and to a large extent the characteristics or traits exhib - ited. Different forms of genes, referred to as alleles, are responsible for the individuality of living things. Some characteristics are determined by alleles of one gene—for instance, the absence of horns or the occurrence of a metabolic disease. In such cases a sin- gle mutation can lead to a deleterious condition. However, most traits of significance involve alleles of more than one gene. Superior characteristics for growth rate or milk yield,so-calledpolygenic traits, re- sult from the combination of alleles of many genes. Animal breeding seeks to improve genetically the fu- ture population of a particular species by increasing the proportion of desirable alleles or the appropriate combination of such alleles.Geneticimprovementre- quires selection of appropriate breeding animals and a mating plan for such animals. Selection and Mating Systems Selection is theprocess of determining which animals are to be used as breeding stock. The simplest form of selection considers only traits of the individual, whereas more complex selection takes account of ad- ditional information on relatives, such as siblings, par- ents, and offspring. The accuracy of predicting ge- netic progress is improved by considering relatives. This process requires reliable measures for desired traits, acquisition of records from numerous animals, and analysis of the records, which has been aided by advances in statistical theory and computational 40 • Animal breeding Global Resources Since the 1940’s, dairy cows in the United States, such as these in Sacramento, California, have been intensively bred for milk production, which has tripled since that time. (Photo courtesy of USDA NRCS) power. The result is a ranking of animals based on their genetic merit for single or multiple traits. Several systemshave been used for mating selected animals. One involves complementarity, whereby in- dividuals with high genetic merit for different traits are mated. Ithas been used toimprove livestock in de- veloping countries by mating animals adaptedtolocal conditions with highly productive ones from devel- oped countries. The beefcattle, swine, and poultry in- dustries make heavy use of crossbreeding, in which animals from different breeds are mated. One of its advantages is the “hybrid vigor” that results. Another system is mating the best to the best. Oneofitshazards is inbreeding, or the mating of relatives, which often results in decreased fertility and viability. Environmental Factors Performance or productivity is determined not only by genetics but also by environmental factors. Cli- mate, nutrition, and management can affect the ex- tent to which the genetic potential of an animal is real- ized. Because the productivity of an animal can be affected deleteriously by heat and disease, climate and other environmental factors can influence ani- mals’ performance. Similarly, the management sys- tem used, whether intensive or extensive, can also af- fect productivity. Accordingly, the most productive animal under one set of conditions may not necessar- ily be the most productive under another. Interac- tions between genetics and the environment must be considered in animal breeding. Post-1940’s Developments Beginning in the mid-twentieth century, reproduc- tive technologies, most notably artificial insemina- tion, contributed to rapid improvement in animal performance. These technologies permit animals with the best genetics to be used widely, resulting in nu- merous offspring from which to select the best breed- ing stock for the next generation. As a result of inten- sive selection and management in the United States beginning in the 1940’s, milk production per cow has more than tripled. The growth rate of chickens has more than doubled, as has egg production. Such increases have occurred concurrently with a higher efficiency in raising animals for human food. Molecular biology and biotechnology hold the po- tential to alter animal breeding processes significantly in the early twenty-first century. Further understand - ing of the genomes of livestock species should permit identification of specific genes that will increase the productivity of these animals. One approach, known as marker-assisted selection, would use genetic mark- ers associated with desirable production characteris- tics to enhancegenetic improvement. If suchmarkers prove to be accurate predictors, they will allow selec- tion of desirable animals long before performance records are available. Transfer of desirable genes, within or between species, may also expedite the gen- eration of superior animals. The goal of animal breed- ing can be expected to remain similar to that of the past—namely the improvement of animal species to better meet human needs—but the precise nature of the improvements desired and the methodologies used to achieve them could be vastly different. James L. Robinson Further Reading Bourdon, Richard M. Understanding Animal Breeding. 