The ruminants are one of the most successful mammalian lineages, exhibiting extensive morphological and ecological diversity and containing several key livestock species, such as cattle, deer, yak, sheep, and goat. The acquisition of the rumen in the ruminants and of the omasum in pecorans (all ruminants except the Tragulidae) allow these animals to use plant material with a higher efficiency than other herbivorous mammals, such as equids.