Adaptations in the digestive system

All carnivores, when fed a whole prey-based diet, consume proteins and fats from the muscle, vitamins from organs and gut contents, minerals from bones, and roughage from the hide, feathers, hooves, teeth, and gut contents. Felids are set apart from other, more omnivorous meat eaters because of their inability to effectively utilize carbohydrates as an energy source. They therefore depend on a higher concentration of fats and protein in their diet, as well as dietary sources of preformed vitamin A and D, arachadonic acid (an essential fatty acid), and taurine.

Herbivores, on the other hand, have adapted numerous methods of utilizing roughage-based diets. Plankton feeders such as the baleen whales have a filtering apparatus that consists of a series of horny plates attached to the upper jaw and then left hanging from both sides. As the whale makes its course through the ocean, water flows over and between

Digestive System Mammal

the plates, and plankton is caught in the plates' hair-like edges.

Ruminants and some non-ruminant herbivores (e.g., sloths, hippos, colobines, large marsupials) utilize pre-gastric microbial fermentation to break down cell wall constituents, while the odd-toed hoofed animals, or perissodactyls, rely primarily on post-gastric fermentation.

Some small herbivores, like rodents and rabbits, have relatively higher nutrient requirements compared with larger herbivores. In order to meet these requirements, they must routinely practice coprophagy to obtain the protein, water, enzymes, vitamins, and minerals provided by the microbes. Coprophagy, which comes from the Greek copros, meaning "excrement," and phagein, meaning "to eat," is of great nutritional importance. If coprophagy is prevented in rabbits, their ability to digest food decreases, as does their ability to utilize protein and retain nitrogen. This process is reversible, however. When coprophagy is allowed again, the rabbits' ability to digest cellulose is restored.

The soft feces that a rabbit re-ingests originate in the cecum, or "blind gut," a large blind pouch forming the beginning of the large intestine. Upon ingestion, these feces are not masticated and mixed with other food in the stomach. In stead, they tend to lodge separately in the base of the stomach. A membrane coats the soft feces, and they continue to ferment in the stomach for many hours. One of the fermentation products is lactic acid.

For most herbivores, the gastrointestinal microbial population is an integral component of the feeding strategies, especially since most of them live on food that make cellulose digestion essential. Some of the most important domestic meat- and milk-producers (cattle, sheep, goats) have specialized tracts that are highly adapted to symbiotic cellulose digestion; they are known as ruminants.

The stomach of a ruminant consists of several compartments, or in more precise terms, the true digestive stomach, the abomasum, is preceded by several large compartments. The abomasum corresponds to the digestive stomach of other mammals. The first and largest compartment of this system is the rumen, which serves as the main fermentation center in which the food, after it has been mixed with saliva, undergoes heavy fermentation.

Both bacteria and protozoans reside in the rumen in large numbers. These microorganisms work to break down cellulose and make it available for further digestion. The fermentation products (mostly acetic, propionic, and butyric acids)

Fermentation Fat Rumen
A camel's (Camelus dromedarius) hump is primarily fat, which is metabolized when food is scarce. (Photo by © Dave G. Houser/Corbis. Reproduced by permission.)

are absorbed and utilized, while gases (carbon dioxide and methane) formed in the fermentation process are released through belching. A cow fed 11 lb (5 kg) of hay a day can give off 1 qt (191 l) of methane each day.

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Gaining Weight 101

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