2d ed. Upper Saddle River, N.J.: Prentice Hall, 2000. Falconer, D. S., and Trudy F. C. Mackay. Introduction to Quantitative Genetics. 4th ed. New York: Longman, 1996. Field, Thomas G., and Robert E. Taylor. Scientific Farm Animal Production: An Introduction to Animal Science. 9th ed. Upper Saddle River, N.J.: Prentice Hall, 2008. Sandøe, Peter, and Stine B. Christiansen. Ethics of Ani- mal Use. Oxford, England: Blackwell, 2008. Schatten, Heide, and Gheorghe M. Constantinescu, eds. Comparative Reproductive Biology. Ames, Iowa: Blackwell, 2007. Van der Werf, Julius, Hans-Ulrich Graser, Richard Frankham, and Cedric Gondro, eds. Adaptation and Fitness in Animal Populations: Evolutionary and Breeding Perspectives on Genetic Resource Management. London: Springer, 2009. Weaver, Robert F., and Philip W. Hedrick. Genetics.3d ed. Dubuque, Iowa: W. C. Brown, 1997. Web Site U.S. Department of Agriculture Animal Breeding, Genetics, and Genomics http://www.csrees.usda.gov/ animalbreedinggeneticsgenomics.cfm See also: Animal domestication; Animals as a medi - cal resource; Biotechnology; Livestock and animal husbandry. Global Resources Animal breeding • 41 Animal domestication Category: Plant and animal resources The domestication of animals began long before re- corded history and has been integral to the develop- ment of human societies. Domesticated species are re- newable resources that provide humans with food, fiber, fuel, power, implements, and other benefits. Background Domestication is the process of bringing a species un- der control of humans and gradually changing it through careful selection, mating, and handling so that it ismoreuseful. Domestication of plantsand ani- mals led to the development of agriculture, permit- ting people to abandon a hunter-gatherer existence. Instead of following available game and edible wild plants, people were able to establish permanent set- tlements. Agriculture gave them more time to de- velop other skills and arts, eventually leading to com- plex societies and civilizations. A critical aspect of domesticating animals is direct- ing breeding toward a specific function. Although taming is probably a first step, domestication is more than accustoming animals to the presence of people. Domestication does not apply to captive wild animals in zoos or circuses, for example; although humans control such animals and their breeding, the latter is not directed toward a useful goal (although one could argue that breeding in captivity helps propagate the animal and perhaps helps maintain genetic diversity). Wild animals are those that have not been domesti- cated, and feral animals are previously domesticated animals that are no longer under human control. Wild relatives of domesticated species are relatively easily domesticated, and feral animals can be readily redomesticated. Characteristics That Favor Domestication In 1865, the English naturalist Francis Galton sug- gested the following six physiological and behavioral characteristics that make some animals good candi- dates for domestication: hardiness, “dominance” so- cial behavior, herd behavior, utility to humans, facile reproduction, and facile husbandry. First, hardiness refers to the ability of the young to tolerate removal from the mother and the presence of humans. The guinea pig is perhaps an extreme example of tolerat - ing removal from its mother; it is born ready to eat solid food. Most mammals, on the other hand, de- pend on their dams’ milk. Primates are poor subjects for domestication because of their helplessness at birth and their relatively long dependence on their mothers for food and nurturing. Second, dominance social behavior, in contrast to territorial behavior, refers to one animal assuming leadership, with the rest of the group acquiescing to it. In domestication, humans co-opt the function of the leader, and animals remain submissive even as adults. Third, herd animals are contrasted to those that are solitary or disperse in response to danger. Domesticated animals are penned or otherwise re- stricted at various times. If they remain together in herds, they are easier to manage. Fourth, utility to hu- mans includes their use for food, fiber, traction, com- panionship, and even worship. Humans would not make the effort to domesticate an animal unless it had some perceived value. However, the purpose for do- mestication may change with time. It is likely that the initial motivation fordomesticating cattle wasfor wor- ship—to capture the strength and aura of these ani- mals, which were revered. Traction became a subse- quent goal, while contemporary utility in Western societies involves meat and milk production. The fifth characteristic is facile reproduction un- der confined conditions: