Mammal Reo. 1992, Volume 22, NO. 2,79-85. Printed in Great Britain

The process of domestication JULIET CLUTTON-BROCK Department of Zoology, The Natural History Museum, Cromwell Road, London SW75BD, U.K.

ABSTRACT Domestication involves both culture and biology. The cultural process of domestication begins when animals are incorporated into the social structure of a human community and become objects of ownership, inheritance, purchase and exchange. The morpho- logical changes that occur in domestic animals come second to this integration into human society. The biological process resembles evolution and begins when a small number of parent animals are separated from the wild species and are habituated to humans. These animals form a founder group, which is changed over successive generations, in response to natural selection under the new regime imposed by the human community and its environment, and also by artificial selection for economic, cultural, or aesthetic reasons. In the wild, the evolution of a subspecies occurs when a segment of a species becomes reproductively isolated by a geographical barrier. With domestic animals, this separation leads to the development of different breeds.

INTRODUCTION The problem of what is domestication is of continuing concern to biologists and archaeologists. During the last century it was a subject which preoccupied Darwin, and the conclusions he came to about the origin of species were greatly influenced by his knowledge of animal breeding. During this century the history of domestication has been much studied, notably by Hilzheimer (1926), Herre (1954), Zeuner (1963), and more recently by Bokonyi (1974, 1989), Ducos (1978, 1989), Mason (1984), Clutton- Brock (1987, 1989) and Hemmer (1990). Yet the process of domestication is still little understood and there are still arguments about its definition. Everyone agrees, however, that domestication is both a cultural and a biological process. It begins when animals are incorporated into the social structure of a human community and become objects of ownership, inheritance, purchase, and exchange. It is complete when the new popu- lation is permanently isolated from the wild species and its breeding, organization of territory, and food supply is under total human control.

Many archaeologists have considered that there was a natural progression first from generalized or broad-spectrum hunting to specialized hunting and herd-following- what Ingold (1980) calls carnivorous pastoralism-of a social herbivore such as the Reindeer or Llama. The next stage is to management of the herds, then to controlled breeding, and finally to artificial selection-for favoured characteristics. However, as emphasized by Ducos (1978, 1989) and Ingold (1980), animals cannot be domesticated unless they are owned, and the social implications of ownership by a human social group are a greater hurdle to domestication than they may seem. Many hunter-gatherer societies never owned domestic livestock, although they did keep Dogs. I t was probably for cultural, as much as for many other complicated reasons, that the native Americans

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did not domesticate the Bighorn Sheep, nor the Australian Aborigines the Kangaroo, nor the ‘Kung San the Eland in southern Africa.

Animals have to be owned to be domesticated but it is also essential that the available species have social behavioural patterns that are compatible with those of humans. This means that the animals must be gregarious, breed readily in captivity, have a wide home range, and a short flight distance. Goats and Sheep fulfil these conditions but gazelle do not, and it is for this reason that Goats and Sheep were first domesticated in western Asia about 9000 years ago and not the gazelle.

ORIGINS OF DOMESTICATION From information on animal behaviour (Schaller, 1977; Eltringham, 1984), the results of numerous excavations (Uerpmann, 1987), and hypotheses about changes in climate and movements of human populations (Cohen, 1977), a picture can be built up of how and why domestication of livestock first took place in western Asia. Animal remains from archaeological sites of 9000 years BP in the Levant show that the earliest Neolithic people obtained most of their meat from hunted gazelle, as they had done for thousands of years. These animals could be driven into enclosed areas or traps to be slaughtered, but whenever they were bunched up together they panicked. The climate was becoming more arid, wild herbivores were decreasing in numbers and plant foods were more difficult to collect. Family groups of hunter-gatherers began to settle around sources of water and to plant seeds gathered from local cereals. These had to be protected from wild gazelle and other grazing animals which were driven away.

Young Goats and Sheep were slower in their reactions to the unknown than the nervous gazelle. They could be tethered, and even led about by children, and at night they could be kept in enclosures and protected from wild predators. By planting cereals and herding their expanding flocks of Goats and Sheep the Neolithic people could survive, and indeed increase in numbers, in an environment that was becoming increasingly arid and depleted of wild herbivores. Once a small group of, say;Goats had been cut off from the wild populations and enfolded into the human community the biological process of domestication began: the animals formed a founder group, which was changed over successive generations, at first in response to natural selection under the new regime imposed by the human community and its environment. Only later was there artificial selection for economic, cultural, and aesthetic reasons.

