Structure And Function Of The Gastrointestinal Tract

Because herbivores are classified by their diet, it is pertinent to explore the specialities of their physiological machinery to prehend and digest food required to sustain life. Specifics of diet and habit interact with location of eyes, length of neck and legs, and structure of the mouth. For example, the lemur has binocular vision, a prehensile tail, and dexterous hands; the beaver is renowned for its teeth and swimming ability; the elephant's trunk and the manatee's lips are magnificent adaptations to food supply; and the sightless mole rat has incisors that function as excavators, and stubby legs to propel loosened soil to the rear of its subterranean habitat.

The wide variety of habitats and food preferences is associated with a myriad of subtle distinctions among herbivores of the shape of the teeth, tongue, and jaw, number and type of molars, presence or absence of functional canines, and the eruption of teeth during life.[1] Jaw structure, articulation with the skull, and musculature provide the fulcrum for cutting, crushing, and grinding. For example, horses have side-to-side jaw movement similar to that of ruminants to facilitate the diminution of grasses,[2] and elephants and kangaroos have front-to-back lower jaw movement to grind highly structured plant cell walls. The structure and composition of teeth with specific functions allow them to break seed coats, rupture highly lignified plant cell walls, or comminute leaves or other physical alteration of diet that predicate digestion. The hypsodont molar teeth of some herbivores are designed to be ground away, yet retain their abrasive surface.[2] The loxodont molars of elephants have pronounced enamel ridges that enhance grinding capacity.[3] Elephants slough and replace these molars several times. The incisors of rodents grow throughout life to replace teeth worn away by gnawing.[3] The diastema between incisors and molars allows space for the jaw and soft tissues of the mouth of grazing herbivores to prehend long pieces of plant material (such as leaves and stems), protrude them to the side, then shorten and ingest them at a deliberate pace. The same diastema allows other herbivores to gnaw through unwanted material (such as soil, bark, and shell) and expel the unwanted material to the side.[1,2]

Digestion and absorption of nutrients can be divided into two strategies or two main structures for the gastrointestinal tract. The strategies are: 1) rapid transit of chyme with high (fruit bat) or low (panda) extraction of nutrients; or 2) slower transit of chyme for increased extraction of nutrients (horse). The two main structures center on the: 1) pregastric (colobus); or 2) postgastric site (rabbit) of simbiotic, microbial fermentation. Within these general divisions are a myriad of subtle adaptations. Most nonruminant herbivores belong in the slower transit, postgastric fermentation category, finding survival and the opportunity to reproduce in a habitat that provides highly structured food material that benefits from microbial fermentation in the gut. In general, fluids have lower retention time than particulates (therefore faster passage rates) in the gut, but some postgastric fermenters have developed methods to selectively retain fluids.[4] The most active method to achieve selective retention is antiper-istalsis in the lower gut.

Table 1 Diets of nonruminant and noncamelid herbivores from various orders of mammals

Order

Common names of herbivorous animals and principal diet

Carnivora

Chiroptera Marsupialia

Edentata

(Xenarthra)

Rodentia

Primates

Artiodactyla Dermoptera

Lagomorpha Perissodactyla

Proboscidea Sirenia

Giant panda—bamboo

Fruit-eating bat Petaurids

(gliders, ring-tailed possums) and koalas—eucalyptus leaves, blossoms, shoots.

Ring-tailed possum is coprophagous

Three-toed sloth—tree leaves

Beaver—bark, leaves, coprophagy

Hamster, lemming, vole—seeds, shoots, roots, fruit

Porcupine—bulbs, roots, berries, tree bark, thistles, leaves Lemur—leaves, flowers, coprophagy Hippopotamus

Flying lemur (colugo)—leaves, buds, flowers, fruits Pika—leaves and stems of forbs and shrubs, grass leaves and seeds, coprophagy Horse, zebra, ass—grass

Elephant—leaves, roots, bark, fruit Dugong—bottom-growing, shallow vegetation, coprophagy

Lesser (red) panda—berries, blossoms, leaves Nectar-feeding bat Kangaroo and wallaby—grasses, shrubs

