Ruminant Animals and Rumination

Definition of Ruminant Animals and Rumination

cattle grazing - a cow is an example of a ruminant animal

Ruminant animals are herbivores that have 4 stomach compartments in their digestive system and are mammals. They are also known as “Animals that chew the cud“.  The term ruminant originated from the Latin word ‘ruminare‘ which means to chew over again. There are numerous species of ruminating animals; some examples are listed below.

A list showing some examples of Ruminating Animals

  1. Cows
  2. Goats
  3. Giraffes
  4. Sheep
  5. Deer
  6. Gazelles
  7. Antelopes
  8. Elks
  9. Buffalos

What are Ruminants?

A ruminant animal is any mammal that has four compartments in its stomach. Ruminants are herbivorous in nature, which means that they eat forage (which are grasses or hay) as food and have the capability to digest the forage compared to other herbivores that have a single stomach compartment (monogastric herbivores).

Characteristics of Ruminants

  • Ruminants have 4 stomach compartments
  • They are herbivores, which means they eat grasses or plants as food.
  • Ruminants are mammals, which are animals that breastfeed their young ones.
  • Ruminating animals have the ability to regurgitate food from the stomach to the mouth for further chewing ( a process known as rumination), and from which they got their names as ruminants – which are also known as ‘animals that chew the cud’.
  • Ruminants can break down cellulose, a complex carbohydrate found in plants that cannot be broken down by non-ruminants.

Classification and Taxonomy of Ruminant Animals

Ruminating animals can be classified into various types based on different criteria.

Taxonomic classification

When classified based on taxonomy, ruminants are under the order: Ruminantia and can be found in 6 different families: Tragulidae, Giraffidae, Antilocapridae, Moschidae, Cervidae, and Bovidae.

Classification of ruminants based on diet (Type of Feeding)

Hofmann and Stewart classified ruminants based on their feeding habits and adaption of their digestive system as:

  • Concentrate selectors
  • Grass eaters
  • Intermediate

The size of their digestive systems differs and causes different adaptions to feeding.

Concentrate selectors

These are ruminant animals that select plants that have concentrated amounts of starch, fats, or protein; such plants must be easy to digest because concentrate selectors have less ability to digest cellulose compared to other ruminants because of their small reticulorumen when compared with their body size. This group of ruminants select the types of trees or plants to eat; examples include deer and giraffes. Deer selects leguminous plants such as peanuts and beans.

Grass Eaters

The grass eaters are also called roughage eaters or bulk eaters. Examples include cows and sheep. They have a lengthy digestive system and a large reticulorumen compared to their body size and love eating fresh grasses and fibrous plants than legumes. Roughage eaters have a better ability to digest cellulose than other ruminants.

Intermediate Eaters

This type of ruminants prefer flowering herbs (forbs, phorbs) such as sunflowers, milkweed, seep monkeyflowers, clovers, and daylilies, and milkweed. Examples of intermediate eaters are goats. They have better digestion of cellulose than concentrate selectors but not as high as that of grass eaters.

Digestive System of Ruminants

The digestive system of ruminant animals comprises various organs with their functions; these organs when listed in order include the mouth, esophagus, stomach, intestines, and large intestine. There are other accessory organs that aid in the digestion of food in ruminants such as the pancreas and gall bladder. You may think the digestive system of ruminants is similar to that of other herbivores or mammals but they are not; ruminant animals differ from non-ruminants in that, the stomach of ruminants has four compartments while that of non-ruminants have a single compartment.

Diagram of the digestive system of a cow as an example of a ruminant animal
Diagram of the digestive system of a cow as an example of a ruminant animal, showing the 4 compartments of a cow’s stomach.

Mouth

When ruminants eat forages during grazing, they do so rapidly without actually chewing into small sizes. During grazing, the ruminant animal only grazes but does not thoroughly chew the forage which is mixed with the saliva and then swallowed through the esophagus.

Esophagus

The esophagus of ruminants is capable of moving food in both directions unlike that of non-ruminants which is only unidirectional. By allowing food to move in both directions, the esophagus of ruminants helps in the regurgitation of food for further chewing and digestion when the animal is resting.