Animals with finicky repro- ductive behavior and/or elaborate courtship rituals make poor candidates for domestication. Sixth, facile husbandry refers to placid behavior and versatility in terms of nutrition. Poor candidatesfor domestication are those animals that are very high strung or depen- dent on a unique feedstuff. Koalas, which eat leaves from only certain eucalyptus trees, are poor candi- dates for domestication. On the other hand, pigs and goats are excellent candidates because they are not fastidious in their eating habits. These six characteris- tics, enunciated more than a century ago, apply strongly to livestock species and somewhat less well to dogs and cats; some argue that cats are not so much domesticated as they are tolerant of humans. History of Animal Domestication Evidence from archaeology suggests that agriculture developed at least ten thousand years ago, after the last ice age. That agricultural development occurred at that point, when the climate was warmer and more stable than it had been, was probably not coincidence. Predictability of the weather is particularly crucial for 42 • Animal domestication Global Resources plant agriculture. Plant and animal domestication ap- parently developed together, suggesting a synergy be- tween the two. Domestication of any animaldidnotoccur at once, but rather over a substantial period of time, perhaps hundreds of years. Accordingly, thedates for domesti- cation have a substantial margin of error. Further, they may be modified as new information becomes available. For some species, domestication occurred independently at more than one location. In the be- ginning, the process may have been almost acciden- tal, as by raising a captured young animal after its mother had been killed and observing its behavior and its responses to various treatments. The domesti- cation of an animalthenspread from the site of origin through trade or war. Animal domestication occurred in various parts of the world. The Middle East, the so-called Fertile Cres - cent, stretching from Palestine to southern Turkey and down the valley of the Tigris and Euphrates rivers, was an important site. There sheep, goats, cattle, and pigs were domesticated by around 8000 b.p. The In- dian subcontinent and east Asia were independent sites for domesticating cattle and pigs, respectively. Llamas, alpacas, and guinea pigs were domesticated in the Andes Mountains of South America. Domesti- cation of cats occurred in Egypt, and rabbits were do- mesticated in Europe. No native animal has been do- mesticated in Australia,withthe possible exception of the emu. Interestingly, few successful domestications occurred after 1000 b.p. Archaeology, coupled with the natural history of domesticated animals and their wild relatives, has been essential in reconstructing the history of domes- tication. Examining the skeletal remains at archaeo- logical sites for changes in morphology and distribu- tions by age and sex has helped researchers deduce the extent of domestication. Lately, traditional ar- chaeology has been supplemented by the methods of molecular biology. Examining extant breeds for their degrees of genetic relationship has been particularly useful in distinguishing single versus dual sites of do- mestication. Early Domesticates: Dogs and Reindeer Dogs (Canis familiaris) were the earliest known do- mesticated animals. They were widespread across the Northern Hemisphere before other animals were do- mesticated. They derive from wolves (Canis lupus), with whom they are completely interfertile. The earli- est known dog is in a burial site in northern Iraq that dates from 12,000 to 14,000 b.p. Other sites, dating from 9000 to 12,000 b.p., have been documented in England, Palestine, Japan,andIdaho. While the origi- nal domestication may have occurred in China (the Chinese wolf has some of the detailed physical fea- tures of domesticated dogs), domestication probably occurred at a number of separate sites. Domesticated dogs accompanied the American Indians when they occupied the Americas in several waves prior to the end of the last ice age. Dingoes were brought to Aus- tralia by trade from Asia long after the Aborigines set- tled Australia 40,000 years ago. While dogs were con- sidered a food animal, they have long been known as companions and guards. Subsequently, they were de- veloped for hunting and herding. Reindeer (Rangifer tarandus) were another early domesticate, dating from around 14,000 b.p. innorth - ern Scandinavia and Russia. Herding reindeer contin - Global Resources Animal domestication • 43 Sheep were one of the first animals to be domesticated. (USDA) ues as a principal occupationofthe Laplanders of Fin - land, Sweden, and Norway. Reindeer are used to pull loads and for clothing and shelter (skins), tools (ant- lers), and food (meat and milk). They are well suited to their environment