Ruminants are herbivorous mammals characterized by a specialized four-chambered stomach. Belonging to the order Ruminantia, they are named so because they ‘ruminate’ or chew their food over and over again till it is ready for digestion. During this process, a number of symbiotic anaerobic microbes ferment the feed in the anterior part of the stomach (foregut fermentation), thereby assisting in the easy assimilation of nutrients.
They represent the most diverse group of ungulates, including cattle, bovines, giraffes, deer, goats, sheep, and antelopes, among others. First appeared in the Early Eocene (except the Artiodactyls with cranial appendages, which appeared in the Miocene), there are around 200 species of ruminants currently divided into six families: Tragulidae, Giraffidae, Cervidae, Moschidae, Bovidae, and Antilocapridae.
The digestive tract of a ruminant includes the mouth, salivary glands, esophagus, a characteristic four-chambered stomach (that occupies about 75% of the abdominal cavity), pancreas, gall bladder, small intestine (duodenum, jejunum, and ileum), and large intestine (cecum, colon, and rectum).
The stomach comprises four compartments: rumen, reticulum, omasum, and abomasum. The first two chambers, collectively called the reticulorumen, are the primary site for food fermentation and have similar tissue layers and textures.
Rumen
Sometimes called the ‘paunch,’ the rumen, the largest of all compartments, is partitioned by muscular pillars into dorsal, ventral, caudodorsal, and caudoventral sacs. It has a high storage capacity, and thus, these animals can consume food in large amounts and chew on them later.
The rumen houses diverse microbial flora (also called rumen bugs), including bacteria, protozoans, yeasts, and even fungi. About 1 ml of ruminal fluid contains 10 to 50 billion bacteria, 1 million protozoans, and numerous yeasts and fungi.
These microbes are either obligate or facultative anaerobes because the environment within the rumen lacks oxygen.
Reticulum
The reticulum, also called the ‘honeycomb’ (because of its honeycomb-like inner lining), is connected by a short tunnel to the next part, the omasum. The rumen and the reticulum are often considered a single organ owing to their similar functions.
Omasum
The omasum is spherical and is also referred to as ‘many piles’ or the ‘butcher’s bible’ owing to the multiple folds in its inner lining, resembling the pages of a book. These folds increase the surface area for the absorption of water, nutrients, volatile fatty acids, and ammonia.
Abomasum
The abomasum is the true stomach of ruminants and is homologous to the monogastric (single-chambered) stomach in non-ruminants such as rhinoceroses, rabbits, horses, and guinea pigs. Glands in the walls of this compartment release hydrochloric acid and digestive enzymes like pepsin, which cause chemical breakdown in the food. It also receives digestive enzymes, like pancreatic lipase, secreted from the pancreas.
Next to the abomasum lie a small and a large intestine. The small intestine is tubular (extends up to 150 ft in length) and contains numerous finger-like absorptive projections called villi along its inner walls. The first section of the small intestine, the duodenum, receives bile juice from the gall bladder. The large intestine is approximately 3 ft in length and contains a long colon, which aids in absorption. It also contains a blind pouch, the caecum, which has little role in their digestion.
Since the forage is rich in silica, their teeth are ever-growing to compensate for the loss from abrasion. The upper incisors are either reduced or absent. Instead, they have a thick browsing or dental pad that helps chew hard plant parts.
The suborder Ruminantia constitutes a crown group within the order Artiodactyla, which includes all even-toed ungulates. As a crown group, it includes only the last common ancestor (LCA) of all extant ruminants and their descendants. This suborder, in turn, belongs to Ruminantiomorpha, a stem group representing all extant ruminants as well as all extinct taxa that are most closely related to them.
The terms Ruminantia and Ruminantiomorpha are often used synonymously, though they differ considerably based on which members (extinct or extant) are taken into consideration. In the context of neoteny, the term Ruminantia is used when referring to living ruminants. However, in paleontology, where extinct species are also included, the term Ruminantiomorpha is more commonly used.
Currently, around 200 species of ruminants are divided into two infraorders and six extant families: Tragulidae, Giraffidae, Cervidae, Moschidae, Bovidae, and Antilocapridae.
