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Chapter 41. Animal Nutrition. Modern humans foraging at a farmer’s market. Four Main Feeding Mechanisms of Animals. SUSPENSION FEEDERS. SUBSTRATE FEEDERS. Feces. Baleen. Caterpillar. FLUID FEEDERS. BULK FEEDERS. Homeostatic regulation of cellular fuel.

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chapter 41

Chapter 41

Animal Nutrition

four main feeding mechanisms of animals
Four Main Feeding Mechanisms of Animals

SUSPENSION FEEDERS

SUBSTRATE FEEDERS

Feces

Baleen

Caterpillar

FLUID FEEDERS

BULK FEEDERS

a few of the appetite regulating hormones

Secreted by the stomach wall, ghrelin is one of the signals that triggers feelings of hunger as mealtimes approach. In dieters who lose weight, ghrelin levels increase, which may be one reason it’s so hard to stay on a diet.

Produced by adipose (fat) tissue, leptin suppresses appetite as its level increases. When body fat decreases, leptin levels fall, and appetite increases.

Ghrelin

Insulin

Leptin

The hormone PYY,secreted by the small intestine after meals, acts as an appetite suppressant that counters the appetite stimulant ghrelin.

A rise in blood sugar level after a meal stimulates the pancreas to secrete insulin. In addition to its other functions, insulin suppresses appetite by acting on the brain.

PYY

A few of the appetite-regulating hormones
variation in alimentary canals

Anus

Anus

Gastric ceca

Crop

Variation in alimentary canals

Crop

Earthworm. The digestive tract ofan earthworm includes a muscular pharynx that sucks food in through themouth. Food passes through the esophagus and is stored and moistened in the crop. The muscular gizzard, whichcontains small bits of sand and gravel, pulverizes the food. Digestion and absorption occur in the intestine, which has a dorsal fold, the typhlosole, that increases the surface area for nutrient absorption.

Gizzard

Esophagus

Intestine

Pharynx

Mouth

Typhlosole

Lumen of intestine

Foregut

Midgut

Hindgut

(b)Grasshopper. A grasshopper has several digestive chambers grouped into three main regions: a foregut, with an esophagus and crop; a midgut; and a hindgut. Food is moistened and stored in the crop, but most digestion occurs in the midgut. Gastric ceca, pouches extending from the midgut, absorb nutrients.

Esophagus

Rectum

Mouth

Crop

Esophagus

Gizzard

(c) Bird. Many birds have three separate chambers—the crop, stomach, and gizzard—where food is pulverized and churned before passing into the intestine. A bird’s crop and gizzard function very much like those of an earthworm. In most birds, chemical digestion and absorption of nutrients occur in the intestine.

Mouth

Intestine

Stomach

Anus

the four stages of food processing

Smallmolecules

Piecesof food

Chemical digestion(enzymatic hydrolysis)

Nutrient moleculesenter body cells

Mechanicaldigestion

Undigested material

Food

ELIMINATION

ABSORPTION

DIGESTION

INGESTION

1

2

4

3

The four stages of food processing
the digestive tracts of a carnivore coyote and herbivore koala compared

Small intestine

Stomach

Small intestine

Cecum

Colon(large

intestine)

The digestive tracts of a carnivore (coyote) and herbivore (koala) compared

Carnivore

Herbivore

ruminant digestion

Rumen. When the cow first chews and

swallows a mouthful of grass, boluses

(green arrows) enter the rumen.

Intestine

1

2

3

4

Esophagus

Abomasum. The cud, containing great numbers of microorganisms,

finally passes to the abomasum for digestion by the cow‘s own

enzymes (black arrows).

Ruminant digestion

Reticulum. Some boluses also enter the reticulum. In both the rumen and the reticulum, symbiotic prokaryotes and protists (mainly ciliates) go to work on the cellulose-rich meal. As by-products of their

metabolism, the microorganisms secrete fatty acids. The cow periodically regurgitates and rechews the cud (red arrows), which further breaks down the

fibers, making them more accessible to further microbial action.

Omasum. The cow then reswallows

the cud (blue arrows), which moves to

the omasum, where water is removed.

ulcer causing bacteria

Bacteria

Mucuslayer of stomach

1 µm

Ulcer-causing bacteria
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