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Comparative Digestive Physiology. Why Do Animals Digest?. Food not ingested in suitable state Physical nature of food determined by: gathering apparatus for uptake type of digestive system . Primary Functions of the Digestive Tract. Transport food – peristaltic contractions

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why do animals digest
Why Do Animals Digest?

Food not ingested in suitable state

Physical nature of food determined by:

gathering apparatus for uptake

type of digestive system

primary functions of the digestive tract
Primary Functions of the Digestive Tract
  • Transport food – peristaltic contractions
  • Digestion
    • Mechanical breakdown
    • Chemical breakdown
  • Absorption
    • 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)
regions of alimentary canal
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
associated structures
Associated Structures
  • Pancreas
  • Liver
  • Gallbladder
  • Salivary glands

Contribute to small intestinal digestion

primitive gastrointestinal tract
Primitive Gastrointestinal Tract
  • Found in monotremes (egg-laying mammals), insectivores (bats, shrews, moles), and dermopterans (colugos)
    • Simple stomach, little or no division between small intestines and large intestines, large intestine simple, presence of cecum, non-sacculated colon
species dependent nutritional adaptations
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 (langurs 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
  • 2.8 billion domesticated ruminants
    • Cattle, sheep, deer, elk, bison
  • Pregastric fermentation
    • Ability to chew cud at frequent intervals distinguishes true ruminant from other foregut fermenters
      • Kangaroo, colobine monkey are not true ruminants
  • Four compartment stomach
    • Reticulum
    • Rumen
    • Omasum
    • Abomasum
classification of ruminants by feeding preference
Classification of Ruminants by Feeding Preference
  • Classes of ruminants
    • Concentrate selectors
    • Intermediate feeders
    • Roughage grazers
concentrate selecting species
Concentrate Selecting Species
  • Properties
    • Evolved early
    • Small rumens
    • Poorly developed omasums
    • Large livers
    • Limited ability to digest fiber
  • Classes
    • Fruit and forage selectors
      • Very selective feeders
      • Duikers, sunis
    • Tree and shrub browsers
      • Eat highly lignified plant tissues to extract cell solubles
      • Deer, giraffes, kudus
intermediate feeding species
Intermediate Feeding Species
  • Properties
    • Seasonally adaptive
  • Feeding preference
    • Prefer browsing
      • Moose, goats, elands
    • Prefer grazing
      • Sheep, impalas
roughage grazing species
Roughage Grazing Species
  • Properties
    • Most recently evolved
    • Larger rumens and longer retention times
    • Less selective
    • Digests fermentable cell wall carbohydrates
  • Classes
    • Fresh grass grazers
      • Buffalo, cattle, gnus
    • Roughage grazers
      • Hartebeests, topis
    • Dry region grazers
      • Camels, antelope, oryxes
structures in mouth
Structures in Mouth
  • Lips
  • Teeth
  • Tongue
  • Salivary glands
  • Functions
    • Grasp food
    • Taste
    • Masticate food
    • Mix with saliva
digestion in the mouth
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 only to reduce the size of the particle to a size small enough to swallow
      • Herbivores must chew continuously (40-50,000 times a day) to increase surface area
  • Seizing and conveying feed to mouth
  • Mechanisms vary with behavior and diet
    • Forelimbs
      • Primates, raccoon
    • Snout
      • Elephant, tapir
    • Tongue
      • Anteater, cow, sheep
    • Lips
      • Horse, sheep, rhinoceros
  • 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
the importance of prehension in diet formulation
White Rhino (“wijd” = wide)

Squared off upper lip used to “crop” grass

Grazes on savannah

Black Rhino

Prehensile upper lip for browsing

Consumes bushes and shrubs in forest

The Importance of Prehension in Diet Formulation
  • Physical reduction of feed
  • Especially important in non-ruminant herbivores
  • Adaptations with teeth
    • Carnivores
    • Herbivores
    • Edentates (sloths, armadilloes, anteater)
      • Relative toothlessness
morphological adaptations for herbivory
Morphological Adaptations for Herbivory
  • All related to finding, ingesting, masticating, and digesting plant cell walls
  • Dental adaptations for herbivory include changes to incisors, molar occlusal surfaces, & masseter
  • Solution for digestive problems is to provide a place in digestive tract for anaerobic bacteria & protozoans (microflora) to colonize
monogastric teeth
Monogastric Teeth
  • Function:
    • Mechanically reduce particle size
    • Increase surface area