and, contrary to other domesti- cated animals, their range has not been extended by domestication. Attempts to establish reindeer indus- tries in Canada and Alaska have not been successful. In contrast, the farming of several other deer species (such as Cervus dama and Cervus elaphus) has recently gained in importance in New Zealand and Western Europe. They are raised for meat (venison) and “vel- vet,” the new growth of antlers, which is the basis for traditional medicines in Asia. Sheep, Pigs, and Cattle Sheep (Ovis aries) were the first of the common food animals to be domesticated. They were derived from wild sheep (Ovis orientalis) and were first domesti- cated in the western Fertile Crescent around 9000 b.p. Goats (Capra hircus), derived from Persian wild goats (Capra aegarus), were domesticated in the central Fer- tile Crescent slightly later, 8000-9000 b.p. Sheep and goats have been used for food, skins, and, in the case of sheep, wool. Both were later selected for milk pro- duction. Pigs (Sus domesticus) probablyoriginatedattwosep- arate sites, the central Fertile Crescent around 8000 b.p. and in eastern Asia around 7000 b.p. Derived from wild pigs, they were primarily raised for meat. Despite Islamic and Judaic restrictions against eating pork, it has long been the principal meat consumedin the world.The most populous country, China, has ap- proximately 50 percent of the world’s pigs. Cattle (Bos taurus and Bos indicus) are derived from now-extinct wild cattle (aurochs, Bos primigenius) that ranged over much of Europe and Asia. They were probably domesticated independently at two loca- tions, the western Fertile Crescent around 8000 b.p. for Bos taurus and the Indian subcontinent around 7000 b.p. for Bos indicus. Initially, the animals were worshiped and used in religious ceremonies. Rever- ence for cattle is still practiced by Hindus in India. Subsequently, cattle were developed for draft (pulling loads), meat, and milk. Their hides are made into leather. Traditional cattle in Africa are derived from initial importations of Bos taurus and subsequent im- portations of male Bos indicus. Other Domesticated Animals Asiatic buffaloes (Bubalus bubalus) were domesticated as the water buffalo in India (5000 b.p.) and as the swamp buffalo in Southeast Asia and southern China (4000 b.p.). While both were developed as draft ani- mals, the water buffalo has also been selected as a dairy animal. In 2008, almost 60 percent of the milk production in India was from buffaloes. In spite of its tropical origin, the Asiatic buffalo is not very heat tol- erant and compensates by wallowing in water or mud. Neither the African buffalo (Syncerus caffer) nor the 44 • Animal domestication Global Resources Eight Major Domesticated Animal Species Worldwide Millions Leading Countries Primary Uses Ruminants Buffalo 174 India, China, Pakistan Draft, milk, meat, hides Cattle 1,355 India, Brazil, China, United States, Argentina Meat, milk, hides Goats 808 China, India, Pakistan, Sudan Milk, meat, hair, hides Sheep 1,081 China, Australia, India, Iran Wool, meat, milk, hides Nonruminants Chickens 16,740 China, United States, Indonesia, Brazil Meat, eggs, feathers Ducks 1,046 China, Vietnam, Indonesia Meat, eggs, feathers Swine 960 China, European Union, United States Meat Turkeys 280 United States, France, Italy, Chile Meat, eggs, feathers Source: Data from Thomas G. Field and Robert E. Taylor, Scientific Farm Animal Production, 9th ed., 2008. American buffalo (more properly, bison, Bison bison) have been domesticated. Yaks (Bos [Poephagus] grun- niens) were domesticated at an unknown timeinTibet or surrounding areas, where they are used as packani- mals and as sources of milk, hair, and hides. Horses (Equus caballus) originated from wild horses in the Caucasus Mountains around 6000 b.p. Originally used for food and skins, they were also de- veloped for draft and, much later, for riding. Because they arrived in the Middle East after the development of written language, their arrival is documented in writing, so scholars do not need to rely only on the ar- chaeological record. Donkeys (Equus asinus) were do- mesticated in the Middle East or Northern Africa (5000 b.p.). Theyare used as pack animals and forrid- ing, as is the mule, an infertile cross between a horse and donkey. Llamas (Lama glama) and alpacas (Lama pacos) were domesticated in Peru by Incas around 6000 b.p. Llamas are from wild guanaco and alpaca from wild vicuña, found at higher elevations. Llamas are used as pack animals, alpacas are valued for their fine wool, and both serve as sources of meat. Camels, the one- humped dromedary (Camelus dromedarius) and two- humped Bactrian (Camelus bactrianus), were domesti- cated in Arabia (4000 b.p.) and Central Asia (3500 b.p.), respectively. Both are pack animals, and the dromedary is also used for meat. The largest animal to be