Introduction

The method of digestion which an animal uses depends on its diet i.e. carnivore, herbivore or omnivore. For example not all mammals are carnivores (e.g. dogs and cats), herbivorous mammals include rabbits, chinchillas, guinea pigs etc.

Generally, carnivores consume animal tissue which is similar to their own; therefore all the body needs to do is break down the tissue and absorb the different components which can then be used in the carnivores own body. Omnivores have very similar digestive systems to carnivores with the addition of a caecum.

Herbivores consume plant matter which is more difficult to break down than tissue. Therefore herbivores have evolved fermentation systems which contain specific microflora, the microflora breakdown the plant material releasing useful nutrients which the herbivore utilises.

Digestion by Diet:

Carnivores

Because meat is easily digested, the gastric system of carnivores is typically short and simple. They are monogastric meaning they have only one stomach (unlike a ruminants’ stomach which has four chambers). Due to the ease at which components required for growth are obtained from food, some carnivores have lost the ability to synthesis them (e.g. cats are unable to synthesis taurine).

The teeth of carnivores are sharp and strong, this makes it easy to rip and tear meat from bones of prey. When possible, the meat is broken down further by the teeth to ensure maximum surface area for digestion by enzymes in the stomach and small intestines. True carnivores do not have digestive enzymes in their saliva.

Due to the lack of salivary enzymes, food spends little time in the mouth of a carnivore, it is shortly swallowed and travels down the oesophagus. The oesophagus is a tube which runs from the pharynx (back of the oral cavity) to the stomach. The walls of the oesophagus are protected from damage by food by stratified squamous epithelium arranged in longitudinal folds, this also allows for expansion as the food travels down to the stomach. Food is passed down the oesophagus by peristalsis which is the contraction and relaxation of longitudinal and circular muscles, pushing food down to the stomach in wave like motion.

The next stop is the stomach, the stomach has multiple roles in digestion, including:

  • A reservoir for food
  • A sterilising chamber, due to the low pH (high acid content – HCL)
  • A churning chamber to mix food with digestive gastric juices a
  • The initial site of protein digestion, primarily by pepsin – secreted by the epithelial lining of the stomach

Food is moved to the next site of digestion, the small intestine, by peristalsis. The small intestine is a long and narrow ‘tube’ with a structure and epithelium that maximises surface area. This is important because the small intestine is the primary site of digestion by enzymes. Food continues to travel along the small intestine by peristalsis. The small intestine can be divided into the duodenum, jejunum and the ileum. The pancreatic duct connects the pancreas to the duodenum – the majority of the digestive enzymes enter the small intestine by this duct. To aid in lipid digestion, bile is secreted by the liver (stored in the gallbladder). Bile emulsifies lipids which gives them a larger surface area, increasing enzyme efficiency.

The small intestine joins to the large intestine, which consists of the caecum, colon and rectum. In carnivores the caecum has no function (as it is used in herbivores/omnivores as a site of bacterial fermentation of plant matter). The colon absorbs minimal nutrients from the ingested food; instead its primary role is the reabsorption of water, vitamins and electrolytes from the mixture of food, saliva and gastric & pancreatic juices passing through. This prevents excessive water loss and therefore dehydration. The remnants are excreted via the rectum and anal sphincters.

Herbivores

Herbivores only consume plant material which is very difficult to digest. No vertebrates make an enzyme capable of breaking down cellulose, the tough sugar that makes up plant cell walls which is unfortunate as its digestion yields glucose. As the diet includes large amounts of fibre the digestive tract of herbivores is comparatively much longer than carnivores, due to fibre being much more difficult to digest.

To overcome this herbivores have developed a symbiotic relationship with a population of microflora that inhabit a specialised region of the gut for fermentation e.g. the caecum or rumen of ruminants. The microflora population of the gut is able to breakdown cellulose and use the glucose for its own metabolic needs. As a waste product of this process, the microflora population releases volatile fatty acids (e.g. acetate, butyrate & propionate) which the herbivore utilises for energy. The production of these fatty acids is known as fermentation (fermentation also produces heat which keeps the animal warm).

There are two types of fermenting herbivores, those which ferment in the foregut and those which ferment in the hindgut. The difference between them is the site of fermentation and the organ used for fermentation; the attributes of the fermentation chamber remain the same however – Anaerobic, plenty of fluid, regulated pH, steady nitrogen supply and the correct temperature.

  • Foregut Fermentation – The majority of foregut fermenters are ruminants (including cow, sheep, goat, ox and deer) who ferment their food before it reaches the ‘true’ stomach. The stomach of a ruminant exists as four chambers which are the rumen, reticulum, omasum and abomasum (true stomach). Non-ruminant foregut fermenters (e.g. camels, llamas and whales) do not have the four distinct chambers; instead they simply have modifications to the gut before the true stomach which allows them to ferment. Ruminants digest food more efficiently than hindgut fermenters as they are able to consume food into the rumen – the site of fermentation, allow microbial digestion and then regurgitate the ‘cud’ and chew it some more. This means by the time the ingested food reaches the abomasum, all the extractable nutrients have been metabolised (some microflora from the rumen may also be digested in the abomasum which increases nutrient intake).
  • Hindgut Fermentation – Hindgut fermenters (e.g. e.g. elephant, horse, guinea pig, rabbit, herbivorous reptiles, e.g. tortoise and herbivorous birds) have a digestive system very similar to carnivores, except due to the large amounts of fibre and other difficult-to-digest components of the diet, the complete digestive tract is much longer. Hindgut fermenters also have a working, enlarged caecum which is the site of bacterial fermentation. The process of fermentation is the same as that of foregut fermenters, however as the caecum is located after the stomach and small intestine, the majority of food reaches the caecum undigested. Bacterial fermentation occurs in the caecum and colon allowing some volatile fatty acids to be absorbed, but then the digested food is excreted (along with the microflora). This is why some hindgut fermenters are seen eating their faeces – the food making up the faeces has been digested by the microflora making it of nutritional value. The ingestion of the faeces allows the restoration of the microflora population.