Four types:

    • Incisors are used for cutting
  • Canine (fangs, eye teeth, tusks) are tearing teeth
  • Premolars and molars (cheek teeth) grind the food
ruminant mouth teeth
Ruminant Mouth - Teeth


  • Reduce particle size


  • Upper dental pad
  • Lower incisors
  • Premolars
  • Molars
teeth specializations
Teeth Specializations
  • Carnivores
    • Canine teeth highly developed and used for tearing
    • Molars are pointed for bone crushing
teeth specializations28
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
jaw teeth specializations
Jaw & 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
ruminant mouth
Ruminant Mouth
  • Lips range from short, relatively immobile in nonselective grazing species to very mobile (prehensile) in selective grazing or concentrate selecting species
  • Chew in a lateral (grinding) motion on one side of mouth at a time
    • Needed to increase surface area of feed particles
    • Feed chewed primarily during rumination in grazing species
jaw muscles and mastication
Jaw Muscles and Mastication
  • Temporalis muscle - develops maximum force on anterior portion of jaw (largest muscle in carnivores and smallest muscle in herbivores)
  • Masseter and medial pterogoid - maximum force for crushing and grinding
  • Lateral pterogoid - allows lateral movement which is important for grinding (highly important in herbivores, but carnivores and many omnivores have almost no lateral movement of jaws)
monogastric tongue
Monogastric Tongue


  • Comprised of three muscles
  • Maneuvers food in the mouth
    • Moves feed to teeth for grinding and to the back of the mouth for swallowing
  • Can distinguish between feed and toxins by papillae or taste buds
ruminant mouth tongue
Ruminant Mouth - Tongue
  • Drinking, chewing and forming boluses
  • Prehension of feed
    • Covered with rough, hook-like papillae that assist in grasping feed
    • Important in nonselective grazing species
  • Taste buds
    • More numerous than monogastric species
    • More numerous on nonselective grazing species
    • Believed that taste is primarily used for food avoidance by grazing species while concentrate selecting species select on the basis of smell
monogastric salivary glands
Monogastric Salivary Glands

Types of Glands:





functions of saliva
Functions of Saliva
  • Moisten feed (salt and water)
  • Lubrication (aids swallowing)
  • Starch and(or) lipid digestion (amylase and(or) lipase)
monogastric salivary glands37
Monogastric Salivary Glands
  • Flow rate affected by:
    • Parasympathetic nervous system
      • Increased tone = Increased flow
      • Increased flow = Increased dilution
    • Sympathetic nervous system
      • Increased tone = Decreased flow
      • Decreased flow = Increased concentration

Ruminant Mouth - Saliva

  • From at least three paired glands
    • Submaxillary, sublingual, parotid (50% of secretions)
  • Aids in mastication, swallowing, forming bolus
    • No digestive enzymes in the saliva of mature ruminants
    • Provides N, P, S and Na for rumen microoganisms
    • Buffering compounds to maintain rumen pH and mucin to prevent bloat


  • Quantity and composition of saliva varies considerably between species
    • Quantity related to level of chewing activity
  • Amount of secretion
    • Dogs minimal (lubrication, no enzymes)
    • Sheep 3-10 liters/d
    • Horse 10-12 liters/d
    • Cattle 130-180 liters/d
deglutition swallowing
Deglutition (Swallowing)
  • Reflex initiated by presence of food in pharnyx
  • Propulsion of food to stomach by esophageal peristalsis
monogastric esophagus
Monogastric Esophagus
  • Horse/Pig:
    • Striated muscles for first 2/3
    • Smooth muscles for last 1/3
    • In horse, esophagus joins stomach at an oblique angle and cardiac sphincter (the valve between the stomach and esophagus) only allows one-way flow
      • MOST horses cannot belch out gas or vomit
  • Dog:
    • Striated muscles throughout allow GREAT control of digesta movement both directions
ruminant esophagus
Ruminant Esophagus
  • Involved in rumination
  • Different from monogastric esophagus
    • Striated muscle along the entire length
      • Provides greater strength
      • Allows some voluntary control
    • Funnel shaped
  • Contains three sphincters active in rumination and eructation
  • Species adaptations
    • Ability to control peristaltic contractions
      • Reverse peristalsis
      • Amount and location of skeletal muscle
      • Regurgitation vs. vomiting
foregut in birds
Foregut in Birds
  • Crop
    • Bottom of the esophagus forms a sac called crop
      • Stores undigested food.
    • Birds with crop gorge when food is available, store it in crop, and slowly digest it later