domesticated, at 4.5 met- ric tons, was the Asian elephant (Elephas maximus); the African elephant has not been domesticated. Domes- ticated elephants have been used for draft and riding for more than 2,000 years. In 2007, Asian elephants numbered less than 40,000 and were approaching en- dangered status. A substantial fraction were working elephants. Burma (also known as Myanmar) alone had 5,700 working elephants,primarilyused for selec- tive logging of teak forests. They drag logs weighing more than a metric ton, making use of trails, rather than the roads needed by mechanized log skidders. Elephants are less destructive of the environment than mechanized equipment. However, because ele- phants are susceptible to heat stroke, they can work only during the cooler partsofthe day (early morning and late afternoon) and not at all during the hottest three months of the year. Thus, they are not as effi- cient as mechanical equipment. Nevertheless, the survival of Asian elephants may depend on their con - tinued use as working elephants. Guinea pigs (Cavia porcellus) were domesticated in Peru around 5000 b.p. They continue to be used as a meat animal in parts of South America. Rabbits (Oryctlagus cuniculus) were domesticated between 600 and 1000 c.e. in France. They are primarily raised for meat and fur, with Angora rabbits producing a valued wool. Cats (Felis catus) are the animals least changed, morphologically, by domestication. In addition, they are quite capable of surviving without human inter- vention. Their domestication occurred relatively late, around 4000 b.p., in Egypt, the home of the African wild cat (Felis catus libyca), which is difficult to distin- guish from domestic tabby cats. The early Egyptians adopted cats enthusiastically, deifying them and pro- hibiting their export. After conversion toChristianity, Egyptians ceased worshiping cats, which were carried to all parts of the Roman Empire and thence to the rest of the world.Cats have been usedfor companions and for rodent pest control. Chickens (Gallus gallus), along with turkeys (from North America), ducks and geese, ostriches (from Af- rica), and emus (from Australia), are nonmammals that have been domesticated. Chickens were probably derived from wild Red Junglefowl in Southeast Asia before 4,000 b.p. Cockfighting was an initial purpose for their domestication. The fowls acquired religious significance and were also used for meat andfeathers. Their selection for egg production has been a rela- tively recent development. Two insects have also been domesticated: honey- bees and silkworms. Honeybees were domesticated shortly after the last ice age and were the principal source of dietary sweetener until two hundred years ago. Also valuable were their wax and venom, the lat- ter used for medicinal purposes. One of ten varieties of silk-producing insects, silkworms were domesti- cated around 5000 b.p. in China, producing fiber used in apparel. Utility of Domesticated Animals As noted in the foregoing section, domesticated ani- mals provide for various basic human needs: food, shelter, clothing, fuel, and emotional well-being. Clearly, their predominant use hasbeen as asource of food in the form of meat, milk, and eggs. As omni- vores, humans included animal products in their diets long before they domesticated animals. Animal prod- ucts are good sources of high-quality protein, miner - als, and vitamins, particularly vitamin B 12 , which is not available in plant materials. Humans have an appetite Global Resources Animal domestication • 45 for foods of animal origin, and as economic circum - stances permit, the consumption of animal foods in- creases. Domesticated animals have long provided shelter and clothing from their hides, hair, wool, and feath- ers. Their bones,horns, and antlers have been used as implements. Animal tissues and blood were the origi- nal sources of many pharmaceuticals that are now manufactured. For example, before the development of synthetic insulin, porcine (pig) insulin was pro- vided to diabetics. Animal hearts, kidneys, and livers have been transplanted into humans. Although these transplants are eventually rejected by the human body, it seems possible that the development of transgenic pigs with human tissue factors could provide a source of permanent transplantable organs. Animals are also used as research subjects, for testing new devices and drugs before they are applied to humans. Although such uses of animals have become somewhat contro- versial, much of the development of modern medi- cine has depended on domesticated animals and on laboratory animals maintained expresslyforresearch, primarily rats and mice. The wastes generated by animals are used to fertil- ize crops. They also provide fuel; dried manure is burned for warmth and to cook food in many coun- tries of the developing world. The development of biogas