Comparative digestive physiology
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Comparative Digestive Physiology. Back in the day…. Digestion is simply breaking our food into its most basic components so that it can be utilized by the cell. (Legos) In a simple, one-celled paramecium, digestion is pretty straightforward.

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Comparative Digestive Physiology

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Comparative Digestive Physiology

Back in the day…

  • Digestion is simply breaking our food into its most basic components so that it can be utilized by the cell. (Legos)

  • In a simple, one-celled paramecium, digestion is pretty straightforward.

  • 1. A small groove on the surface of the cell pulls in water using cilia (tiny hairs)

  • 2. “food” is pushed into a bubble called a vacuole.

  • 3. As the cell pushes the food inside, enzymes are secreted by the vacuole bubble which breaks carbohydrates into simple sugars and proteins into amino acids.

  • 4. When all of the available nutrients have been absorbed, the vacuole is released.

  • Intake – food is absorbed into the “mouth”

  • Formation of vacuole (“food bubble”)

  • The vacuole is moved through the cell

  • Simple sugars, amino acids, and nutrients are absorbed from the vacuole

  • Vacuole is expelled.


  • As our bodies became more complex with trillions of cells, digestion had to do the same.

  • Early multi-celled organisms could no longer simply absorb and diffuse nutrients from their environment.

    • If you are surrounded on all sides by cells, you cannot just absorb your food.

  • This created the need for the Alimentary Canal, a long hollow tube that runs through our bodies.

  • This enabled our bodies to grow much larger than a few cells.

Regions of Alimentary Canal

  • Foregut functions

    • Ingestion and storage of feeds

  • Midgut functions

    • mechanical, chemical & enzymatic digestion of feed

    • nutrient absorption

  • Hindgut functions

    • water & ion re-absorption

    • formation, storage, excretion of feces

Primary Functions of the Digestive Tract

  • Transport food – peristaltic contractions

  • Digestion - reduce feed particles to molecules that can be absorbed into the blood

    • mechanical breakdown by chewing

    • chemical breakdown by HCl and digestive enzymes

  • Absorption - allows nutrients to pass through membranes of GIT to the blood stream

    • passive diffusion and active transport

  • Synthesis - true protein, FA, starch, vitamins

  • Excretion – elimination of waste products

    • via bile (toxins, microbes etc)

    • via rectum (Ca, Mg, P)

Primitive Gastrointestinal Tract

  • Found in monotremes (egg-laying mammals), & insectivores (bats, shrews, moles)

    • Simple stomach, little or no division between small intestines and large intestines, large intestine simple, presence of caecum, non-sacculated colon

Species-dependent Nutritional Adaptations

  • Includes involvement of:

    • Teeth

    • Jaws and jaw musculature

    • Alimentary canal

      • Stomach - May be simple or become sacculated to compartmentalize functions for prolonged storage of feed and utilization of bacterial fermentation (langures and ruminants)

        • May also become voluminous for storage of large amounts of feed (vampire bats)

      • Large intestine - varies substantially in length, compartmentalization, and complexity among species


  • Functions

    • Grasp food

    • Taste

    • Masticate food

    • Mix with saliva

  • Why would taste be necessary for digestion? Think, Pair, Share

  • What is saliva? TPS

Digestion in the Mouth

  • Prehension

    • Bringing the food to the mouth

      • Upper limbs, head, beak, claws, mouth, teeth and lips

  • Mastication or chewing

    • To crush the food, increase surface area and allow enzymes to act on molecules

      • Carnivores need only to reduce the size of the particle

      • Herbivores must chew continuously (40-50,000 times a day)


  • Domestic mammals use lips, teeth and tongue

    • Relative importance varies by species

    • Horses

      • lips when eating from manger

      • teeth when grazing

    • Cows and sheep have limited use of lips

      • Use long rough tongue to grasp forage

    • Pigs use snout to root in ground and pointed lower lip to convey feed into mouth

  • Birds use beak and tongue

  • Drinking varies as well

    • Most mammals use suction

    • Dogs and cats use tongue to form ladle


  • Physical reduction of feed

  • Especially important in non-ruminant herbivores

  • Adaptations

    • Carnivores

      • Large canines and incisors, tearing but little to no chewing activity

    • Herbivores

      • Specialized molars, lots of chewing and grinding

    • Edentates (sloths, armadilloes, anteater)