Rumination in animals (chewing the cud)

The process of food regurgitation in ruminant animals is commonly termed as rumination or chewing the cud. This process of rumination is necessary for aiding the digestion of the forage. The harvested forage is first taken and temporarily stored in the first compartment of the stomach prior to regurgitation. When regurgitated for further chewing, the bolus of food is now termed as the cud. The cud when thoroughly chewed is then swallowed back into the stomach.

Stomach

The stomach of ruminant animals such as cows is divided into 4 compartments namely: the rumen, reticulum, omasum, and abomasum. The rumen is the first compartment of the stomach and it is linked to the reticulum, which is the second compartment of the stomach. There is a synergy between the rumen and reticulum because they are connected and for this reason, they are often referred to as the reticulorumen.

The third compartment of the ruminant stomach is called the Omasum while the fourth compartment is known as the Abomasum.

When the cud is swallowed back into the ruminant stomach, it is separated into solid and liquid portions.  Most of the liquid portions are then rapidly transported to the omasum and abomasum for absorption while the solid part of the cud slowly goes to the rumen, from where it is then fermented by the action of some microbes to release energy precursors. Some nutrients of the chewed cud are absorbed in the rumen after fermentation while the remaining cud is then transported to the small intestine.

The 4 compartments of the Ruminant Stomach

Rumen

The rumen is the primary area of microbial fermentation. It is also known also as the paunch. It is the largest stomach compartment in ruminants. This is also the larger part of the reticulorumen which is the first chamber in the alimentary canal of ruminant animals and it is located at the base of the esophagus.

Fermentation in the rumen

Ruminant animals eat plants that have a complex carbohydrate known as cellulose; for the ruminant to digest cellulose, it needs to ferment the ingested forage. This fermentation occurs in the rumen with the aid of some microorganisms present in the rumen. These microbes are numerous and consist of different species of bacteria, protozoa, and fungi. In fact, 1 ml of rumen fluid is estimated to contain about 10 to 50 billion bacteria, 1 million protozoa, and several fungi.

Since the environment inside a rumen is anaerobic, most of these microbial species are obligate or facultative anaerobes that can decompose complex plant materials such as cellulose, hemicellulose, starch, and proteins.

The hydrolysis of cellulose by these microorganisms results in sugars that are further fermented to acetate, lactate, propionate, butyrate, carbon dioxide, and methane.

Reticulum

This is the second chamber in the alimentary canal of a ruminant animal; it is also referred to as the honeycomb due to the honeycomb appearance of its lining. Anatomically, the reticulum is the smaller portion of the reticulorumen. The reticulum and the rumen make up 84% of the total volume of the ruminant stomach.

Omasum

This is also named the bible, the fardel, the manyplies, or psalterium. The omasum is the third compartment of the stomach of ruminants that receives chewed cud and absorbs some fatty acids. The omasum also receives food from the reticulum through the reticulo-omasal orifice and provides food to the abomasum through the omaso-abomasal orifice.

Abomasum

The abomasum of ruminants (also called the maw, rennet bag, or reed tripe) is the true stomach in ruminants (because it has a similar function as the non-ruminant stomach); It is the fourth and final stomach compartment in ruminants. The abomasum of young ruminants is a source of an enzyme known as rennet which is used as an animal by-product for the production of cheese. It is secretory in nature and is the equivalent of the monogastric stomach. Its primary function is in the hydrolysis of microbial and dietary protein using hydrochloric acid in order to prepare these protein sources for further digestion and absorption in the small intestine. As bacteria enable fermentation of the cud in the rumen, they consume about 10% of the carbon, 60% of the phosphorus, and 80% of the nitrogen that the ruminant ingests. In order to reclaim these nutrients, the ruminant then digests the bacteria with the help of the enzyme known as lysozyme in the abomasum.

Small Intestine

The small intestine of ruminants is the organ responsible for most absorption; it is lined with finger-like projections known as the villi that increase the surface area for absorption of nutrients.

In the intestine, the partially digested and fermented cud is now referred to as the digesta; while in the small intestine, the digesta is acted upon by the secretions of the accessory organs (pancreas and the liver); these secretions raise the pH of the digesta from acidic value of 2.5 to a neutral or slightly alkaline pH of 7 to 8. This pH change is necessary to activate the enzymes of digestion in the small intestine. Pancreatic ribonuclease helps to degrade bacterial RNA in the small intestine to provide release nitrogen.

