omestication can be summarized as follows.

Size of body
The early stages of domestication of any species of mammal are almost always accompanied by a reduction in size of the body. This is so generally true that it is used as the main criterion to distinguish the skeletal remains of domestic from wild animals when these are retrieved by archaeological excavation of early prehistoric sites. During the later stages of domestication animals that are either very much larger or very much smaller than the wild progenitor are selected for and breeds developed from them, for example, shire horses and Shetland ponies.

	Thinking Point
	Some domesticated breeds, such as dachshund hounds and bulldogs, have been deliberately bred in ways that have produced extreme forms of their natural characteristics. Is this practise cruel and 'unnatural', or are domesticated animals so different to their wild counterparts anyway, that such judgments have no relevance?

Outward appearance and pelage characters
During the late Mesolithic and early Neolithic periods when humans were experimenting with the breeding of early domestic dogs and livestock animals it is probable that those animals that were easy to distinguish and markedly different from the wild species would be especially favoured; they could be identified easily and they would increase the prestige of their owner. Thus a puppy that had a curled-up tail, or lop ears, or more white on its body than was usual would be nurtured more carefully and better fed than its litter mates. In this way greater diversity would be introduced into the stock of domestic animals. A disproportionate lengthening of the ears has occurred in most common domestic mammals with the exception of the horse, whilst an extra long tail is found in many breeds of sheep. A tightly-curled tail is common in many breeds of dog, occasionally in the cat, and in many breeds of pigs (see Figure 1).

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Figure 1: A tightly-curled tail is common in many breeds of pig.

Great variation is found in the wool and hair coats of most domestic animals, this being related to the climate where the breed has been developed as much as to human preference. For example, dogs and sheep from the alpine regions have extraordinarily thick woolly coats whilst those bred in the tropics may be almost hairless. In domestic long-woolled sheep the character for self-shedding with the onset of summer has been lost and the wool is continuously growing which means that, although the fleece has to be shorn by a shepherd each year, none of the wool is lost.

In the horses the hair of the mane and tail is much elongated; in the wild horse and in asses and zebras the mane is short and stands erect rather than falling to one side as in all breeds of domestic horse.

Internal characters and dentition
There is a general tendency in well-fed domestic mammals for there to be a layer of fat under the skin and bands of fat through the muscle; in the past this was particularly favoured in beef and mutton. In wild animals surplus fat is usually stored around organs such as the kidneys rather than through the muscle. Sheep and pigs are prone to the development of excessive amounts of fat and in the sheep an anciently evolved character is the fat tail (see Figure 2). This was described thus by Herodotus in about 450 BC, as (translated by Rawlinson, 1964, see Figure 2):

There are also in Arabia two kinds of sheep worthy of admiration, the like of which is nowhere else to be seen; the one kind has long tails, not less than three cubits in length, which, if they were allowed to trail on the ground, would be bruised and fall into sores. As it is, all the shepherds know enough of carpentering to make little trucks for their sheep's tails. The trucks are placed under the tails, each sheep having one to himself, and the tails are then tied down upon them. The other kind has a broad tail, which is a cubit across sometimes. (III, 113)

Juliet Clutton-Brock

Figure 2: Fat-tailed sheep in Iraq.

In nearly all species of domestic mammals, in which it has been measured, it has been found that the brain is smaller in size relative to the size of the body than it is in the wild progenitor. In Domestication: The Decline of Environmental Appreciation (1990), Hemmer discussed how brain size differs in geographically separate species of wild mammals, for example wolves and wild cats, and he believes that the domestic form is descended from the wild subspecies with the smallest cranial capacity. For the dog this is the wolf from western Asia, while for the domestic cat it is the wild cat from north-east Africa and western Asia. This fits with the archaeological evidence for the earliest finds of domestic cat, and for dogs from around 14,000 years ago.

