Hindgut Fermenters - Anatomy & Physiology
Introduction
Hindgut fermenters are evolved to eat a herbivorous diet. Such a diet includes large quantities of insoluble plant carbohydrates, such as cellulose. Mammals cannot digest these insoluble carbohydrates as they lack the essential enzymes, such as cellulase. However it is important that they do digest these carbohydrates as there is insufficient quantity of soluble carbohydrates in plant material. Some microbes do have the enzymes to digest these insoluble carbohydrates and so hindgut fermenters hold a symbiotic relationship with these microbes. Hindgut fermenters have anatomical adaptations to allow for an expanded microbial population. The products of fermentation are volatile fatty acids. It is important to supply a source of fibre in their diet as it stimulates peristalsis in the gut and prevents a build up of gas.
Cellulose Structure
Cellulose exists in the cell walls of plants. Mammals are unable to digest cellulose due it's structure. Cellulose is a polymer of glucose where each glucose monomer is joined to the next by a β-1,4 glycosidic bond. Mammalian enzymes only have the capability to break α-1,4 glycosidic bonds, which are seen in starch and glycogen.
Comparison with foregut fermenters
Hindgut fermenters have an enlarged hindgut as opposed to foregut, as the ruminants do, for microbial fermentation to take place. Hindgut fermentation provides advantages and disadvantages.
Advantages: Soluble carbohydrates, such as glycogen, are available to the animal before they are available to the microbes. Therefore the brain, which can only utilise glucose, receives a good, constant supply with relatively less time and energy utilised than in the ruminant. Foregut fermenters receive nearly all of their energy in the form of VFAs, which have to be converted to glucose by gluconeogenesis in the liver to be able to be used by the brain.
Disadvantages: Microbes in the foregut can convert non-proteinaceous sources of nitrogen, like ammonia and urea to all of the amino acids. Microbial protein is available to the ruminant when the microbes die and pass down into the abomasum and small intestine. Therefore ruminants can survive on a poor quality source of of nitrogen. Microbial protein is not available to hindgut fermenters because when the microbes in the large intestine die, they get excreted as there is no further opportunity for their digestion. Microbes in the foregut synthesise vitamins, which are also available to the animal further on in the digestive tract. Again, they are not available to the hindgut fermenter. Microbes in the foregut can detoxify some poisonous compounds. Poisonous compounds can be digested and absorbed before they reach the microbes in the hindgut.
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