At least 75 million ruminants are found worldwide, inhabiting all continents except Antarctica. Of these, almost 90% are found in Eurasia and Africa; however, they are now domesticated in Australia. Over 3.5 billion ruminants are globally domesticated, with cattle, goats, and sheep accounting for around 95% of this population.
Being a diverse group, they inhabit a wide range of habitats, ranging from open plains to forests across the tropics to the Arctic.
These ungulates are exclusively herbivorous and feed on grass, hay, and leguminous plants like clover and alfalfa. They also consume high-fiber roughage, such as corn stalks and grains. When domesticated, they are fed with supplements rich in proteins and minerals.
Depending on what they eat, there are different types of ruminant feeders.
They obtain nutrients from plant-based food through fermentation. Ruminants either graze or browse on their forages and consume them with their mouth. Some, like cattle, wrap their tongue around the plants, tear the foliage, and shove them into their mouth. On average, cattle have been found taking around 25,000 to over 40,000 such bites on foliage while grazing.
The digestion occurs sequentially in the multi-chambered stomach.
Once the food is in their mouth, it is broken down with the help of the saliva produced by salivary glands. They produce large quantities of saliva (for example, 100 to 150 l of saliva is produced per day in cows) containing salivary lipase and amylase, which digest fats and starch. The saliva acts as a buffering agent by providing the fluid base for further digestion in the stomach. As the food mixes with saliva, it separates into a solid and a liquid layer. The solid part clumps together to form a mass called bolus or cud. This cud is constantly chewed, ingested, and regurgitated for further mixing with saliva, a process called rumination. This upward movement of the food is facilitated by the bidirectional movement of the esophageal muscles.
Gradually, the cud moves into the rumen, where the anaerobic microbes ferment fiber, especially cellulose and hemicellulose, into volatile fatty acids (VFAs) like acetic acid, propionic acid, and butyric acid. Additionally, protein and non-structural carbohydrates, like sugars, starches, and pectin, undergo fermentation, too. The partly digested cud (digesta) then moves to the next chamber, the omasum, and finally to the abomasum.
In the abomasum, the digesta mixes with the digestive acids and enzymes and is passed on to the small intestine. Here, most of the nutrient absorption takes place, followed by digestion of the fermenting microbes from the reticulorumen.
The digesta then enters the small intestine, where it mixes with secretions from the pancreas and liver. These secretions elevate the pH of the digesta from about 2.5 to as high as 8, making it easier for the digestive enzymes in the small intestine to function on an alkaline substrate. With time, nutrients are absorbed by the villi in the intestinal wall, and the food slowly moves to the next part of the gut through muscular contractions.
As the digesta passes into the large intestine, large quantities of water are reabsorbed in the colon, along with further fermentation by the microflora in the hindgut. Finally, nitrogenous metabolic wastes are expelled through the rectum.
As a large group, lifespans vary among the different ruminants. For instance, cattle live around 14 to 25 years in the wild, while they survive up to 30 years in protected environments. Wild goats live between 12 and 22 years, whereas sheep live for a maximum of 12 years.
Deer have an average lifespan of 3 to 6 years, though they live much longer when in captivity.
These animals contribute to the carbon cycle with the help of the microbes in their rumen. These microbes are methanogens, producing methane gas as a byproduct of cellulose fermentation in their body. This gas is then released into the atmosphere and, in a span of about 10 to 12 years, breaks down into carbon dioxide, which plants utilize for photosynthesis. However, methane, being a greenhouse gas, adds considerably to the global rise in temperature (global warming).
According to reports, enteric fermentation in ruminants accounted for about 26% of the total greenhouse gas emissions from agricultural activity in the United States, as well as about 22% of the total methane emissions in the U.S. Based on a 2013 report by the Food and Agricultural Organization of the United States, 43% of the total greenhouse gas emissions from global agricultural activities is caused by such fermentation.
Some ungulates, like hippopotamuses and camels, have three-chambered stomachs and are thus called pseudoruminants.
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