Horses have a shared history with man going back millennia to their domestication around 4000 B.C. Yet only in very recent years have scientists begun to turn the tools of modern science on this remarkable animal that has been so wrapped up in human dreams and legends. Now modern scientific research is beginning to explain long-standing mysteries about the true nature of the horse. How well can horses really see? What causes breakdowns in racehorses? How intelligent are they compared to other animals, and are some breeds smarter than others? Does nature or nurture matter more in creating a great sport horse? What causes cribbing and other vices? In this beautifully illustrated, compelling narrative, Budiansky tells the story of the origins, behavior, intelligence and language of the horse.
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Stephen Budiansky, formerly U.S. Editor of Nature, is currently Senior Writer at U.S. News & World Report. He is the author of Covenant of the Wild and Nature's Keepers: The New Science of Nature Management. He lives in Leesburg, Virginia.Excerpt. © Reprinted by permission. All rights reserved.:
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 J. B. S. Haldane observed, "You can drop a mouse down a thousand-yard mine shaft; and, on arriving at the bottom, it gets a slight shock and walks away, provided the ground is fairly soft. A rat is killed, a man is broken, a horse splashes."
As we shall see, there were some compensating evolutionary virtues that made large size attractive, but that does not alter the initial fact of the relative rarity, and improbability, of animals the size of horses.
The speed of the horse is a rarity, too; indeed, it has few equals in speed over long distances. The cheetah, the fastest land animal, can hit 100 kilometers per hour for extremely short bursts over a distance of a hundred meters or so; a horse's sustained racing gallop is nearly 70 kilometers per hour.
At the same time, the horse's dietary needs are astonishingly easy to meet for such a large animal. The black rhino, an animal whose body weight is comparable to that of a large draft horse, requires huge quantities of green twigs and legumes. Large carnivores of similar weight, such as polar bears or lions, will eat up to 40 kilograms of meat in a single meal. Yet the horse has adapted to eating the poorest-quality forage, containing the lowest concentration of protein, of any large herbivore. It thrives on grasses that a cow would starve to death on.
And unlike many large grazing animals, horses break down the otherwise indigestible cellulose of stems and leaves in a digestive organ known as the cecum. While ruminants such as sheep and cattle have to rest for hours after eating, the hoofed herbivores that possess a cecal digestive system (horses, tapirs, and rhinos) can eat and run.
It is another remarkable and happy coincidence that the horse possesses a diastema, or gap between the front incisors and the rear grinders (figure 1.3). Control of the horse by man would have proved far more difficult without this anatomical convenience, which allowed the effective placement of bridle and bit.
Horses possess one final anatomical convenience from our point of view: almost unique among hoofed animals, they lack horns or antlers.
The Nature of Evolution
The sheer abundance of horse bones, and especially horse teeth, in the fossil record has made the horse the single most frequently cited paradigm of evolution. There are more than half a million specimens of fossil horses in museum and academic collections in North America alone.
Explaining evolution has never been easy, and educators and museum curators quite naturally seized on the well-documented fossil history of the ever popular horse as Exhibit A. Practically everyone who has visited a science museum or taken an elementary biology course has seen the evolutionary sequence of fossil horses from tiny eohippus (more properly known by its scientific name, Hyracotherium) to modern Equus. Starting as a small, squat, dog-sized, four-toed creature 55 million years ago, the horse step-by-step turned into the tall, fleet, elegant, single-hoofed animal of modernity.
But this simple, linear picture of evolution has led to a couple of unfortunate misunderstandings. One is the idea that evolution really does run in a straight line. This notion, known technically as orthogenesis, was assumed by many early biologists, and is still the popular conception of how evolution works: each species in an animal's fossil family tree gives rise to a (presumably superior) replacement. "The orthogenetic template has...influenced millions of lay people, many of whom visit natural history museums with turn-of-the-century exhibits that convey 100-year-old ideas," says paleontologist Bruce MacFadden, an expert on fossil horses. In fact, paleontologists now know that evolution is full of branches, dead ends, and blind turns.
The recent history of modern equids is no exception. Analysis of the similarity of mitochondrial DNA among the seven modern species of the genus Equus (three zebras, two asses, the horse, and the now-extinct, zebralike quagga of South Africa, which was hunted to its death in the late-nineteenth century) has allowed scientists to reconstruct a branching family tree that is surely far more paradigmatic of evolution than any straight-line model (plate 1). Mitochondrial DNA is the genetic material found in a portion of the cell known as the mitochondria, which is responsible for generating energy and which has the peculiarity of reproducing itself separately from the rest of the cell. Mitochondrial DNA is inherited solely from the mother. Thus, changes in mitochondrial DNA can occur only slowly, by accumulated mutation, not by the recombination of male and female genes through mating in every generation. By comparing this DNA in related species and estimating the mutation rate at 2 percent every million years, biologists have calculated when the various modern Equus species diverged from one another. (The extinct quagga's mitochondrial DNA was extracted from a bit of muscle tissue in a preserved hide at the Museum of Natural History in Mainz, Germany; it appears to have diverged from Equus zebra 3 to 4 million years ago.)
The other, related misperception that the orthogenetic model of evolution has engendered is that evolution has a purpose, or goal. It is commonplace, and perhaps inevitable, for people in love with horses to see this 55-million-year history as a process of "perfecting" the horse. We often read that the horse is "perfectly" adapted to running, for instance, and it is hard for us not to see modern Equus as superior to its forebears.
In fact, the evolutionary explosion of the horse in North America beginning some 20 million years ago gave rise to a multiplicity of other branches, too, with as many as 13 genera existing simultaneously. Some species were larger than their predecessors, but some were smaller (figure 1.4). Some moved toward the "modern" horse diet of grasses, but others specialized in the "primitive" diet of browse (figure 1.5). Some showed a trend toward the one-toed hoof of the modern equid, others did not.
The first step toward understanding how evolutionary forces shaped the modern horse is to understand that the purpose of evolution is not progress or perfection in the long run, but survival in the short run. And that in turn involves a complex interaction of genes and the environment. It is simply wrong to say that the modern horse is "superior" to its extinct predecessors. Many of those predecessors that we so cavalierly dismiss as failures, or as inferior stepping stones on the path to perfection, were in fact brilliant successes that flourished for millions of years -- until an unpredictable change in climate finally did them in. The evolutionary success of the modern horse owes more to its having been a lucky guesser than a pinnacle of progress. That is especially borne out by the fact that abrupt climatic changes at the end of the Ice Age some 15,000 years ago (possibly exacerbated by overhunting by humans) drove the modern horse to extinction in North America and within a hair's breadth of extinction in Europe and Asia as well. Were it not for domestication, Equus caballus would have gone the way of Hyracotherium and all the other ancestral horses that are testimony to the inevitability of extinction.
A new trait, however meritorious it may prove in the long run, cannot become fixed ...
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