In details

Compared digestion


Mammals need a nutritious and abundant diet because the metabolic costs of maintaining temperature are very high, but according to the diet, the digestive tract may have specific adaptations.

We carnivores The digestive tract is simple because the proteins, lipids and minerals found in meat do not require specialized digestion. We omnivores The stomach is a sac of muscular walls with glands producing hydrochloric acid and enzymes. The stomach wall is not destroyed by these fluids due to the protection of mucin, another gastric secretion. Plants contain complex carbohydrates such as cellulose. So in herbivores The intestine is proportionally larger because vegetables are less nutritious and difficult to digest. Since no vertebrate produces enzymes capable of hydrolyzing this polysaccharide, many herbivores harbor bacteria in several compartments, notably in the cecum or in the stomach itself, which in this case is sub-compartmentalized:

  • rumen - where bacteria capable of fermenting cellulose are located, which reproduce at a rate sufficiently high to compensate for those "lost" with the displacement of the bolus. The contents of this compartment (bacteria and plant material) are regularly regurgitated in the mouth when the animal, in a safe place, chews the ingested food hastily;
  • reticle - Also rich in fermentative bacteria, it receives the food bolus after being stuffed in the mouth, allowing a greater area of ​​attack on bacterial cellulases;
  • omaso - food paste containing a large quantity of fermentative bacteria is "concentrated" due to water reabsorption;
  • abomasum - stomach compartment in other mammals, secretes acids and proteases that complete digestion in the traditional way.

Thus, these animals ingest larger amounts of food, which remain long in the digestive tract. This digestive method is very effective for a low protein diet because the bacteria themselves are also digested making them a protein source for the ruminant (a cow can get about 100 g of protein per day from digesting its endosymbiotic bacteria) .

Some non-ruminant herbivores, such as rabbits and hares, also contain their own fermentative flora, usually in specialized diverticula - cecum. However, as the caecum opens in the large intestine, the absorption of nutrients digested by microorganisms is ineffective and incomplete. To compensate for this, many of these animals ingest their feces - coprophagia. There are usually two types of stool in these cases, one composed exclusively of debris and one that is ingested directly from the anus, which is made up of cecal material, which will then pass through the stomach and small intestine, and its nutrients will be absorbed.

Carbon dioxide and methane are by-products of the fermentative metabolism of these bacteria, and a typical ruminant (a cow, for example) can produce up to 400 liters of methane per day. This fact makes domestic livestock the second most important cause of the greenhouse effect on Earth (right after industry).

The size of the animal is decisive in the type of diet, and thus in the type of digestive system that will present. In small mammals the area / volume ratio is high, meaning they lose a lot of heat to the environment. Thus, they must have high caloric needs and high metabolism. Since they cannot tolerate slow digestion such as herbivores, mammals weighing less than 500 g are almost all insectivores.

In contrast, larger mammals generate more heat and lose less heat, tolerating a longer food collection process (carnivores attacking large prey) or slow digestion (herbivores).

In addition, animals over 500g could not collect enough insects during the day. The only exception is mammals that feed on large amounts of colonial insects (ants or termites).

The human digestive tract can be considered typical of the mammalian class. Food introduced into the mouth progresses in the tube through involuntary peristaltic movements. Although digestion begins in the mouth, it is in the stomach and small intestine that it is processed, with the intervention of a variety of enzymes. These are produced by gastric and intestinal glands, as well as attached organs such as salivary, pancreas and liver glands (the bile does not, however, have enzymes). Absorption is facilitated by the presence in the small intestine of folds covered with glove-finger-shaped intestinal villi whose epithelial cells still have microvilli. This whole set greatly increases the contact area between the food and the wall, facilitating absorption by diffusion or active transport.

Animal Habitat Digestive tube Compartments Annex Bodies Digestion Type
Plainly Fresh water Incomplete Gastrovascular Cavity - Intra and extracellular
Annelids Aquatic or terrestrial Complete Pharynx, esophagus, crop, gizzard, intestine with tiflosole - Extracellular
Insects Terrestrial Complete Pharynx, esophagus, crop, stomach, intestine and rectum Salivary glands, gastric caeca Extracellular
Cartilaginous fish Salty water Complete Pharynx, esophagus, stomach, small intestine and large intestine Salivary glands, liver and pancreas Extracellular
Bone fish Aquatic Complete Pharynx, esophagus, stomach, small intestine and large intestine Liver and pancreas Extracellular
Amphibians Fresh and terrestrial water Complete Pharynx, esophagus, stomach, small intestine and large intestine Liver and pancreas Extracellular
Reptiles Terrestrial Complete Pharynx, esophagus, stomach, small intestine and large intestine Salivary glands, liver and pancreas Extracellular
Birds Terrestrial Complete Pharynx, esophagus, crop, proventriculum, gizzard, small intestine and large intestine Salivary glands, intestinal caeca, liver and pancreas Extracellular
Mammals Aquatic or terrestrial Complete Pharynx, esophagus, stomach, small intestine and large intestine Salivary glands, intestinal caeca, liver and pancreas Extracellular