Chewing the Cud – How does rumination work?
Ruminants are herbivorous mammals that have a four-chambered stomach. This bizarre adaptation allows them to digest otherwise indigestible plant matter by means of a process called rumination. Let's process this from the beginning.
The problem with cellulose
Most animals cannot digest cellulose, but ruminants thrive on eating plant-cellulose such as tough leaves and stalks. Clearly they need some help in getting these tough fibers through their digestive system and that's where rumination comes into play.
The first stop
As the animal grazes or browses, the fodder is mixed with copious amounts of saliva. Ruminants can produce up to 150l of saliva daily. This mush then passes down their oesophagus to the first stomach, called the rumen.
The rumen works on the meal together with the reticulum (stomach 2) to separate liquids from solids. These two organs are barely distinguishable and together and are known as the reticulorumen.
Solid matter remains in the rumen, while liquids drain into the reticulum. More of that later. Inside the rumen, these animals have billions of bacteria, yeasts, protozoa and fungi which are vital for breaking down plant fibers and starting the digestive process off. In exchange, they extract much of the carbon, phosphorus, and nitrogen that the animal has ingested.
Undigested matter forms into a bolus (cud) which is then regurgitated, re-chewed and swallowed again and again, and again. This process continues until the tough fibers are broken down enough to pass through to the reticulum, taking some of the microbial organisms with them. To compensate for all this chewing, ruminants usually have a thick dental pad instead of upper incisors, and their teeth never stop growing.
The liquids flow from the reticulum into the omasum (stomach 3) and the abomasum (stomach 4) which get to work extracting water, sugars, fatty acids, and other nutrients from the mush. The abomasum functions in the same way as our stomachs, and also digests any nutrient-rich bacteria to claim back the minerals they took in the reticulorumen.
The small and large intestines are the next stop, where the last stages of digestion and fermentation occur before excretion.
It is interesting to note that a by-product of rumination is methane gas, and that domestic ruminants such as cows are responsible for 15-20% of global methane production.Who would suspect that there are such complex things going on inside that masticating impala that you would never have digested!