  • Monogastric
    • One compartment
      • Varies in size by species
  • Ruminant
    • Four compartments
      • Reticulum
      • Rumen
      • Omasum
      • Abomasum
gastric digestion
Gastric Digestion
  • Functions
    • Reservoir for controlled release of digesta to small intestine
      • Horse has small capacity – requires increased number of smaller sized meals
    • Mixing food
    • Mechanical breakdown of feed
    • Hydrolytic digestion by acid and enzymes
      • Mainly protein
    • Kill bacteria
    • Secrete intrinsic factor: needed for vitamin B12 absorption
    • Hormone production
stomach regions
Stomach Regions
  • Esophageal
    • Non-glandular
  • Cardiac
    • Secretes mucus
  • Fundic
    • Parietal cells
    • Chief cells
  • Pyloric
    • Mucus
gastric pits
Gastric Pits
  • Formed by numerous folds in the epithelium
  • Glands empty into the gastric pit
  • Many types of glands may empty into one gastric pit
stomach secretions

Decreases pH (~2-3)

Denatures protein

Kills bacteria

Activates pepsinogen


Protects lining from acid and enzymes

No “autodigestion”



Activated form is pepsin

Hydrolyzes protein

Rennin (abomasum)

Clots milk


Some species

Stomach Secretions
gastric motility and emptying
Gastric Motility and Emptying
  • Motility aids mixing, mechanical and hydrolytic reduction of feed to chyme
    • acid pulp
  • Emptying is stimulated by distension of antral wall and presence of liquid chyme
control of gastric secretions and gastric motility
Control of Gastric Secretions and Gastric Motility
  • Cephalic phase
  • Gastric phase
  • Intestinal phase
cephalic phase
Cephalic Phase
  • Vagal reflex
    • Parasympathetic innervation
    • Increases gastric motility, enzyme secretion
    • Small increase in HCl secretion
gastric phase
Gastric Phase
  • Local reflex, depends on presence of feed in stomach
  • Mainly mediated by gastrin
    • Increases HCl secretion
intestinal phase
Intestinal Phase
  • Stimulated by duodenal distension, pH, osmolarity, nutrients (fat)
  • Cholecystokinin (CCK) is released by the small intestine
    • Decreases HCl secretion and gastric motility
gastrointestinal hormones
Gastrointestinal Hormones
  • Gastrin
    • Origin: Stomach, Abomasum
    • Stimulus: Food in stomach
    • Function: Stimulates HCl & pepsinogen secretion, increases stomach motility
  • Secretin
    • Origin: Duodenum
    • Stimulus: Acid
    • Function: Stimulates pancreatic secretions. Slows stomach motility and acid production
gastrointestinal hormones57
Gastrointestinal Hormones
  • Cholecystokinin (CCK)
    • Origin: Duodenum
    • Stimulus: Fat & protein in duodenum
    • Function: Stimulates bile and pancreatic secretions
      • Also regulates appetite and feed intake
  • Gastric Inhibitory Protein (GIP)
    • Origin: Duodenum
    • Stimulus: Fats and bile
    • Function: Inhibit stomach motility and secretion of acid and enzymes
ruminant stomach
Ruminant Stomach


  • Reticulum
  • Rumen
  • Omasum
  • Abomasum
reticulo rumen

Although structurally they appear as a single continuous compartment, functionally they are distinctly different