DISPERSAL OF THE EARLIEST DOMESTIC ANIMALS An important part of the domestication process was the dispersal of the founder group away from its core area. The Dog was the first animal to join human societies, around 12,000 BP (Davis & Valla, 1978; Clutton-Brock, 1984a) and it appears to have had a fairly drastic effect on some wild species. For example, it could well be that hunting with the first domestic Dogs in western Asia greatly contributed to the increasing scarcity of wild animals such as the gazelle. And in Australia, the extinction of the large marsupial carnivores has been correlated with the introduction of the Dingo (Diamond, 1984). This must have occurred after 10,000 BP, when Tasmania was separated from the Australian continent by the sea breaking through the Bass Straits, because there are no records of Dogs on Tasmania. However, it must have happened before the domestication of the Pig which was never taken to Australia as it was to New Guinea and the Pacific Islands (Groves, 1981).

During the period of early agricultural settlement, between about 9000 and 5000 BP in different parts of the world, small numbers of livestock must have slowly diffused

The process of domestication 8 1

many thousands of miles until there were Goats, Sheep and Cattle over the whole of Europe and Asia, as well as in parts of Africa. Because the distribution of the wild ancestors of the Goat and Sheep were restricted to western Asia it is relatively easy to trace their introduction, as domestic livestock, into the archaeological record, as they moved west and northwards. The earliest remains of Sheep in western Europe, for example, have been identified from around 8000 BP on the Mediterranean coast of France (Geddes, 1985).

The presence of these new, human-protected species became a threat to the endemic large herbivores, such as the Wild Horse and the Aurochs in Europe, and they may well have been the cause of extinction of the Pleistocene large mammals which were lingering as relicts on the Mediterranean islands: the Dwarf Hippos and Elephants on Cyprus, for example, and species of Deer unique to the islands. Support for this view can be seen on the Balearic islands where it seems that Sheep and Goats did not arrive until around 4000 BP and this coincided with the extermination of the endemic caprine, Myotragus balearicus. This could have happened from a disease to which the Myotragus had no immunity, such as foot-and-mouth disease (caused by pathoviruses) or anthrax (Bacillus anthracis), which was carried by the domestic livestock (Burleigh & Clutton- Brock, 1980; Clutton-Brock, 1984b).

FEATURES OF EARLY DOMESTICATION The problem for the archaeozoologist is: how can the bones retrieved from an exca- vation be distinguished as wild or domestic? Reduction in size of the bones is the most obvious characteristic of nearly all early domestication. It is still rather a puzzle why this is of such general occurrence; it is found in the transition stage from wild to domestic in nearly all large mammals. The animals would have become smaller as a natural response to the alteration in the feeding regime of captivity and in particular with the loss of night-time grazing. Also, maybe the early farmers would have selected the smallest and most docile animals for a breeding stock and would have killed off large males that behaved in a dominant manner. The largest animals may also have been killed for meat, at around 2 years old, before they could breed, thereby causing unintentional selection for small size.

With the reduction in overall size there was also a reduction in the size and shape of horns, particularly in the Goat. The ancestor of all Domestic Goats is the Bezoar Goat Cupra aegagrus of western Asia. In the wild this Goat has long scimitar-shaped horns in the male and short straight horns in the female. However, rather early on in the domestication process the male horns were often rather small and twisted. This change in shape could have been a consequence of sampling atypical genes from the ancestral population, as well as from inbreeding (homozygosis) in the small numbers of early Domestic Goats. It could then have been subject to artificial selection because billy Goats with small, twisted horns could do less damage in fighting, and they might also have been more docile than the big-horned males.