generators, in which animal wastes are con- verted to methane gas, is a more efficient way to gen- erate fuel. It has the added advantage that the residue can be used as fertilizer for crops. Domesticated animals provide power to cultivate crops, pull carts or carry loads, lift water, and skid logs. In the last decades of the twentieth century, use of draft animals increased around the world. This slightly reduced the need for petroleum. Neverthe- less, a return to heavy use of animal powerisunlikely. Domesticated animals also serve as insurance and bank, particularly in developing countries, areas sub- ject to drought, flooding, and pests, and where infla- tion is rampant. In some areas, animals are the cur- rency of exchange. Animals are used as a walking larder, especially when refrigeration and other means of preserving food are unavailable. Because storage of crops for more than a year is difficult and results in large losses, having animals may buffer a bad crop year. Risk management favors more small animals over fewer large animals, although this may not be most efficient from a resource management perspec - tive; minimizing risk also favors a variety over similar - ity of animals. Until other forms of insurance and banking are fully reliable, animals will continue to be used to alleviate risk. Finally, animals contribute to human well-being by providing companionship and recreation. Animals such as dogsandcats are companions to humans,pro- viding emotional support particularly important for the young, elderly, and infirm. The use of animal com- panions to promote recovery from illness has been demonstrated to be effective. Guide dogs contribute to an independent lifestyle for blind persons, and monkeys have been trained to provide help to para- plegics and quadriplegics.Animals also provide recre- ation for humans, as in the form of rodeos, polo playing, riding, backpacking, and racing. Noncon- sumptive uses of animals for companionship and rec- reation make definitivecontributions to the qualityof human life. Numbers of Domesticated Animals In the early twenty-first century, according to the ninth edition of Scientific Farm Animal Production (2008), by Thomas G. Field and Robert E. Taylor, world estimates for the major domesticated animal species were 16.7 billion chickens, 1.4 billion cattle, 1.1 billion sheep, 1 billion pigs, 0.8 billion goats, 1 bil- lion ducks, 280 million turkeys, 174 million buffaloes, 55 million horses, 41 million donkeys, 19 million camels, and 12 million mules. (For comparison, hu- man beings number about 7 billion.) Domestication brought these animals under human control and vastly increased their numbers and range. Domesti- cated animals represent a renewable resource that contributes substantially tothewell-beingofhumans. James L. Robinson Further Reading Budiansky, Stephen. The Covenant of the Wild: Why Ani- mals Chose Domestication. New York: W. Morrow, 1992. Reprint. New Haven, Conn.: Yale University Press, 1999. Clutton-Brock, Juliet. A Natural History of Domesticated Mammals. 2d ed. New York: Cambridge University Press, Natural History Museum, 1999. Diamond, Jared.“WhyIsaCowLikeaPyramid?”Natu- ral History 104, no. 7 (July, 1995): 10. Field, Thomas G., and Robert E. Taylor. Scientific Farm Animal Production: An Introduction to Animal Science. 9th ed. Upper Saddle River, N.J.: Prentice Hall, 2008. 46 • Animal domestication Global Resources Price, Edward O. Animal Domestication and Behavior. New York: CABI, 2002. Roots, Clive. Domestication. Westport, Conn.: Green- wood Press, 2007. Smith, Bruce D. The Emergence of Agriculture. 1995. Re- print. New York: Scientific American Library,1998. Web Site Food and Agriculture Organization of the United Nations Agricultural Statistics http://faostat.fao.org/site/573/default.aspx#ancor See also: Agricultural products; Animal breeding; Animal power; Animals as a medical resource; Live- stock and animal husbandry. Animal husbandry. See Livestock and animal husbandry Animal power Categories: Energy resources; plant and animal resources By using animals’ muscle power for traction and transport, humans expanded the efficiency of these processes immensely. Animal power, essential for heavy hauling or rapid travel until the mid-1800’s, remains important to much of the world’s agriculture. Background The dog was thefirstanimaldomesticated, tamed and bred from wolf ancestors. Archaeological sites show- ing this development date back approximately eleven thousand years in both northern Europe and North America. The first dogs may have helped Stone Age hunters chase and exhaust game. They also may have pulled snow sleds and hauled loads via travois, as they did for American Indians in later centuries. If dogs were so employed in this era—and it has not been proved that they were—these would be the first inten- tional uses of animal power as an