  • Honeycomb lining
    • No secretions
  • Formation of food bolus
  • Regurgitation initiated here
  • Collects hardware (nails, wire)
  • Digestion and fermentation vat
    • 40-50 gallons
    • No secretions
  • Contains anaerobic microbes (25-50 billion bacteria/mL fluid)
    • Also protozoa, fungi
    • Produce VFA, protein
  • Papillae lining
    • Increase surface area
  • Absorption of VFA
    • Passive diffusion
  • Laminae/manyply lining
    • Muscular folds
    • No secretions
  • Reduces particle size
  • Absorption of water
    • ~60% removed
  • Absorption of VFAs
    • ~2/3 of VFAs entering or 10% of total produced
    • Prevents buffering of the abomasum
  • True gastric stomach - four gallons in a cow
    • Three regions (cardiac, fundic, and pyloric)
    • Digestive secretions
      • Proteolytic enzymes and HCl
  • pH decreases

from 6 to 2.5

    • Denatures proteins
    • Kills bacteria

and pathogens

    • Dissolves minerals
    • Gastric digestion
small intestine
Small Intestine
  • Composed of 3 segments (proximal to distal)
    • Duodenum
      • Releases bile and pancreatic secretions
      • Active site of digestion
    • Jejunum
      • Active site of nutrient absorption
    • Ileum
      • Active site of nutrient absorption
        • Most water, vitamins & minerals
      • Some bacterial presence
        • Fermentation

The pH of the small intestine increases towards 7.0 as food moves from the duodenum to the ileum

specialized cells lining villi
Specialized Cells Lining Villi



  • Absorptive epithelial cell
    • Contain brush border on lumen/apical side
    • Brush border:
      • Enzymes
      • Nutrient transport molecules
  • Goblet cell
    • Secretes mucus
specialized cells lining villi67
Specialized Cells Lining Villi

Anti-microbial compounds

  • Endocrine cell
    • Secrete hormones into bloodstream or local cells
  • Paneth cell
    • Secretory granules with anti-microbial properties

CCK, Secretin, etc.

small intestine absorptive surface
Small Intestine – Absorptive Surface
  • Villi
  • Enterocyte
  • Brush border
  • Cell migration from crypts to tips of villus
    • 2-3 days
small intestine structure
Small Intestine - Structure
  • Lumen
  • Mucosa
  • Villi
  • Crypts
  • Lacteal
  • Enterocyte
  • Brush border

Intestinal Wall



nutrient absorption in the small intestine
Nutrient Absorption in the Small Intestine
  • Principal site of absorption of amino acids, vitamins, minerals and lipids
    • Glucose and other sugars in monogastrics
  • Generally, most absorption occurs in the proximal (upper) part of the small intestine but some absorption occurs in all segments
    • Duodenum, jejunum and ileum
  • Digestion and absorption within SI is rapid
    • Within 30 minutes of entering SI
nutrient absorption
Nutrient Absorption
  • Variety of mechanisms
    • Diffusion
    • Facilitated diffusion
    • Active transport
    • Pinocytosis or endocytosis
  • Dependent upon
    • Solubility of the nutrient (fat vs. water)
    • Concentration or electrical gradient
    • Size of the molecule to be absorbed
  • Water and small lipid molecules pass freely through membrane
  • Move down concentration gradient to equalize concentrations
facilitated diffusion
Facilitated Diffusion
  • Carrier loads particle on outside of cell
  • Carrier releases particle on inside of cell
  • Reverse

Allows equalization of concentrations across membrane

active transport
Active Transport
  • Carrier loads particle on outside of cell
  • Carrier releases particle on inside of cell
  • Carrier returns to outside to pick up another particle
active transport78
Active Transport
  • Unidirectional movement
  • Transports nutrients against concentration gradient
pinocytosis or endocytosis
Pinocytosis or Endocytosis
  • Substance contacts cell membrane
  • Membrane wraps around or engulfs substance into sac
  • Sac formed separates from the membrane and moves into cell
secretions entering si
Secretions Entering SI

Secreted from within SI

  • Intestinal mucus
  • Brush border enzymes
  • Pancreatic juices
    • Produced & stored in pancreas
  • Bile
    • Produced in liver
    • Stored in gallbladder
    • Horse has no gallbladder
      • Direct bile secretion into duodenum
      • Cannot store bile—continuous intake of food