One example of the process of domestication in action has been described by Belyaev & Trut (1975). Using the silver variety of the Red Fox Vulpes vulpes, on a fur farm in Russia, they began by observing the behaviour of 6-8-week-old Fox cubs. Those that were most responsive to being called, to being hand-fed, and handled were then chosen as breeding stock. After 15 years of selection for docile behaviour, the Foxes would come when they were called, the most responsive wagged their tails in greeting, and they barked on seeing people. In every way, they were claimed to behave like domestic Dogs

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and, what is more, the breeding cycles were altered so that the females came into oestrus earlier and sometimes twice a year.

CRANIAL CAPACITY It has been known for half a century or more that domestic animals have relatively smaller brains than their wild counterparts, and recently Groves (1989) and Hemmer (1990) have been able to quantify the reduction in a large number of mammals. Hemmer maintains that the smaller brains of the domestic forms have fewer folds and furrows in the neocortex than the brains of the wild species and this is responsible for loss of perception in the domesticates and a lowering of reaction to stress. Lowering of the stress reaction also results from reduction in weight of the adrenal gland in many domesticates.

Removal of the need to be constantly on guard against predators is perhaps the main factor contributing to behavioural changes in the domestic animal. The animal has to become adapted to the new conditions of the human environment and its perceptions of the world around it, what is described by Hemmer (1990) as the ‘merkwelt’, are much altered.

NEOTENY Neoteny is probably the best-known characteristic of domestication although the mechanism is little understood. Neoteny is the retention of juvenile characters and behaviour into the adult domestic animal. All domestic animals show some degree of retention of juvenile characteristics which may in part result from selection for individuals that are docile and easy to manage, i.e. they have slow reactions and a low perception of their environment (Hemmer, 1990). However, there can also be secondary selection to superimpose fierceness, as in the Pit-Bull Terrier, which has a neotenous head but an aggressive temperament.

The Pekingese Dog can be seen as the ultimate example of conscious and un- conscious selection for the appealing baby animal that solicits the ‘cute response’. With its round head, soft fur, short face, large round eyes, and floppy ears, the Pekingese goes far towards providing its owner with a substitute baby. As with every domestic Dog, however, the adult Pekingese will struggle to behave, to the best of its ability, like a Wolf.

COATCOLOUR Probably one of the first effects of breeding in captivity was the birth and survival of animals, whether they were Wolves, Goats, or Cattle, that were single-coloured. In the wild, if animals with an aberrant coat colour or markings did survive they would be unlikely to breed. But under domestication they would have been greatly favoured because they would be different from the wild species, they would be remarkable, and more than likely they would be more placid than their wild-coloured relatives.

As reviewed by Hemmer (1990), it has been shown in a variety of mammals including Foxes, Rats, Cats and Fallow Deer that there is a close link between coat colour and temperament. Blotched Tabby Cats are more placid than the wild-type striped Tabby which is why they are found more commonly in the urban environment, while striped Tabbies are more common on farms (Todd, 1977). Golden Labradors are more com- monly used for Guide Dogs for the blind than Black Labradors and in general they are more docile. Light-coloured Fallow Deer (Hemmer, 1990) and amber Foxes (bred for their furs, see Keeler, 1975) become tamer than the wild types. This correlation between

The process of domestication 83

coat colour and temperament should not be found surprising when it is remembered that, throughout history, red-headed people and chestnut horses have always had the reputation of being highly strung.

FOUNDER EFFECTS The spread of animal populations can quite often occur from very small numbers or even from a single pregnant female. For example, the vast herds of feral horses that inhabited the grasslands of South America from the sixteenth century onwards, are thought to have all derived from about five individuals. Or, a later example can be seen in the world population of captive Golden Hamsters, which are all descended from one female with 12 young, caught in Aleppo in Syria in 1930.

It is probable that domestication of most mammals with a very widespread distri- bution of the wild progenitor, like the Wolf or the Aurochs, occurred many times over in different parts of the world and that the initial captive stock was bred from very small founder populations. At later times single aberrant animals could have been the founders of new forms that became very widespread, like the Asian humped cattle called Zebu, or the Fat-tailed Sheep. Selective breeding would enhance characters that originally appeared as a chance mutation. A good example of this can be seen in the double dew-claws of the Pyrenean Mountain Dog which are today included as a standard of the breed. It has been claimed by breeders that this doubling of the first digit (dew claw) on the hind foot, helped the Dogs to climb in the snow of the Pyrenees, their place of origin. However, a mounted skin in The Natural History Museum, London, of an early example of the breed brought to England from the Pyrenees, does not have this character. It is most probable that the double dew-claw is the result of a recent mutation that has spread through the breed and in now considered to be obligatory.

