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Digestion in herbivores
David Attenborough outlines how bison and other ruminants have become specialised to their high cellulose diet. The bison of North America are completely vegetarian. Like all vegetarians they have teeth that are specially modified for the job. Their front teeth, or incisors, are 'nippers' and good for shearing off grass. At the back of their jaw are grinding molars that are open rooted so they can keep on growing as the enamel wears down. The molars have ridges that help break down plant matter and their jaw can move from side to side to help with grinding. However, to be able extract the maximum energy from their food the bison digests it twice over. Firstly, the chewed grass goes down into their chambered stomach which is full of bacteria and single-celled creatures. No mammal can digest the cellulose walls of plant cells on its own, so the bison enlists the help of these micro-organisms to ferment and dissolves it, turning it into a product that they can absorb. But, not before the half-digested mush, or cud, is brought up from the stomach for a second chewing. This is known as 'chewing the cud'. First broadcast in the series 'Life on Earth' in 2009.
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