Enters from ducts into SI

intestinal mucus
Intestinal Mucus
  • Secreted by glands in wall of duodenum
    • Brunner’s glands
  • Acts as lubricant and buffer to protect duodenal wall
  • Green, viscous liquid
    • Alkaline ph (neutralize acidic chyme)
  • Secreted by liver via bile duct to duodenum
    • Stored in gall bladder (except in horses)
  • Functions to emulsify fats
  • Composition
    • Bile salts (glycocholic and taurocholic acids)
    • Bile pigments (bilirubin and biliverdin)
    • Cholesterol
  • 95% reabsorbed and returned to liver
nutrient digestion lipids
Nutrient Digestion - Lipids

Large Lipid Droplet

Action of bile salts

Lipid emulsion


Bile salts & pancreatic lipase

and colipase

Water soluble micelles

pancreatic juice
Clear, watery juice

Enters duodenum via pancreatic duct

Aids in fat, starch, and protein digestion









Pancreatic Juice


importance of pancreas for digestion
Importance of Pancreas for Digestion
  • Produces enzymes responsible for
    • 50% of carbohydrate digestion
    • 50% of protein digestion
    • 90% of lipid digestion
  • Produces sodium bicarbonate for neutralization of chyme in duodenum
activation of pancreatic enzymes
Activation of Pancreatic Enzymes
  • Enterokinase
    • Secreted from crypts in duodenum
    • Trypsinogen trypsin
  • Trypsin then converts:
    • Trypsinogen trypsin
    • Chymotrypsinogen chymotrypsin
    • Procarboxypeptidase carboxypeptidase
overview of digestive enzymes


Chymosin (rennin)








Brush Border (SI)







Overview of Digestive Enzymes
ruminant small intestine
Ruminant Small Intestine
  • Similar in structure and function to monogastric
  • Differences are subtle but important
    • Limited ability to digest starches and sugars
      • Little to none presented except in exceptional circumstances (high-grain feeding)
small intestine93
Small Intestine

Digesta pHFunctions

Duodenum 2.7 - 4 Enzymes

pH change

Flow rate regulation

Jejunum 4 – 7 Enzymes


Ileum 7 - 8 Absorption

Limited fermentation

  • Rate of pH increase through small intestine is slower than monogastrics
    • Better for peptic activity
    • May limit pancreatic protease and amylolytic activity
pancreatic secretions
Pancreatic Secretions
  • Secretion pH is 7.2-7.8
  • Enzymes
    • Amylase
    • Lipase
    • Proteases
      • Trypsinogen converted to trypsin
      • Chymotrypsinogen converted to chymotrypsin
      • Procarboxypeptidase converted to carboxypeptidase
    • Nucleases
activity of pancreatic enzymes
Activity of Pancreatic Enzymes
  • Concentration of enzymes in pancreatic juice comparable to monogastrics
  • Activity is lower and may be affected by:
    • Less juice secreted/kg BW
    • Low digesta pH
    • High rate of passage
  • Limited activity particularly a problem for intestinal digestion of starch escaping ruminal digestion
    • For ruminants fed high grain diets, less than 50% of starch reaching small intestine is digested
  • Secreted with pancreatic juice in the common bile duct of sheep
  • Secreted in the bile duct of cattle
large intestine
Large Intestine
  • Composed of three segments
    • Cecum
    • Colon
    • Rectum
  • Function
    • Fermentative digestion
      • No enzyme secretion
      • Relies on microbes or secretions washed out of the SI
    • Absorption of remaining water, volatile fatty acids (VFAs) from microbial fermentation and minerals
    • Digesta storage

