5 Digestive processes
The earliest ruminant was probably an ancestor of the present-day chevrotain. The chevrotain skeleton appears to have remained virtually unchanged for the past 30 million years and, although there are now only four species confined to the jungles of Africa and Southeast Asia, they once had a worldwide distribution. So, chevrotains are placed in the suborder Ruminantia within the order Artiodactyla, to which other deer, antelopes, cattle, sheep and goats also belong. A second suborder, the Tylopoda, contains the camels and llamas; these also have a fermentation chamber within a complex stomach, and are therefore ruminants. So you can now transfer camels from the 'Not known' column of your Table 2 to the 'Ruminants' column. We'll be returning to camels later, too.
What process is going on when an animal 'ruminates'?
When an animal ruminates, it retrieves lumps of its food from one compartment of its stomach, gives them a second chewing and then swallows them again into a separate chamber of its stomach where microbes continue to work on digesting the cellulose [p. 94].
The word 'ruminant' is derived from the Latin word ruminare, to chew again. To allow for the food to be chewed twice, the stomach of ruminants is complex, with four separate sections.
Look again at Figure 3 and list the four sections of the stomach of a ruminant.
They are, in order: the rumen, reticulum, omasum and abomasum. (Technically speaking, only the abomasum is a true stomach; the remaining three sections are swellings of the oesophagus, which is the muscular tube that conveys food from the mouth into the stomach itself.)
When a ruminant is feeding, it does so in a series of quick bites, giving the food no more than a cursory chew between its molar teeth, mixing it with large quantities of saliva (several hundred litres per day in domestic cattle) and then swallowing it into the first of the chambers, the rumen. Here powerful muscles churn it with the microbes that start the fermentation process. The food ferments, generating methane and carbon dioxide which are eructed (burped!). The microbes start to break down the cellulose of the cell walls into sugars, thereby releasing other nutrients from inside the cells. The microbes use some of these nutrients for their own metabolism, and in doing so generate fatty acids, which the ruminant can absorb into its blood through the wall of the rumen and can use in its own metabolism. Large pieces of plant material float on top of the fluid in the rumen and are passed to the reticulum, which has honeycomb partitions in its walls. Here the food is formed into balls called 'cuds'.
Eventually the animal takes a break from feeding, selects a resting place where it can keep watch for predators and spends some time ruminating - the cuds are regurgitated and the animal chews the material again, mixing it with saliva and breaking it down into smaller particles. This process gives a bigger surface area for the microbes to continue digestion of the food when it is swallowed again.
Only when the material is very finely ground, does it pass into the omasum, where strong muscular contractions churn it up further. Finally, it enters the true stomach or abomasum, where the normal digestive enzymes get to work to break down the remains of the food and also to digest many of the microbes that have continued along with the food. Digestion continues in the small intestine, and absorption of the digested food into the blood begins through the wall of the small intestine. If any tough plant material has still not been broken down, there is a further opportunity for fermentation, and absorption in the caecum (the bulge from the side of the lower part of the digestive tract). Any material that still remains undigested is expelled from the body as faeces.