energy resource. Other important tasks using their senses and group instincts to help humans—tracking, scavenging, and guarding—probably meant that dogs were seldom kept primarily for their muscle power. The next successfully domesticated animals were sheep, goats, pigs, and cattle. This process is shown in remains and artifacts from Jericho that document the origins of agriculture. Centuries after grain was first cultivated, people began to keep livestock. At first the animals were probably loosely controlled and were seen as “walking meat larders” and occasionally pro- viders of milk, fiber,andleather. Once the process was under way, around 8000-7000 b.c.e., people must have experimented with riding and other ways to use the animals in their farming. Of these anciently domesticated species, only cat- tle proved to have the combination of strength and malleability to do useful work under human direc- tion. Shifting from a plow pushed by a man or woman to one pulled by an ox multiplied the traction enor- mously and enabled much more food to be produced with the same investment of human time. This was a major step in the ongoing “agricultural revolution” that created a growing population, town life, and a material surplus to support specialized trades. Using cattle in the fields also called forth other innovations. Harnesses and/or yokes had to be created to control the animals, implements had to become larger, and castration of young male animals had to be practiced to produce oxen that were both strong and docile. Later (sometime around 3000 b.c.e.), the wheel was invented. Hitching such animals to wheeled carts and wagons, humans could travel farther and more easily and could move bulkier goods. Draft animals (ani- mals usedfor hauling) thus served not only as a direct resource in agriculture and transportation but also as a source of synergy, expanding their owners’ geo- graphic and trade horizons and inspiring further inventions. While most of the evidence for this sequence of events exists in the ancient Near East, some of the same steps took place independently, perhaps several times, elsewhere in Eurasia. Forexample, the working cattle native to Asia—water buffaloes and yaks—were bred from wild species different from European do- mestic cattle’s ancestors. Horses and Related Species Horses and their kin, the most versatile of hauling and riding animals, were domesticated later. Wild horses roamed much of the world during the last ice age but had become extinct in the Americas by 10,000 b.c.e. Global Resources Animal power • 47 and rare in Western Europe and the Mediterranean region about the same time. How much this disap- pearance was due to climate changeand how much to humans’ overhunting is uncertain. Many prehistorians believe that horses were first tamed and trained for riding north of the Black Sea, where they survived in large numbers. Hence they were reintroduced into Europe and western Asia, be- tween 3000 and 2000 b.c.e., by successive invasions of mounted tribesmen from the central Asian steppes. However, as with much of prehistory, the evidence is unclear. Horses never disappeared completely in Eu- rope, and they may havebeen domesticated in several places from local stock. The donkey or ass, which is native to North Africa, was brought into use in the same millennium. Don- keys loaded with packs or wearing saddlecloths are shown in Egyptian friezes from 2500 b.c.e.; they also appear in early Sumerian and Assyrian records. Horses and asses were already being interbred at this time to produce vigorous offspring, notably the mule, with the traits of both species. From then until the end of the nineteenth century, the equids were the most widely used animalsin the world for transporting peo- ple and goods. They also became immensely impor- tant in agricultural processes. Because of their gait, asses cannot be ridden at high speeds, but their adaptability to harsh conditions makes them good pack animals and beasts-of-all-trades on small farms. Mules, hybrids from mare mothers and donkey sires, combine the horse’s strength with the ass’s stamina. Mules have been knowntocarry 450 kilograms each, going as far as 80 kilometers between water stops. Horses’ special qualities include speed, herd hier- archical instincts that dispose them to follow human leadership, and relative intelligence. Horses have been bred to strengthen various traits: the Arabian and 48 • Animal power Global Resources An Indian family sits atop a cart pulled by two bulls. People throughout the world have relied on draft power for centuries. (AFP/Getty Im- ages) the modern thoroughbred for speed; the medieval war horse and the modern shire and Clydesdale for strength and stamina; the Shetland pony for multiple tasks in damp and ferocious weather. As an example of the speed gained from using horses, over short dis- tances (up to 5 