The belly of the camel is enormous, distending out from behind the chest, most of it a giant 100 litre sack of rotting vegetation, sometimes called the rumen. The heat generated by the fermentation is so great that the rumen has to be situated immediately under the belly muscles to let the heat dissipate to the outside. The contents are repeatedly churned by waves of muscle contractions, and every now and then, some of the green sludge is belched back into the mouth, ground between the teeth, soaked in saliva and gulped back down again. Often, in an evening, the only sound to disturb the quiet of a desert camp is the sound of camels chewing the cud.
The rumen is not a stomach since it has no glands and secretes no enzymes. Only the saliva has enzymes. The rumen is a fermentation vessel charged with bacteria and protozoa that break down cellulose and assimilate it while discarding volatile fatty acids as the end product of their energy metabolism.
Since no vertebrate has ever evolved an enzyme to digest cellulose, the camel is happy to leave it to the microbes. The camel only has to crop the vegetation during the day and then eruct and ruminate at leisure in the evening. It takes up to 50 hours to completely process the contents of a rumen. Evolution has never developed a fast way of digesting cellulose.
The camel hasn’t yet gained profit from its efforts. It has simply provided sanctuary for the microbes. But now their time is up. The fine slime is squeezed into the true stomach and blasted with enzymes to disintegrate the microbes. The stomach needn’t be large; most of the hard work has already been done.
The products of this dual digestion, first by the microbes and then by the stomach enzymes, are passed into the small intestine where the nutrients are finally absorbed. The cellulose and any dietary starch have all been converted into volatile fatty acids which constitute the principal energy source for the camel. Sugars just can’t make it this far without being converted to fatty acids. The microbes also provide the camel with its principal source of protein which are converted into amino acids by the specialised enzymes of the small intestine.
And in a final coup for the camel’s digestive system, any nitrogenous material that has been converted to urea, which would normally be excreted by the kidneys in a less exacting animal, is reabsorbed by the blood and passed into the saliva to be swallowed into the rumen where the microbes convert it back into protein. No wonder camels smile so smugly.
The remaining waste is dried out in the colon and finally expelled as pellets, dry enough to be used immediately in a Bedouin fire to make the evening bread.
With such a powerful digestive system, the camel can survive on shrubs that would starve other animals.
Nevertheless, given the chance, they are choosy grazers and browsers, preferring fresh shoots and leaves. They have large flappy lips that can search and grasp; a row of incisors on the bottom jaw that can nip plant stalks against the upper palate; large canines for grasping and breaking branches; and a tough palate for crushing thorny plants; while dangerous toxins are often neutralised by the microbes in the rumen.