      • Relative toothlessness

Teeth and Mastication

  • Teeth are essential for proper chewing

    • Distinguishing difference between carnivores and herbivores

    • May regulate the amount of forage an herbivore is able to consume

      • Problems with older animals

      • E.g. Usually Elephants die from starvation at old age, not old age itself

Feldhammer Fig. 6.1

Fig. 6.6

Utilizing Cellulose

  • Advantages

    • Ultra-abundant in the environment

    • Easily obtained – no need to “hunt” plants

    • Plant cell walls & fiber high in energy

  • Disadvantages

    • Indigestible by mammalian digestive enzymes

    • Cellulase is found only in bacteria & some protozoans

Structural Adaptations

of Teeth in Mammals

Teeth Specializations

  • Carnivores

    • Canine teeth highly developed and used for tearing

    • Molars are pointed for bone crushing

Teeth Specializations

  • Omnivores

    • grinding teeth patterns on posterior teeth (molars)

    • piercing and ripping cusps on anterior teeth (incisors)

    • Tongue - used to move feed to teeth

Teeth Specializations

  • Non-ruminant herbivores (horse)

    • incisors for nipping, molars slightly angled, jaws move circularly (vertical and lateral)

  • Ruminants

    • no upper incisors, have dental pad, molars allow only lateral movements

    • Different classes - roughage eaters, transition types, selective eaters all differ in tongue mobility and cleft palate


  • Quantity and composition of saliva varies considerably between species

    • Quantity related to level of chewing activity

  • Amount of secretion

    • Dogsminimal (lubrication, no enzymes)

    • Sheep3-10 liters/d

    • Horse10-12 liters/d

    • Cattle130-180 liters/d


  • Monogastric

    • One compartment

      • Varies in size by species

  • Ruminant

    • 4 compartments

      • Reticulum

      • Rumen

      • Omasum

      • Abomasum

Adaptations to Feed Sources

  • Gastric capacity and structure

    • Capacity is greatest inpregastric fermentors

      • Stomachs act as reservoir

    • Small stomach in carnivores is related to high nutrient density of the diet

    • Distribution and composition of epithelial lining varies between species and dietary adaptations

Adaptations to Feed Sources

  • Intestinal length and functions

    • Small intestine

      • Less variable among species than stomach and hind gut, but generally shorter in carnivores than in herbivores (why? Nutrient density)

    • Large intestine

      • Importance of hind gut fermentation dictates variation in structure and size

      • Some hind gut fermentation occurs in most species (i.e. second stomach)

Digestive Tracts


Small Intestine


Large Intestine

Rule Size = Function

Digestive Enzymes

  • Young animals produce little sucrase, maltase, amylase

  • Ruminants produce no sucrase

  • Adult pigs lack lactase

  • Activity changes with age

    • Lactase

    • Sucrase, maltase

Monogastric Animals

  • Single, simple stomach structure

  • Mostly carnivores and omnivores

    • Very simple - mink, cat and dog

    • Cecal digestion - horse, rabbit, elephant or rat

    • Sacculated stomach - kangaroo

GIT Classifications - Dog

  • Monogastric carnivore with limited post-gastric fermentation

    • Simple stomach, not capable of effective utilization of forage-based (high fiber) diets

    • Unable to digest some of the substances in grains, fruits and vegetables

    • Similar to cat

GIT Classifications - Pig

  • Monogastric omnivore with limited post-gastric fermentation

    • Simple stomach, not capable of effective utilization of forage-based (high fiber) diets

    • Unable to digest some of the substances in grains, fruits and vegetables

    • Similar to human




(2 gal)

Large Intestine

(16’, 2 gal)



Small intestine

(60’, 2.5 gal)


(10”, 0.5 gal)

Human Digestive Tract

GIT Classifications - Horse

  • Monogastric herbivore with extensive post-gastric fermentation

    • Simple stomach incapable of utilization of forage-based (high fiber) diets

    • Extensive fermentation after primary sites of digestion and absorption



Small Colon (12’, 3 gal)

Small intestine

(70’, 12 gal)