Degree of development is species dependent

monogastric cecum
Monogastric Cecum
  • Located at junction of small and large intestine
  • Function similar to rumen in ruminants
    • Microbial activity and digestion of feeds
      • Contains a microbial population similar to the rumen
        • Cellulolytic & hemicelluloytic bacteria
  • Since cecum is located AFTER major site of nutrient absorption (small intestine), then microbial cell proteins are not available to the animal
    • Fecal loss
monogastric large intestine
Monogastric Large Intestine
  • Function:
    • Absorption of liquid
    • Mass movements move fecal matter to anus
    • Usually only a few times a day
      • Associated with defecation
  • Cellulolytic – digest cellulose (forages)
  • Amylolytic – digest starches and sugars (concentrates or grains)
  • Other types:
    • Proteolytic
      • Clostridium
    • Organic acid utilizers
    • Methanogens
      • Produce CO2, H2, formate, CH4
ruminant large intestine
Ruminant Large Intestine
  • Fermentative digestion
    • Bacteria similar to rumen, but no protozoa
    • Digestion in colon may account for as much as:
      • 27% of cellulose digestion
      • 40% of hemicellulose digestion
      • 10% of starch digestion
    • Only important in conditions that increase the amount of fermentative carbohydrate entering the large intestine
      • Increased rate of passage of forages
      • High grain diets
    • May account for as much as 17% of total VFA absorption
    • VFAs are efficiently absorbed, but primarily used as energy source for large intestinal mucosa cells
ruminant large intestine102
Ruminant Large Intestine
  • Absorption of ammonia-N
    • May account for as much as 30 to 40% of the net transport of N into body fluid
    • Absorbed N may be used for:
      • Synthesis of nonessential amino acids
      • Recycling of N to the rumen
        • Important on low protein diets
    • Regulated by:
      • Increased by increasing N concentration of diet
      • Decreased by increasing the amount of carbohydrate fermented in the large intestine
  • Mineral absorption
  • Water absorption
    • 90% of water entering the LI is absorbed
  • Muscular area of large intestine used for storage of feces and ultimately for defecation
    • Feces includes sloughed cells, undigested food and microbial matter
digestive adaptations to varying feed sources
Digestive Adaptations to Varying Feed Sources
  • Gastric capacity and structure
    • Capacity is greatest in pregastric fermentors
      • Stomach acts 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
digestive adaptations to varying feed sources105
Digestive Adaptations to Varying 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
    • Large intestine
      • Importance of hind gut fermentation dictates variation in structure and size
      • Some hind gut fermentation occurs in most species
adaptations of the digestive tracts
Adaptations of the Digestive Tracts


Small Intestine


Large Intestine

Rule Size = Function

adaptations of digestive enzymes
Adaptations of Digestive Enzymes
  • Young animals produce little sucrase, maltase, amylase
  • Ruminants produce no sucrase
  • Adult pigs lack lactase
  • Activity changes with age
    • Lactase
    • Sucrase, maltase
utilizing cellulose
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
fermentative digestion
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
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
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
      • Ingested protein is broken down by microbes to ammonia and carbon
      • Some ammonia resulting from microbial degradation will be absorbed across rumen wall and excreted in urine
      • 20% of the nitrogen converted to nucleic acids
disadvantages of pregastric fermentation113
Disadvantages of Pregastric Fermentation
  • Ruminants are susceptible to toxins produced by rumen microbes
    • Nitrates to nitrites
    • Urea to ammonia
    • Nonstructural carbohydrates to lactic acid
postgastric fermentors
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
foregut vs hindgut fermentation
Foregut vs. Hindgut Fermentation
  • Foregut
    • More efficient per unit volume of food
    • Slower digestive process
    • Animal may starve with a full rumen
    • Size restricted
  • Hindgut
    • More efficient relative to time
    • Faster rate of passage

Feeding Habits of Mammals& Taxonomic Distribution

Feeding HabitNumber of OrdersPercent of Species

  • Herbivory 10 40
  • Frugivory, Graniv., Nectivory 5 4
  • Carnivory 4 12
  • Planktonivory? (Krill feeders) 2 <1
  • Insectivory 10 33
  • Omnivory 7 10
classification of animals by preferred ingested feedstuffs
Classification of Animals by Preferred Ingested Feedstuffs
  • Carnivore – consume animal products
    • dogs, cats
    • komodo dragon, tigers, eagles, sharks, polar bear
  • Herbivore – consume plant products
    • cattle, sheep, goats, horses
    • giant panda, gorilla, elephant, ostrich, green iguana, giraffe, American bison
  • Omnivore – plant and(or) animal products
    • pigs
    • opossum, raccoon, blue jays, black bear, human
specialized carnivores
Specialized Carnivores

