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Bison of North America are completely vegetarian.

Like all vegetarians they have teeth that are specially modified for the job.

The front teeth are 'nippers' and good for shearing off the grass or the browse.

At the back of the jaw are grinding molars that are open rooted so they can keep on growing as the enamel wears down.

The molars also have ridges that help break down the cellulose in the plant.

Also, the jaw can move from side to side to help with grinding. Bison must spend up to nine hours a day gathering all the grass they need to sustain their bulk.

Like a rabbit, a buffalo digests its food twice over but in a rather neater way.

The chewed grass goes down into the stomach which serves as a fermentation vat and is full of bacteria and single-celled creatures. The tiny organisms are so small that a million of them could fit in a tea spoon.

No mammal can digest the cellulose walls of plant cells but the micro-organisms can. They produce a ferment that dissolves the cellulose and turns it into a substance that the bison can absorb.

But the bodies of the microbes also contain valuable protein and this will also be digested, but not until the half-digested mush - or cud - is brought up from the stomach a lump at a time for a second chewing.

Many grass-eaters chew the cud like this. It can be done while resting or looking out for danger.

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