defined as one that has been bred in captivity for purposes of economic profit to a human community that maintains total control over its breeding, organization of territory, and food supply. Not all adult mammals can flourish or will breed under such drastic alterations to their natural way of life, although all young mammals can be tamed when nurtured under the right conditions.

A young animal if removed from its mother and its natural environment to be reared in captivity, for example in a zoo, will, if it survives, adapt to its new way of life. This adaptation will include changes in its physical anatomy and in its behaviour. Some of these changes are obvious, some are barely perceptible, but if the animal breeds in captivity they will be more marked in the next generation. The mammalian body is a much more plastic and changeable structure than might be thought, and even the skull and skeleton of a tame animal may exhibit distinct differences from that of its wild parent.

	Thinking Point
	Many domestic animals have been bred to retain juvenile characteristics in adult life. Do their baby faces help to inspire in their human owners an innate desire to care for them?

Future generations of the tamed animal, whether these lived in the prehistoric period or at the present day, will be subjected not primarily to natural selection but to artificial selection by humans for characters that may be favoured for economic, cultural, or aesthetic reasons rather than for survival of the species. Over a long period of selective breeding dramatic alterations may occur in the appearance of the domesticated animal although these will be always constrained by genetic barriers. It is for this reason that the effects of taming and domestication produce the same general physical changes in widely different groups of mammals. Many of these changes originate from the retention of juvenile characters into adult life rather than from genetic alteration and they therefore exhibit the same form in mammals as different as the pig and the dog. Examples of such changes are the deposition of fat under the skin, the shortening of the jaws, and the curled tail, all of which will be discussed in further detail in Session 3.

Domestication causes an imbalance and disruption in the rate of growth of different parts of the organism, resulting in morphological proportions in the adult animal that differ from those of its wild counterpart. The mechanism of how this happens is not well understood but it is likely to be due to stress and to hormonal changes as a result of the animal's emotional and physical dependence on humans. Domesticated animals that return to living in the wild, that is they become feral, will usually revert by natural selection to a physical form that is closer to the wild species as a consequence of the loss of this dependence. Feral animals can be defined as those that live in a self-sustained population after a history of domestication. The present-day feral pigs of Australia or North America are good examples of populations of animals that after a relatively short period conformed to a uniform type more closely resembling the wild boar than their highly domesticated antecedents.