Large Colon

(12’, 19 gal)



(4’, 8 gal)

Stomach (3.5 gal)

GIT Classifications - Sheep

  • Ruminant herbivore with extensive pre-gastric fermentation

    • Highly developed sacculated stomach capable of extensive and effective utilization of forage-based (high fiber) diets

    • Extensive fermentation before primary sites of digestion and absorption

    • Similar to cattle and goats



Large intestine

(33’, 8 gal)



(3’, 3 gal)

Rumen (paunch)

(43 gal)




(5 gal)



(2.5 gal)

Small intestine

(150’, 16 gal)


(4 gal)

GIT Capacity - Volume

  • Carnivores

    • stomach (70%) > SI = LI (15%)

    • GIT surface/body surface: 0.6:1

  • Omnivores

    • stomach = SI = LI (33%)

    • GIT surface/body surface: intermediate

  • Herbivores

    • Ruminants

      • stomach (70%) > SI (20%) > LI (10%)

      • GIT surface/body surface: 3:1

    • Non-ruminants

      • stomach (10%) < SI (30%) < LI (60%)

      • GIT surface/body surface: 2:1

Capacity of Digestive Tracts

Fermentative Digestion

  • All mammals have some fermentative capacity that allows for utilization of ingested fiber

  • The comparative importance of fermentation is related to the fraction of total digesta contained in fermentative compartments of the gastrointestinal (GI) tract

Advantages of pregastric fermentation

  • Make better use of alternative nutrients

    • Cellulose

    • Nonprotein nitrogen

  • Ability to detoxify some poisonous compounds

    • Oxalates, cyanide, alkaloids

  • More effective use of fermentation end-products

    • Volatile fatty acids, microbial protein, B vitamins

  • Allows wild animals to eat and run

Disadvantages of pregastric fermentation

  • Fermentation is inefficient

    • Energy

      • Loss % of total caloric value

        Methane 5-8

        Heat of fermentation 5-6

      • Relative efficiency is dependent on the diet NDF.

    • Protein

      • Some ammonia resulting from microbial degradation will be absorbed and excreted

      • 20% of the nitrogen in microbes is in the form of nucleic acids

Disadvantages of pregastric fermentation

  • Ruminants are susceptible to ketosis

  • Ruminants are susceptible to toxins produced by rumen microbes

    • nitrates to nitrites

    • urea to ammonia

    • nonstructural carbohydrates to lactic acid

    • tryptophan to 3-methyl indole

    • isoflavonoid estrogens to estrogen coumestans

Pregastric Fermenters

Postgastric Fermentors

  • Cecal fermentors

    • Mainly rodents, rabbits and other small herbivores

    • Often associated with coprophagy

  • Colonic fermentors

    • Includes true herbivores (e.g., horse), omnivores (e.g., pig and human), and carnivores (e.g., cat and dog)

      • Horse has some expanded cecal fermentation in addition to greatly expanded colonic fermentation

    • Degree of colonic sacculation is related to importance of fiber digestion and fermentative capacity

Postgastric Fermenters


  • If an animal eats a more nutrient dense diet, they will have a less developed digestive tract.

  • Animals that eat hard-to-digest diets have well developed, compartmentalized digestive tracts.

  • Imagine a spectrum with cows and other ruminants on one end and earthworms on the other. Most multi-celled animals fall somewhere in between these extremes.


  • The following factors affect what an animal can eat in its diet:

    • Teeth shape and prehensive strategies

    • Saliva production

    • Complexity of digestive tract (e.g. presence of compartments)

    • Production of enzymes

    • Presence and amount of anaerobic bacteria

    • Length of small intestine

    • Presence of a cecum


  • Examples of digestive strategies:

    • Carnivore – e.g. dog or cat – simple, short GIT. Few compartments. Production of enzymes for protein breakdown and vitamin synthesis.

    • Omnivore – humans and pigs; slightly more complex and longer GIT. More post-gastric fermentation. Longer small intestine.

    • Pregastric Fermenter – e.g. cow. Compartmentalized stomach. Fermentative capability. Highly advanced GIT. Large capacity.

    • Post-gastric Fermenter – e.g. horse or rabbit. Little digestion until large intestine’s cecum.

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