kilometers) a horse can travel in the range of 48 kilometers per hour. A horse carrying rider and saddle might make a trip of 480 kilometers in sixty hours, and the time can be shortened by fre- quent changes of mount. A personin top condition— for example, a soldier accustomed to long marches— typically can walk around 65 kilometers a day. Horse-related technology also developed continu- ously over time, adding to the effectiveness of horse and rider, and horse and vehicle. Bit and reins, stir- rups (not adopted in Europe until the early Middle Ages), horseshoes, saddle and carriage designs, and modern veterinary medicine, all brought new capaci- ties to the horse power on which humankind relied. Not only was the horse an essential energy re- source, but its presence also repeatedly changed his- tory and society. The rise of cavalry as a mobile force in warfare and the change to a horse-based economy and culture by North American Plains Indians when horses were reintroduced by Spanish invaders are only two of the many transformations wrought by horse power. Other Animals as Energy Sources Humans have attempted to put many kinds of large animals to work, but only a few other species have proved useful. Of these, the most important have been those already adapted to extreme climates and terrains. The camel iscalled the “ship ofthe desert” because of its ability to travel for long distances between water holes. Some desertnomad tribes organize their wayof life around the use of camels. Normally employed as pack and riding animals, camels were also occasion- ally used inwar in the ancientworld, partly tofrighten the enemy’s men and horses. Llamas, members of the camelid family native to South America, serve as pack animals in the Andes mountain region. Reindeer, adapted to living in an Arctic environ- ment, can find forage on the barren tundra and sur- vive temperatures of −50° Celsius. Laplanders who live in far northern areas have used them in the roles filled by cattle and horses in warmer climates, includ - ing riding, pulling carts, and carrying loads as pack animals. Elephants, native to the Indian subcontinent and to Africa, have been trained in both regions tolift and carry extremely heavy objects, although their use as riding beasts has been largely confined to ceremonial occasions and entertainment. Animal Power Today From prehistory through the nineteenth century, much of the work of civilization depended upon ani- mal power. With the coming of steam power and the internal combustion engine, animals gradually be- came less essential for transport and traction, at least in the developed world. Yet as late as the 1930’s, horses or mule teams, rather than tractors, were used by many American farmers. In Asia and Africa, most of the farmland is still worked with draft animals. For a small farmer of lim- ited means or in an isolated area, animal power has several advantages. Unlike machines, animals do not need complex networks to supply their fuel or parts for repair. Theyproduce their ownreplacements,and their malfunctions sometimes heal themselves with- out special knowledge or tools on the owner’s part. Their by-products can be recycled into agricultural use. For these reasons, and because they bring less dev- astation to land and air,somemembers of “back to the Earth” movements in the United States choose animal power. Heavy horses are also used in selected logging operations to avoid the clear-cutting and other envi- ronmental damage that machines bring. Emily Alward Further Reading Ableman, Michael. From the Good Earth: A Celebration of Growing Food Around the World. New York: H. N. Abrams, 1993. Chamberlin, J. Edward. Horse: How the Horse Has Shaped Civilizations. New York: BlueBridge, 2006. Chenevix Trench, Charles. A History of Horsemanship. Garden City, N.Y.: Doubleday, 1970. Clutton-Brock, Juliet. Domesticated Animals from Early Times. Austin: University of Texas Press, 1981. _______. Horse Power: A History of the Horse and the Don- key in Human Societies. Cambridge, Mass.: Harvard University Press, 1992. _______. A Natural History of Domesticated Mammals.2d ed. New York: Cambridge University Press, Natural History Museum, 1999. Greene, Ann Norton. Horses at Work: Harnessing Power Global Resources Animal power • 49 . from the combination of alleles of many genes. Animal breeding seeks to improve genetically the fu- ture population of a particular species by increasing the proportion of desirable alleles or. Herding reindeer contin - Global Resources Animal domestication • 43 Sheep were one of the first animals to be domesticated. (USDA) ues as a principal occupationofthe Laplanders of Fin - land, Sweden,. simplest form of selection considers only traits of the individual, whereas more complex selection takes account of ad- ditional information on relatives, such as siblings, par- ents, and offspring.