User:Laander05/Hindgut fermentation

Hindgut fermentation is a digestive process seen in monogastric herbivores, animals with a simple, single-chambered stomach. Cellulose is digested with the aid of symbiotic bacteria. The microbial fermentation occurs in the digestive organs that follow the small intestine: the large intestine and cecum. Examples of hindgut fermenters include proboscideans and large odd-toed ungulates such as horses, elephants, and rhinos, as well as small animals such as rodents, rabbits and koalas. In contrast, foregut fermentation is the form of cellulose digestion seen in ruminants such as cattle, deer, moose, and giraffes which have a four-chambered stomach, as well as in sloths, macropodids, some monkeys, and one bird, the hoatzin. Hindgut fermenters have the capability of being larger than foregut fermenting ruminants, suggesting digestive advantage.

Large Animals
The largest extant and prehistoric megaherbivores, elephants and indricotheres (a type of rhino), respectively, have been hindgut fermenters. Study of the rates of evolution of larger maximum body mass in different terrestrial mammalian groups has shown that the fastest growth in body mass over time occurred in hindgut fermenters (perissodactyls, rodents and proboscids). While foregut fermentation is generally considered more efficient, and monogastric animals cannot digest cellulose as efficiently as ruminants, hindgut fermentation allows animals to consume small amounts of low-quality forage all day long and thus survive in conditions where ruminants might not be able to obtain nutrition adequate for their needs. Hindgut fermenters are able to extract more nutrition out of small quantities of feed. The large hind-gut fermenters are bulk feeders: they ingest large quantities of low-nutrient food, which they process more rapidly than would be possible for a similarly sized foregut fermenter. The main food in that category is grass, and grassland grazers move over long distances to take advantage of the growth phases of grass in different regions.

Small Animals
Research on small cecum fermenters such as flying squirrels, rabbits and lemurs has revealed these mammals to have a GI tract about 10-13 times the length of their body. This is due to the high intake of fiber and other hard to digest compounds that are characteristic to the diet of monogastric herbivores. In smaller hindgut fermenters of the order Lagomorpha (rabbits, hares, and pikas), cecotropes formed in the cecum are passed through the large intestine and subsequently reingested to allow another opportunity to absorb nutrients. Cecotropes are surrounded by a layer of mucus which protects them from stomach acid but which does not inhibit nutrient absorption in the small intestine. Coprophagy is also practiced by some rodents, such as the capybara, guinea pig and related species, and by the marsupial common ringtail possum. This process is also beneficial in allowing for restoration of the microflora population, or gut flora.

Fermentation Physiology
Hindgut fermenters generally have a cecum and large intestine that are much larger and more complex than those of a foregut or midgut fermenter. Unlike in foregut fermenters, hindgut fermentation occurs after the stomach and small intestine in monogastric animals, which limits the amount of further digestion or absorption that can occur after the food is fermented.

Cecum
The cecum is the first portion of the colon immediately following the ileum that, in addition to its fermentation roles, is responsible for the absorption of salt and water that were not absorbed in the small intestine. It is blind-ending, meaning that the end of the cecum does not continue into any other organs. The microbes residing in the cecum utilize undigestible fibers and other dietary elements to, in turn, provide metabolic products to the host animal. One common common type of microbial product is short-chain fatty acids (SCFAs), which can be utilized in energy production. The cecum of the omnivorous rat is dominated by the phylum Firmicutes, and Bacteroidetes and Proteobacteria are highly represented as well. The cecum of the herbivorous horse is dominated by Firmicutes as well, but it is followed by Fibrobacteres.

Proximal, Distal, and Sigmoid Colon
The sections of the colon following the cecum have their own unique roles in digestion. In addition to absorbing electrolytes and water, they also absorb vitamins K and B7 (and some other B Vitamins), some of which are cellulose digestion byproducts of their microbial inhabitants. However, the large intestine is not capable of digestion or absorption of any protein, sugar, or fat that these microbes may produce. In rats, Firmicutes, particularly Lactobacillus, and Proteobacteria dominate the sections of the colon following the cecum as well.

Microbes
Gastrointestinal microbiomes are highly variable and shaped by species genetics, diet, environment, stress, age, sex, and many other factors. Microbes found in the digestive organs of living creatures and can act as protective agents that strengthen the immune system. In a study that compared a variety of hindgut fermenters to ruminants, the most abundant phyla of Firmicutes and Bacteroidetes were comparable, but their UniFrac beta-diversity metrics showed distinct differences. At the family level, Ruminococcaceae, Erysipelotrichaceae, Lachnospiraceae, and Prevotellaceae were identified as core members of the microbiome of hindgut fermenters. Many of these phyla participate in the metabolism of complex carbohydrates from plant polysaccharides via glycoside hydrolase, carbohydrate esterase, polysaccharide lyase, and other enzymatic digestions. A large database of these enzymes often utilized for analyses refers to them as CAZymes. The biproducts of these digestions can be the primary source of energy for many herbivores. The biproducts can also feed other microbial populations sharing the colonic space.

Insects
In addition to mammals, several insects are also hindgut fermenters, the best studied of which are the termites, which are characterised by an enlarged "paunch" of the hindgut that also houses the bulk of the gut microbiota. Digestion of wood particles in lower termites is accomplished inside the phagosomes of gut flagellates, but in the flagellate-free higher termites, this appears to be accomplished by fibre-associated bacteria.