CLIMATIC SELECTION AND THE DEVELOPMENT OF BREEDS An important result of the spread of domestic animals is that they are removed from the biotope to which they have been best adapted by evolution and are taken far away from the natural border of the species. For example, the progenitor of the domestic Donkey, the Wild Ass Equus africanus is one of the very few large mammals to be adapted to a true hot-desert environment. It has all the characteristics of a desert mammal with its slender body, long legs, long ears, a wispy tail, short, pale coat, and a digestive system adapted to survive on dry shrubs and very little water. These characteristics are exploited in the domestic Donkeys of western Asia which live extraordinarily hard lives as pack animals in desert conditions. But the Donkey will also flourish in Ireland, more than a thousand miles from its natural species border. It needs rather more shelter than a Horse but it can live well by grazing on lush grass and can survive the cold with its short limbs, stocky body, and long coat. These adaptations have been brought about by natural selection for survival in the new environment, under human protection.

Climatic selection has also influenced the development of the different breeds of Horse as reflected in the horsebreeders’ terms, ‘cold-blooded’ for the northern heavily built Horses and ‘hot-blooded’ for the lightly built Arabs. All breeds of Horse have originated from one wild species, Equus ferus, which ranged over the whole of Europe and Asia at the end of the Pleistocene, although it is probable that over this vast area there were several subspecies of the Wild Horse. These included two that survived into recent times: a western subspecies, the Tarpan Equus ferus ferus which has been extinct since the last century, and an eastern subspecies Przewalski’s Horse Equusferus

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przewalskii from Mongolia, which is at present being reintroduced to the wild from captive-bred animals.

The effects of climatic selection on domestic animals appears to be identical to the well-known correlations in size and body-shape that can be seen in subspecies of wild animals across a geographical cline. For example the Scottish Wild Cat Felis silvestris grampiu, with its small ears, stocky limbs, and heavy body, contrasts with the graceful African Wild Cat Felis silvestris lybica, and domestic Cats of the same regions reflect these body shapes.

Evolution of a subspecies can also occur, without such gross climatic selection, when a population of animals becomes reproductively isolated from the rest of the species. With domestic animals, under artificial selection, this separation leads to the development of different breeds. A breed can be described as a group of animals that has been selected by humans to possess a uniform appearance that is inheritable and distinguishes it from other groups of animals within the same species. A new breed is established by reproductive isolation and it becomes subject to genetic drift in the same way as a subspecies in the wild. The founders of the new breed contain only a small fraction of the total variation of the parent species. It is a genetically unique population which, by shifting of gene frequencies, may, in Mayr’s words, ‘give rise to an evolutionary novelty’ (Mayr, 1970). A certain amount of inbreeding of the founders will stablize the phenotype while too much inbreeding will reduce fertility, as exempli- fied by the Longhorn breed of Cattle at the end of the eighteenth century when Robert Bakewell tried to carry improvement to excessive lengths by continuous inbreeding (Trow-Smith, 1959; Hall & Clutton-Brock, 1989).

This way of looking at the development of domestic animals, in terms of founder populations and genetic drift, helps to explain how a species such as the Wolf can give rise to both a Great Dane and a Pekingese.

CONCLUSION In writing about domestication in 1969, Berry stated that a domesticated form is like a local ecotype of a species (Berry, 1969). Those were the days, even more than today, when biologists who worked on domestic animals were thought of as a bit odd, their subject being outside the natural world. Today, it is time to recognize that the study of all aspects of domestication is essential for the understanding of human and animal behaviour and welfare. Furthermore, it should be accepted that domestication is evolution, and it is for this reason that despite a number of well-argued cases for the withdrawal of domestic animals from formal zoological nomenclature (e.g. Groves, 1971) I favour its retention (Clutton-Brock, 1987). The Wolf is a wild species of canid with the name Canis lupus. It is the ancestor of a new species, the domestic Dog which is reproductively isolated from all other canids and was quite rightly named by Linnaeus Canis familiaris.

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