The gall bladder is also triggered by the presence of fats in the digesta to release bile which is needed for further digestion of any fats or lipids. Most of the digested nutrients are absorbed in the small intestine while the remaining materials are moved to the large intestine.

Large Intestine

In the large intestine, the main function is the absorption of water and the formation of fecal matter which are then excreted through the rectum.

Differences between Ruminant and Non-ruminant Animals

  1. Ruminants graze food rapidly in the mouth without thorough chewing before swallowing whereas non-ruminants need to chew food thoroughly before swallowing.
  2. The esophagus of non-ruminants is unidirectional, which means it only transports food in one direction – that is, from the mouth to the stomach; whereas ruminants’ esophagus is bidirectional which means that they can move food from the mouth to the stomach and also from the stomach back to the mouth (a process known as rumination).
  3. Ruminants have 4 stomach compartments whereas non-ruminants have a single stomach compartment.
  4. The upper incisors of ruminants are reduced or sometimes absent
  5. Fermentation of cellulose in ruminating animals always occurs in the foregut while cellulose fermentation in non ruminants such as monogastric herbivores with the ability to ferment cellulose occurs in the hindgut. Some pseudoruminants such as the hippopotamus also ferment cellulose in the foregut but are not true ruminants because they have 3 chambers in the stomach.
  6. Ruminants can process cellulose (which is a complex carbohydrate) with the aid of microbes found in the reticulorumen; non ruminants cannot process cellulose due to the absence of the microbes needed for its digestion.

Pseudo-ruminant Animals

Some mammals are actually pseudo-ruminants as they have a 3 compartment stomach instead of 4 like true ruminants do. Examples include chevrotains (mouse deer), hippopotamus, rabbits, hamsters, guinea pigs, and camels. Pseudoruminants are capable of fermenting cellulose just as ruminants do; the fermentation also takes place in the foregut just as that of ruminants but not all pseudo ruminants chew the cud.

Monogastric Herbivores (Hindgut fermenters) vs Ruminants

Some monogastric herbivores such as rabbits, hares, rhinos (rhinoceros), pikas, and horses have a single stomach chamber but can ferment and digest cellulose in their enlarged cecum, the same way as ruminants do in their rumen. This set of monogastric herbivores ferment cellulose in their hindgut and not foregut as the ruminants do.

Economic Importance of Keeping Ruminant Animals

The products and by-products of ruminant animals have tremendous nutritional and economical value to man.

Some of the main ones are outlined below:

  • Meat and milk of cattle, sheep, goats, and other ruminants are good sources of animal protein. They are better sources of some amino acids than plant-based protein.
  • They serve as sources of raw material used in industries e.g. leather goods; goat hair used to make carpets, bags, and rope; wool is a raw material for making clothes for human use.
  • Some countries export dairy products such as milk and milk products, as well as meat, to earn foreign exchange.
  • They serve as a source of income for subsistence farmers. In Nigeria, cattle, sheep, and goats are kept at subsistence level by local farmers.
  • The dung from these animals can be used as a source of organic fertilizer.
  • The skin of the Red Sokoto breed of goats in Nigeria commands a high premium at the international market because of its superior quality for making leather products such as bags, wallets, shoes, etc.
  • In the South-Western part of Nigeria and several other countries such as Jamaica, goats are relished as meat and barbecued during ceremonies.
  • Generally, under an organized production system, ruminant animals are slaughtered during festive seasons all over the world. Blood and bones obtained from this are often recycled and processed into blood meal, or bone meal. These are used as components of animal feed.

FAQ

How many stomachs does a cow have?

A cow has a single stomach with 4 compartments; these four compartments of a cow’s stomach allow it to regurgitate grazed food, an ability which makes cows and other animals with such ability to be termed as ruminants (or animals that chew the cud).

What are two characteristics of a ruminant animal?

The two main characteristics of a ruminant animal are the ability to chew the cud and they have 4 chambers in the stomach.

Is a dog or cat a ruminant animal?

Dogs and cats are not ruminant animals because they do not chew the cud. Dogs or cats are mammals because they breastfeed their young just as ruminants do but they cannot regurgitate food and chew and do not have 4 stomach compartments.

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