Wombat—grasses, roots, bark

Two-toed sloth, ground sloth—leaves, fruits, buds

Squirrel—leaves, seeds, nuts Capybara

Cane rat

Colobus monkey—leaves, buds, flowers, fruits

Rabbit—grass, leaves, coprophagy

Rhinoceros—grass, stems, leaves of woody vegetation

Hyrax—leaves, grasses, bark

Manatee—aquatic and marine vegetation, algae, coprophagy

Gopher—roots, tubers, grass leaves

Chinchilla

Langur—leaves, fruits, flowers

Kangaroo rat—seeds, vegetable material Gerbil, sand rat—seeds

Guinea pig Gibbon—ripe fruit

Spring hare—bulbs, corms, green shoots Mole rat—roots, tubers, rhizomes, bulbs, coprophagy Flying squirrel

Tapir—tree leaves, aquatic vegetation, fruit, grass

Table 2 Nonruminant and noncamelid herbivores from various orders of mammals

Order Common names of herbivorous animals

Order Common names of herbivorous animals

Table 2 Nonruminant and noncamelid herbivores from various orders of mammals

Carnivora

Giant panda

Red panda

Chiroptera

Fruit eating bat

Nectar feeding bat

Marsupialia

Ring tailed possum

Kangaroo

Wallaby

Wombat

Koala

Edentata (Xenarthra)

Three toed sloth

Two toed sloth

Ground sloth

Rodentia

Beaver

Squirrel

Gopher

Kangaroo rat

Spring hare

Hamster

Capybara

Lemming

Gerbil

Mole rat

Porcupine

Cane rat

Chinchilla

Guinea pig

Flying squirrel

Primates

Lemur Gibbon

Colobus monkey

Langur monkey

Wooly monkey

Orangutan

Artiodactyla

Hippopotamus

Dermoptera

Flying lemur

Lagomorpha

Pika

Rabbit

Perissodactyla

Horse

Zebra

Tapir

Rhinoceros

Proboscidea

Elephant

Hyrax

Sirenia

Dugong

Manatee

In addition to major components of the gastrointestinal tract, herbivorous mammals have in common with other mammals the usual complement of digestive enzymes, tissue structures, and nutrient transport mechanisms.[4] The stomach of herbivores has four types of gastric epithelium: stratified squamous, cardiac gland, proper gastric, and pyloric gland.[4] With the exception of the fruit bat, the stomach of herbivorous mammals is large enough and compartmentalized enough to accommodate microbial fermentation. Nonruminant herbivores with substantial pregastric fermentation include the Colobus monkey, Langur monkey, brush-tailed porcupine, hamster, three-toed sloth, and the kangaroo. Most herbivores also have sites for postgastric fermentation.[4] The site of postgastric fermentation is caudal to the small intestine and may include a large cecum (rabbit) or large proximal colon (horse, elephant). Other nonruminant herbivores with substantial postgastric fermentation include the wombat, koala, opposum, capybara, night monkey, rhinoceros, dugong, and rock hyrax.[4] The hippopotamus is unusual because it has a very complex stomach, yet a simple hindgut without a cecum and a short colon. The panda is also unusual because it has neither pregastric or postgastric fermentation, but relies on rapid transit of bamboo selected for its high-nutrient content.[4]

Fermentation in the gut is accomplished predominantly by anaerobic bacteria. Bacteria and other microbes produce short-chain fatty acids (acetate, propionate, and butyrate); these acids are absorbed and provide 8 44% of the energy required for maintenance of nonruminant, noncamelid herbivores.[4] In the case of pregastric fermenting herbivores, the animal may also gain significant nutrient supply of protein and lipids upon digestion of the microbial biomass carried with chyme

(see articles on ruminants and camelids elsewhere in this encyclopedia).

Coprophagy (eating one's own feces) is a common behavior among herbivores (Table 1). Coprophagy provides these animals with important nutrients (vitamins, protein) that are produced by the simbiotic microbes in their gut. A special subgroup of coprophagic herbivores are those that produce cecotrophs, special fecal pellets formed in the cecum that have high nitrogen content (rabbits, hares, chinchillas, guinea pigs, ringtail possums, and several rodent species). Cecotrophs are excreted once or twice during a 24-hour period, and are the fecal components preferentially ingested.[4]

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