The foregut fermenter herbivores are a lot more efficient as the food is digested on the first pass through the digestive system. Unfortunately for hindgut fermenters digestion is more difficult; however they do have the ability to expel their microflora population which is useful during times such as hibernation.

Omnivores

Omnivores consume both meat and plant matter; they have a digestive system very similar to carnivores but also have a working caecum (not as well adapted as in herbivores). Due to this flexibility they are able to consume a wide diet, which has also prevented them losing the ability to synthesise certain products in the body (as in carnivores).

The process of digestion is extremely similar to carnivores, except a few minor adaptations which allow them to digested plant matter – although not as efficiently as herbivores.

Digestion by Species:

Many species have digestive systems very similar to those shown above; however there may be slight tweaks to the systems between the species, below are some examples.

Birds

Birds do not have teeth and so cannot chew; they are able to break up food however by using their beak. Only some species of bird (e.g. sparrow) are able to produce saliva with the amylase enzyme (for digestion of carbohydrates prior to the stomach).

When a bird swallows food, it passes down the oesophagus into a structure called the crop. The crop is primarily a storage area for food consumed by the bird, differing in size between species however certain adaptions in some species allow it to produce ‘crop milk’ which is rich in protein and fat and fed to the young during their first few days of life. Another adaptation found in one species so far, is that the crop acts as a foregut fermentation chamber.

The stomach of a bird exists as two parts, the proventriculus and the gizzard (ventriculus). As the bird is unable to chew, powerful muscles in the gizzard allow it to grind food up, the presence of grit in the gizzard aids this process.

From this point, the digestive system is similar to other species – small intestine, large intestine, etc.  until it reaches the end of the tract. At the end of the large intestine, the digestive tract opens into the cloaca which is simply a common exit, shared with the urinary and reproductive tracts. Shortly before the cloaca there is a pair of caecum or caeca, unlike the single caecum found in other species. (The caeca are rudimentary or absent in species such as hawks and parrots).

Rabbits

Rabbits are herbivorous hindgut fermenters able to rapidly pass food through their digestive system and quickly eliminate fibre. Due to this ability, he rabbit has remained small and agile, able to quickly escape predators.

Rabbits have a very typical herbivorous digestive system, the only differences are they are unable to vomit and have a large caecum.

One variation, which rabbits are well known for is that they consume their own faeces – coprophagia. This is due to them being hindgut fermenters and losing a fair amount of nutrients and vitamins in the faeces. The rabbit produces two forms of faeces:

  1. Hard, fibrous –No nutritional value
  2. Soft, caecotrophs – High protein content as well as vitamins B & K and volatile fatty acids. The caecotrophs are not digested/damaged in the stomach acid as a layer of mucus surrounds them and protects them, this allows all the nutrients and vitamins to be absorbed in the small intestine

Myomorphs (Rodents)

Myomorphs are omnivorous and have a very typical digestive system. Due to a structure between the oesophagus and cardiac region of the stomach however, it is almost impossible for them to regurgitate food.

Most of the rodents in this group lack a caecum or similar specific organ involved in the fermentation of cellulose, hamster however do have a foregut – similar to ruminants, which has a high pH and a large microflora population.

All myomorphs, like rabbits show some degree of coprophagia for the same reasons – to consume the vitamins and nutrients lost in the faecal pellets.

Sciuromorphs (Chipmunks)

Also an omnivore and has a very similar digestive system to myomorphs.

Hystricomorph (Guinea Pigs/Chinchilla)

The guinea pig is an herbivorous hindgut fermenter, which a large caecum for its body size – containing up to 65% of the total contents of the digestive tract at one time. As will myomorphs, guinea pigs exhibit coprophagia.

Chinchillas are very similar to guinea pigs, they have evolved however to survive on the nutritionally poor yet highly fibrous grasses of the Andes, this means over indulgence of highly nutritious treats (captive kept chinchillas) can cause fatal constipation or diarrhoea.

Chelonians (Turtles, Tortoises & Terrapins)

Chelonians lack teeth so have to use their horny, beak like structure to cut up food. The small intestine of chelonians is relatively short when compared to mammals. Like birds, reptiles have a common exit from the body – the cloaca (urinary, digestive and reproductive systems exit from here).

Snakes

Snakes are carnivorous and possess many teeth which are regularly replaced. Due to the shape of the snake (elongated) the digestive system remains the same, but all the organs are also elongated appropriately.

Lizards

The diets of lizards vary greatly, so the digestive system adapts accordingly, from herbivorous to insectivorous. Variations occur in the efficiency of the caecum (herbivorous/omnivorous lizards).

Fish

Predatory, carnivorous fish have ‘throat teeth’ located just before their oesophagus used for catching and holding prey. Structure of the digestive system are more tube shaped than in other species and can vary in length greatly, depending on the diet of the fish.

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