Thursday, May 11, 2006


Herbivores are referred to animal that are adapted to feed on plant material, “herbivores are classified into sub groups depending on the type of feed they specialised on”.1 There are those that feed on fruits known as frugivores and folivores those that eat leave. However this does no mean that they do not eat other parts of the plant they mainly eat those selected parts fruits and leaves. Some is influenced by diet that the animal have which vary with season, so then they chose different diet when is available. [1]

Diet in which most of herbivores feed on is different depending on the environment they live in some become more abundant during some seasons. Other herbivores select their diet to feed on, because of the accessibility they have to that foliage. Small animal cannot choose the diet of big trees that found in the forest where there is predators so they have to choose area where they can access. While large animals such as giraffe chose foliage that are higher as their body can stretch to the height of the foliage.

Chosen diet has problems on most of herbivores, but these depend on the type of stomach that the herbivores species have. Some have true ruminants while other are non ruminants, this make those with true ruminants to have better digestion than non ruminants. Herbivores that have long digestive track minimize the problem that they have on digesting the cellulose from plant materials. Cellulose is the main cause of problem in the digestive system of herbivores.

The plant material that most herbivores feed on contain cellulose, it have high energy source and organic substances. And most of herbivores digest food through enzymes digestion; it is difficult for enzymes to break down cellulose because of its high organic substances and energy. so this pose problem in most of herbivores, those that have long digestive track reduce the problem by having extended chewing that the ruminants keep the foliage and fermented it before it comes back to the mouth for chewing. This cellulose is also broken down by micro-organisms that are present in the ruminants; this process is known as fermentation. [2]

This process of fermentation happens after the species have finish grazing, the foliage has been stored in the ruminats.the bacteria and protozoa digest it, during leisure the foliage return to the mouth for chewing and this simplify the breaking down of cellulose. Those problems that happen to herbivores during digestion tend to be reduced. But with the involvement of bacteria and protozoa in the process of fermentation it reduce the problem in herbivores with ruminants while in non- ruminant is still a problem. [2]

The process of fermentation reduce the problem, but in vegetarians takes time for digestion and this were suggested to be reduced by herbivores that have long digestive track. This is because they don’t have bacteria in the stomach. and for animal to have long digestive track their bodies need to grow big, so most of the animal that feed on plant material evolve giant so that they can have long digestive track. Having long digestive track help herbivores to extend their diet to tough plants, which there is less competition. [4]

Because of cellulose they eat, metabolism rate increase and it was suggested that animal with high metabolic process can grow rapidly. It mainly happens in animal that live on land. In terrestrial environment competition of food resource are great, so those that evolve long digestive track and become giant tend to feed on tough plant which other cannot feed on. While other feeds on the foliage that are high where other cannot access it. This reduce the problem that they where having as they cannot compete with other for food and they end up eating fresh material that do not require fermentation much. For example, elephant and giraffe, they reach 18 feet high and they can access high foliage, while elephant also reach 18 feet high with weight around 8 to 10 tonnes ,this enable them to use great bulk of tall trees.[3]

This enables many large herbivores to feed on high foliage, while small herbivores feed on grass or small foliage. Adopting large bodies for these herbivores helps then to maintain body temperature through high metabolic rate, and also to be less vulnerable to predators. This because having large body also increase the strength that can prevent small predators to attack them.

In nutshell, diet that herbivores choose to feed on has impact on digestion and this influence the change in body structure. They change their body structure to solve the digestion problem they encounter. And also to have better access to food resources as there is competition, large animal can access to high foliage. But all changes that herbivores have of becoming large is because of the problem they met, failing to digest cellulose pose an impact, that’s why we find large animals on land .


1. Herbivores. The world’s greatest encyclodictionalmanacapedia [Internet] [cited 2006 May 10] Available from:

2. Findlay A.L.R.: The Gastrointestinal System: an introduction Carnivores, Omnivores and Herbivores [Internet] 1998-February [cited 2006-May11]:

3. Chang.A. Is there any evolutionary advantage to gigantism? Did sauropods continue to grow throughout their lives, like some reptiles and fish do? [Internet] 2000-October-16[cited 2006-May-10]

4. Kaplan.M.1995. A very brief overview of digestion in green iguanas and other herbivorous lizard species. [Internet].2003-August-11[cited 2006-May-11].Available from:

Mr Elelwani Muanalo
NISL- Ecological Informatics Student
CSIR Pretoria
Tel: +27 12 841 2133
Fax: +27 12 842 7024.
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