Key Terms
  1. Digestion: the physical or chemical process of breaking down complex insoluble food into small soluble substances that are suitable for the body to absorb
Digestive system: The system of organs responsible for digestion. The digestive system includes salivary glands, tongue, oesophagus, stomach, small intestine, large intestine, liver, gall bladder, pancreas and rectum.
Herbivores: Organisms that consume plants only
Carnivores: Organisms that consume other animals. They are also known as meat-eaters.
Microbial fermentation: the breakdown of food which involves the action of bacteri
Fore-gut fermentation: microbial fermentation which occurs in the stomach. This causes cellulose to digest before the food reaches the intestines.
Hind gut fermentation: microbial fermentation which occurs in the caecum. Cellulose is digested in this organ which is located at the end of the small intestine.
Ruminant fore-gut fermented: animals that regurgitate the food in the first part of their stomachs and re-chew it so that food particles may be further reduced in size.
Ingestion: the removal of undigested food from an organism


What do herbivores eat?
*Plants are the main diet of herbivores.
*They are high in fiber and starch which provides the main energy source in their diet.
*Since some parts of plant materials such as cellulose and cell wall thickenings are hard to digest and that cellulose does not provide enough energy unless microbes are present, the digestive tract of herbivores are adapted, so that food may be digested properly.

The Digestive tracts of herbivores

*Many large herbivores need microbes in their guts to help them in digesting cellulose
*They have long and complex digestive tract to allow microbial fermentation to occur
*The digestibility of food is closely related to the complexity and the length of the guts.

Digestive Tracts are long because...
*They provide adequate space to hold and store large amount of food which must be eaten
*Allow maximum opportunity for microbial fermentation to occur
  • Allow time for the absorption of nutrients

Digestive tracts are complex because…
  • The high-fiber diets they eat are difficult to digest

Small herbivores such as fruit eaters and nectar feeders have shorter and simpler digestive tracts because…
*Plant tissues in their diets are easier to digest compared to that of large herbivores
*Their diets contain large quantities of sugar and little or no fiber

Fore-gut Fermentation
*Microbial fermentation which occurs in the stomach.
*Cellulose is digested before the food reaches the intestines

*Microbes provide energy by releasing short chain fatty acids
*Some microbes digested by the host as a source of amigo acids
*The stomachs of ruminant fore-gut fermented (e.g. cattle) are very complex and are composed of 4 parts
-2 chambers where microbial fermentation occurs
-1 part for storage
-1 part functions as a true stomach
*Microbes have plenty of time to digest cellulose as food is stored in the microbial fermentation chambers for a long time

Hindgut Fermentation
*microbial fermentation occurs in the cecum which is found at the end of the small intestine
*Cellulose is digested in the cecum

*Microbes cannot be digested as a source of amigo acids as they are in the large intestine where the digestion of protein no longer takes place
*Microbes are lost during the process of ingestion

What do they eat?
*other animals
*animal matter is more easy to digest as animals cells do not have cell walls
*animal matter is low in fiber but high in protein
*has a high energy content (higher than plants)
*the muscles, skin and internal organs of the prey provide fat and protein in their diet
*the cartilage and bones provide small amount of fiber

The digestive tracts of carnivores
*short and unspecified gut as their diet is easy to digest compared to plant materials
*simple stomach
*little undigested food is digested as they do not consume much fiber
*small cecum or no cecum

  • identify data sources, gather, process, analyse and present information from secondary sources and use available evidence to compare the digestive systems of mammals, including a grazing herbivore, carnivore and a predominantly nectar feeding animal
Herbivore (sheep or New Zealan)
Carnivore (dog)
Nectar Feeder (honey possum)
Major food source and chemical
composition of
-fibre and starch
-chemical composition: sugars, proteins, oils, other nutrients
-meat, bones
-protein, fat
-nectar & pollen
-high sugar content (sucrose, glucose, fructose)
-no canines or incisors in upper jaw
-flat molars (for grinding grass)
-chewing is essential
-powerful jaws
-well-developed canine teeth for tearing meat
-molars for crushing meat
-few, small teeth
-thin & flexible lower jaw
-pointed canines, incisors
-flattened cheek teeth
Microbial Action
-microbes in guts digest cellulose
-sheeps are foregut fermenters
-microbial fermentation in large intestine
-hindgut fermenters
-microbes are excreted
-complex-has 4 chambers (rumen, reticulum, omasum, obomasum)
-Rumen: 1st chamber, sheep regurgitates once rumen is full, bacteria & protozoa break down cellulose, 70% of cellulose is transferred to bloodstream
-Reticulum: 2nd chamber, allows regurgitation
-Omasum: 3rd chamber, bacteria & micro-organisms breakdown food
-Obomasum: 4th chamber, ‘real stomach’ secrete hydrochloric acid & enzymes to break down proteins & fat
-small-contains hydrochloric acids which dissolves food
-large-has 2 chambers
-lining of stomach releases mucus
-stores nectar
-Long and complex
-about 17 meters long
-over 50% of food consumed is excreted
-about 6 meters long
-digestion of food takes place
-nutrients enters bloodstream
-simple & short because the plant tissues in their diet is relatively easy to digest (no fibre)
-digestion of pollen takes place

1) Define herbivore and carnivore.
2) Explain why the digestive tracts of herbivores are long and complex.
3) Describe the process of microbial fermentation in fore-gut fermenters.
4) Explain why the digestive tracts of carnivores are short and simple.
5) Compare the digestive systems of a grazing herbivore and a carnivore.

To find out more:
Use pages 150 -153 of the Biology in Focus Textbook
or go to the following websites: