• Digestive tract is interface between animal and environment
• All adaptations are compromises; a particular digestive tract anatomy enables an animal to do some things well and other things poorly
• Tongue – Has 3 primary roles
– Collecting food – sticky tongue of woodpeckers, long thin tongue hummingbirds
– Manipulating food in mouth
• Muscular tongue of finches and parrots for handling seeds
• Fish eating birds have stiff papilla
• Filter feeding birds have bristles that mesh with lamellae on bill to form sieve
– Swallowing – may have papilla to direct food items toward the back of the mouth
• Beak
– Excellent example of adaptation to diet
– Galliform beaks undifferentiated
– Highly specialized beaks such as hummingbirds, crossbills, and snail kite
– Prepare food for swallowing
– Remove poorly digested portions (e.g. seed hulls, snail shells, bones)
• Figure 6-2B from King and McLelland
• Taste buds – Birds have fewer than mammals, but they are functional
– Help bird assess the chemical content of food
– Example of grouse and acorns
• Salivary glands
– Lubricate food for swallowing
– Larger in birds that eat dry foods
– Amylase activity not significant
– Other functions:
• Gray jays use saliva to make balls of food to store in trees
• Sticky tongue of woodpeckers
• Swallow and swift nests
• Bird-nest soup
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• Esophagus
– Thin walled, highly folded tube, larger than in mammals
– Foods often in large pieces
– May serve as storage organ
– Crop
– Storage enables birds to consume a large amount of food and process later
– Food for young
Fig. 2.12 from Stevens and Hume 1995
• Stomach – 2 parts
• Thin walled sacks in carnivores
• Proventriculus
– Glandular, produces acids and enzymes
• Ventriculus
– Muscular part of stomach (Gizzard)
– Grinds food into small particles (teeth)
– Large in herbivores
• Small Intestine –
Primary site digestion and absorption
• Pancreatic secretions
– Bicarbonate for buffering
– Proteases
– Amylase
– Lipase
• Liver secretions
– Bile salts
• Ceca – blind pouches at junction of small and large intestine
• Contain microbes
– Fermentation
– Vitamins
– Nitrogen metabolism
– Water and electrolytes
• Large intestine
– Absorbs water and stores feces
• Cloaca
• Herp GI-tract
• Larval form of amphibians often long undifferentiated tube
• Adult tract shorter
• Carnivorous reptiles have short tract
• Herbivorous species may have sites in large intestine for fermentation
Fig. 2.10 from Stevens and Hume 1995
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• What advantage do most mammals have over most birds when it comes to digestion?
• Don’t have same weight restrictions
• Lips
– Assist in harvesting food and preparing for biting
• Tongue
– Many of the same functions as birds
• Teeth
– Incisors – Remove manageable sized bites
– Canines – Help carnivores capture and kill prey. Social function in many species.
– Molars and premolars – Reduce particle size and mix food with saliva
• Saliva
– Promotes taste by making chemicals soluble
• Enzymes break down starch
– Buffers acids in the rumen (bicarbonate)
– Proteins bind tannins
• Esophagus – tube from mouth to stomach
• Forestomach fermentation sites
– Ruminants most common, but also in camels, peccaries, hippos, kangaroos, leaf eating monkeys, hyraxes, and tree sloths
• Fermentation releases short-chain fatty acids (2, 3, or 4 carbons) which are energy source for animal
• Ruminants regurgitate food and chew it repeatedly.
• Rumen – Large sack with papillae. May have folds and sections
• Reticulum – Smaller sack with honeycomb appearance on inside. Is continuous with rumen
• Omasum – Chamber with folds like the pages of a book. Helps regulate which food particles are released from rumen
• Abomasum – Site of acid digestion
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• Stomach of monogastic species is both a storage and digestive organ
– Carnivore stomach can hold large amounts of food
• Small intestine has same functions as we discussed in birds
• Ceca – Large blind pouch. Well developed in monogastric herbivores, esp. rodents, lagomorphs, horses
• Large intestine
– Site of fermentation in many herbivores
– Absorption of water and electrolytes from feces
Figures of mammalian tracts from Stevens and Hume 1995
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