DIGESTION IN HERBIVORES
Plants form the diet of herbivores and and therefore their digestion holds up the key for the availability of nutrients for these animals1. Plants cells have cellulose which is the primary structural component plant cells is a stable organic compound that is not easily digested. Digestive enzymes which the mammals produce cannot be able to breakdown cellulose to obtain glucose. As herbivores had to feed on the plants they had to overcome the problem of cellulose digestion. Cellulose can be broken down mechanically by chewing for a long time or fermentation by bacteria which dissolves cellulose. All herbivores stomachs have bacteria which dissolve cellulose but the process takes time. Various mechanical methods are employed by different herbivores to breakdown cellulose2.
Among the adaptations for digestion to evolve are exhibited by rabbits and rodents who re-eat their faeces which is soft, and normally deposited in the burrow2. This process is called coprophagy and ensures that the plant material consumed is processed twice and nutrients are extracted. After this second eating process is the faeces discarded outside the burrow as dry pellets2.
Elephants eat leaves, fibrous and woody material which poses a problem in their digestion, first to make it small and secondly to breakdown cellulose2. To handle this problem elephants have molars as their only teeth in their mouths. These molars are massive grinders which pound and crush the food with much power. The molars are continuously replaced as they wear out, by new ones growing on rear of the jaw and moving to replace those that are worn out2. Elephants feed on a lot of food and it takes along time before it is fully digested (up to two and a half weeks). To be able to store this food for those two weeks the elephants have to have big stomachs which also aid in the breakdown of the mostly woody material in the process of fermentation by bacteria in the elephants stomach. The big stomach serves as holders for the bacteria broth there for digestion. 2
Ruminants also evolved a different way to overcome this problem. Ruminants are animals that digest their food in two steps; first they chew the food for the first time roughly to break it to make them easy to swallow and then regurgitate and chew it a second time2, 3. The second chewing is called chewing the cud and animals practising ruminacy include cattle, deer, goats, sheep, camels, giraffes, buffalo, wildebeest, and antelope3. A ruminants stomach is divided in to four compartments namely; rumen, reticulum, omasum, and abomasums2. Grass and other foliage are clipped by use of lower incisors and pressing it against the tongue or gums of upper jaw with no front teeth. It’s briefly chewed by the molars before being swallowed the food into the rumen where there’s a broth of bacteria and is churned by this muscular stomach while bacteria break the cellulose3. The food moves to the reticulum where it is divided into solid and liquid before being brought back in to the mouth2. The food is then brought to the mouth in balls (bolus) and is chewed again and ground by the molars by forward, backwards and sideways movements combined with up and down movements3. The now well ground food and in semi liquid form goes to the omasum through the rumen where water is removed before going to the omasum3. All the final digestion and absorption takes place in this stomach2.
Chewing the cud normally takes place in safety and in a relaxed form thought to be a form of adaptation to escape predators and so the animal have to eat in a hurry. Also due to limited food and there’s competition makes the animals adopt this method first to fill their stomachs and then digest it later. Due to the many number of stomachs has made the animals grow bigger in size. They also take big amounts of food like the cows, wildebeests buffalos and the giraffe. This form of breaking down of cellulose is more advanced than that of the elephants2.
By being able to digest cellulose the animals are assured that their glucose requirements are provided for in their meals. This has come with various modifications to the body structure like teeth, stomach and an increase in their sizes2.
1. Wikipedia contributors. Herbivore [Internet]. Wikipedia, The Free Encyclopedia; 2006 Apr 30, 00:26 UTC [cited 2006 May 11]. Available from: http://en.wikipedia.org/w/index.php?title=Herbivore&oldid=50816562.
2. Knight, R. BCB Biodiversity chapter 2 The Invasion of the Land (Cited 2006 May 11) http://planet.uwc.ac.za/nisl/biodiversity/Chapter2/page_234.htm
3. Wikipedia contributors. Ruminant [Internet]. Wikipedia, The Free Encyclopedia; 2006 Apr 22, 23:58 UTC [cited 2006 May 11]. Available from: http://en.wikipedia.org/w/index.php?title=Ruminant&oldid=49674058.
VINCENT MUCHAI WAIRIMU
Biodiversity and Conservation Biology
University of the
Private Bag X17 Bellville