Aphidivore - feeds on aphidsApivore - feeds on beesErucivore - feeds on caterpillarsInsectivore - feeds on insectsLarvivore - feeds on larvaeMyrmevore - feeds on antsMucivore - feeds on fliesPiscivore - feeds on fishPupivore - feeds on pupaeRanivore - feeds on frogsSanguivore - feeds on bloodZoosuccivore - feeds on liquid animal secretions of decaying animal matter

specialized herbivores
Specialized Herbivores

Ambivore - feeds on grasses and broad leaf plantsExudativore - feeds on gums, resins, and sapFolivore - feeds on foliage (leaves or trees)Forbivore - feeds on forbs (i.e. flowering plants)Frugivore - feeds on fruitGraminivore - feeds on grassesGranivore - feeds on grainsGumivore - feeds on gums secreted by some plantsLignivore - feeds on woodMellivore - feeds on honeyNectarivore - feeds on nectarNucivore - feeds on nuts (agouti)Phytisuccivore - feeds on (tree) sapRadicivore - feeds on roots

classification by type of digestion or site of digestion
Classification by Type of Digestion or Site of Digestion
  • Monogastrics or non-ruminants
  • Ruminants


  • Pre-gastric fermentation (cranial)
  • Post-gastric fermentation (caudal)
monogastric animals
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
ruminant animals
Ruminant Animals
  • Ruminant – herbivores possessing multiple digestive tract compartments for feed breakdown before feed reaches the “true” stomach
      • True ruminants - cattle, sheep, goats
      • Pseudo-ruminants - camels, llamas, alpacas, vicunas
git capacity volume
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

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)


GIT Classifications - Kangaroo

  • Monogastric omnivore with limited pre-gastric fermentation
    • Sacculated stomach,capable of utilization of forage-based (high fiber) diets
    • Able to digest some of the substances in grains, fruits and vegetables

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 Classifications

  • Avian is modified monogastric
    • Beaks replace lips and teeth
    • Crop (enlarged area of esophagus) stores and softens feed prior to entering stomach
    • Proventriculus – glandular stomach
    • Gizzard – muscular part of stomach
    • Branched cecum –postgastric fermentation
    • Cloaca – both fecal and urinary waste
      • Uric acid rather than urea (insoluble)
avians poultry
Avians (Poultry)


  • No teeth, rigid tongue
  • Poorly developed salivary glands
    • Saliva contains amylase
  • Beak is adapted for prehension and mastication
avians poultry141
Avians (Poultry)


  • Enlarged area called crop
    • Ingesta holding and moistening
    • Location for breakdown of carbohydrate by amylase
    • Fermentation

Proventriculus (stomach)

    • Release of HCl and pepsin (gastric juices)
    • Ingesta passes through very quickly (14 seconds)
avians poultry142
Avians (Poultry)

Gizzard (ventriculus)

  • Muscular area with a hardened lining reduces particle size
    • Muscular contractions every 20-30 seconds
    • Includes action of grit
    • HCl and pepsin secreted in proventriculus

Small intestine

  • Similar to other monogastrics
  • No Lacteals
avians poultry143
Avians (Poultry)

Ceca and large intestine

  • Contain two ceca instead of one as in other monogastrics
  • Large intestine is very short (2-4 in) and empties into cloaca where fecal material will be voided via the vent
    • Water resorption
    • Fiber fermentation by bacteria
    • H2O soluble vitamin synthesis by bacteria



Crop (2”)

Small Intestine (55”)


Gizzard (2”)


Ceca (7”)

Large Intestine (4”)



feeding behaviors
Feeding Behaviors
  • Impact feed choices
    • Neophobia (avoidance of new feed sources)
      • Contact testing (based on taste and other sensory information collected in mouth) prior to swallowing
      • Early introduction of a variety of feeds limits this problem
    • Chimpanzees select feeds based on easily digestible carbohydrate content (sugars and starches) rather than fat or protein content
    • Grazers and browser select early growth grasses and plants vs. mature growth
    • In confinement feeding situations, grazers consume concentrates first and then forages in ration based on particle size (basis for creating “total mixed rations”)
feeding behaviors146
Feeding Behaviors
  • Impact feed intake (avoiding under- and over-consumption of feed)
    • Hiding feed in bear exhibit encourages search and gather behaviors, limiting intake and reducing the stress of captivity
    • Predator behavior towards “prey” meals vs. “bowl” meals
    • Grazing animals prefer to eat forage at ground level rather than in elevated feed bunks