounce back from repeated calamities.
Large animals face other evolutionary risks, not least of all gravity. Thomas McMahon, a biomechanician at Harvard University who has made an extensive study of the biology of size, notes that when an animal falls the damage it does to itself is directly proportional to its height, or length. (McMahon's argument in a nutshell: The strength of bones is proportional to their cross-sectional area. An animal twice as tall or twice as long as another will have bones that are twice as thick in all of their dimensions; their cross-sectional area will thus be 2 x 2 = 4 times as great [see figure 1.2]. If an animal's body length is proportional to L, the cross-sectional area of its bones will be proportional to length squared, L2. On the other hand, the energy of a falling body is proportional to its mass, and mass typically increases with the cube of an animal's length, or L3, because an animal twice as long as another will, roughly speaking, also be twice as wide and twice as tall, so its total volume will be eight times as great: 2 x 2 x 2 = 8. So if the energy of an impact is proportional to length cubed, and the ability to resist damage is proportional to length squared, the ratio of the two is [L3/L2], which equals just plain L. The old adage is true: the bigger they are, the harder they fall.) As the renowned British zoologist
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