<P> Domestic mammals form a large part of the livestock raised for meat across the world . They include (2011) around 1.4 billion cattle, 1.2 billion sheep, 1 billion domestic pigs, and (1985) over 700 million rabbits . Working domestic animals including cattle and horses have been used for work and transport from the origins of agriculture, their numbers declining with the arrival of mechanised transport and agricultural machinery . In 2004 they still provided some 80% of the power for the mainly small farms in the third world, and some 20% of the world's transport, again mainly in rural areas . In mountainous regions unsuitable for wheeled vehicles, pack animals continue to transport goods . Mammal skins provide leather for shoes, clothing and upholstery . Wool from mammals including sheep, goats and alpacas has been used for centuries for clothing . Mammals serve a major role in science as experimental animals, both in fundamental biological research, such as in genetics, and in the development of new medicines, which must be tested exhaustively to demonstrate their safety . Millions of mammals, especially mice and rats, are used in experiments each year . A knockout mouse is a genetically modified mouse with an inactivated gene, replaced or disrupted with an artificial piece of DNA . They enable the study of sequenced genes whose functions are unknown . A small percentage of the mammals are non-human primates, used in research for their similarity to humans . </P> <P> Charles Darwin, Jared Diamond and others have noted the importance of domesticated mammals in the Neolithic development of agriculture and of civilization, causing farmers to replace hunter - gatherers around the world . This transition from hunting and gathering to herding flocks and growing crops was a major step in human history . The new agricultural economies, based on domesticated mammals, caused "radical restructuring of human societies, worldwide alterations in biodiversity, and significant changes in the Earth's landforms and its atmosphere...momentous outcomes". </P> <P> Hybrids are offspring resulting from the breeding of two genetically distinct individuals, which usually will result in a high degree of heterozygosity, though hybrid and heterozygous are not synonymous . The deliberate or accidental hybridizing of two or more species of closely related animals through captive breeding is a human activity which has been in existence for millennia and has grown for economic purposes . Hybrids between different subspecies within a species (such as between the Bengal tiger and Siberian tiger) are known as intra-specific hybrids . Hybrids between different species within the same genus (such as between lions and tigers) are known as interspecific hybrids or crosses . Hybrids between different genera (such as between sheep and goats) are known as intergeneric hybrids . Natural hybrids will occur in hybrid zones, where two populations of species within the same genera or species living in the same or adjacent areas will interbreed with each other . Some hybrids have been recognized as species, such as the red wolf (though this is controversial). </P> <P> Artificial selection, the deliberate selective breeding of domestic animals, is being used to breed back recently extinct animals in an attempt to achieve an animal breed with a phenotype that resembles that extinct wildtype ancestor . A breeding - back (intraspecific) hybrid may be very similar to the extinct wildtype in appearance, ecological niche and to some extent genetics, but the initial gene pool of that wild type is lost forever with its extinction . As a result, bred - back breeds are at best vague look - alikes of extinct wildtypes, as Heck cattle are of the aurochs . </P>

The mammals form a class called the mammalia within the animal kingdom