Furthermore, it is a remarkable fact that the decrease in brain size, measured as the cranial capacity, remains small even in populations of feral mammals. According to Hemmer (1990), the dingo which has lived in feral populations for thousands of years has a brain size that is much smaller than all the subspecies of wolves and is similar to that of European domestic dogs. In another study of cranial capacities, 'Feral Mammals of the Mediterranean Islands: Documents of Early Domestication' from The Walking Larder: Patterns of Domestication, Pastoralism, and Predation (J. Clutton-Brock, ed.) (1989), Colin Groves showed that the brains of domestic breeds of sheep are much reduced in size relative to those of all species of wild sheep and the brain of the European mouflon (which is an anciently feral breed originating from Corsica and Sardinia) is also reduced.

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Figure 3: Skull of a pig of the Middle White breed, length c. 37cm.

It is in the skull, and to a lesser extent in the skeleton, that the most obvious changes can be seen. Within a very few generations of breeding in captivity the facial region of the skull and the jaws become shortened, this being common in many species but is most apparent in early domestic dogs. At first there is no corresponding reduction in size of the cheek teeth which are genetically much more stable than the bones of the skull. This causes a crowding or compaction of the premolars and molars, a character that is used to distinguish the remains of the earliest domestic dogs from those of wild wolves.

Shortening of the jaws and facial region is an example of the retention of a juvenile, or even foetal, character in the fully grown animal. As well as being common in dogs it is also found, sometimes to an exaggerated extent, in breeds of pig such as the Middle White (see Figure 3), and occasionally in cattle such as the South American breed known as Niatu (which may now be extinct).

Following the reduction in size of the jaws, especially in the dog, there is a reduction in the size of the teeth which becomes permanent in the domesticated animal so that, for example, in the Great Dane (see Figure 4), a breed of dog that is much larger than its progenitor the wolf, the teeth are still considerably smaller and have a less complicated cusp pattern than is found in the wolf. Similarly, the tympanic bulla (bony case of the ear drum) of the dog's skull is considerably smaller than in the wolf.

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Figure 4: Skull of a Great Dane, length c. 26cm.

The horns of cattle, sheep, and goats show great diversity under domestication and this is often reflected in changes to the skull. The horn consists of a keratinous sheath overlying a bone core which is an out-growth of the frontal bone. Excessively large or long horns, as in the Ankole cattle of East Africa or the lyre-horned cattle of ancient Egypt, have been presumably selected for aesthetic or ritual reasons and for the enhancement of status within the human community that has bred them.

Changes in the skeleton of domestic mammals are not often so marked as in the skull and are usually only related to alteration in overall size of the body; however, the muscle ridges and facets for articulation of the joints are reduced in domestic animals that are not much exercised, whereas in draught animals such as the ox or the horse they may be exaggerated.

Castration
The physical changes brought about by castration are pronounced. After a young male animal has had its testes removed changes occur in its growth pattern so that its bones continue to grow in length rather than in girth, and more fat is laid down in the body tissues. In a castrated ox, for example, the body will be fatter than in a bull fed on the same diet but the limbs and the horns will be longer and more slender (Armitage & Clutton-Brock, 1976).

The behaviour of all domestic animals is greatly altered by the removal of the testes in the male (castration), and to a lesser extent by the surgical removal of the ovaries in the female, known as spaying. It is probable that at least the former operation has been carried out since the beginning of livestock husbandry for castration is an essential aid in the human control of animals. As is well known, it turns an unmanageable, aggressive bull into a placid and submissive draught ox, whilst a yowling, fighting tom cat becomes a fat and contented pet.

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Figure 5: Skull of a Pekingese, length c. 8cm.

Behaviour
With the exception of the domestic cat, all domestic mammals are derived from wild species that are social rather than solitary in their behaviour. As with certain physiological characteristics such as gestation period, blood group, and chromosome number, the structure of the social behavioural patterns of domestic animals is probably little changed from those that evolved in their wild progenitors so that, for example, however different a Pekingese dog may look from a wolf its behaviour is still recognizably wolf-like. Any changes in behaviour that do occur appear to be a result of the retention of the juvenile, submissive behaviour of the young animal to its parent. This is obviously of great advantage to the human owner who wishes to retain dominance over the animal and it is probable that it has been highly selected for, and is correlated with the retention of juvenile features in the anatomy (see Figure 5).