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        THE DIGESTIVE SYSTEM OF VERTEBRATES: CD 

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2. Classifications, diets & distribution:

Introduction:

    Many characteristics of the vertebrate digestive system are related to the taxonomic classification, habitat, or diet of the species. Therefore, a general familiarity with these animals is necessary. The vertebrates are believed to have first evolved approximately 500 million years ago and the present-day species are distributed among the eight classes listed in Table 2.1a, Table 2.1b. Vertebrates can be categorized according to the composition of their diet, such as insectivores, piscivores, frugivores, nectivores, granivores, and herbivores, the size of ingested food particles (microphagous or macrophagous), or whether they feed on living or dead material.  However, species that feed exclusively on invertebrates or other vertebrates are often labeled under the general category of carnivores and those that feed on a combination of plants and animals are referred to as omnivores. Although plant-eating animals are often referred to as herbivores, some of these species feed exclusively on the highly nutritious portion (seeds, fruit, nectar, pollen). Therefore, the term herbivore is generally reserved for those that feed mainly on the leaves, stems, and petioles of plants, which requires a more efficient mechanism for reduction of plant material to a small particle size and a larger gut capacity for the microbial fermentation of cell walls.

Table 2.1a.  Classification of Vertebrates (part 1)                        

(*) = number of orders. Extinct groups of vertebrates are excluded. Fish are classified according to Nelson (1984), and amphibians according to Duellman & Trueb (1986). (from Stevens & Hume 1995)

Table 2.1b.  Classification of Vertebrates (part 2)

(*) = number of orders. Extinct groups of vertebrates are excluded. Reptiles are classified according to Evans (1986), and mammals according to Vaughn (1986). For avian orders see Storer (1971a). (from Stevens & Hume 1995).


Fish:

    Almost half of the present-day vertebrates are found in the four classes of fish; Cephalaspidomorphi, Myxini, Chondrichthyes, and Osteichthyes. The first two classes consist of the cyclostomes (lampreys and hagfish), which are the lowest craniate vertebrates. Chondrichthyes contains the chimera and five orders of elasmobranches with a cartilaginous skeleton. Osteichthyes contains 46 orders of teleosts or bony fish.


Fish - Cyclostomes and Cartilaginous Fish:

    Most fish are carnivores or omnivores. Cyclosomes are parasitic on other fish. Chimeras have a blunt snout and thread-like tail. The elasmobranchs consist of the sharks, skates and rays. Although sharks are noted for their imposing array of teeth, they also include the basking and whale shark, which are both the largest fish and microphagous, filter-feeders.

  Cyclostomes:  hagfish,  pacific lamprey

  Sharks:  leopard shark,  basking shark

  Skate & ray:  barndoor skate,  bluntnose stingray                        

Fish - Teleosts:

    The teleosts include the lungfish and lobe-finned fish, which demonstrate some of the characteristics required for transition to land, the Brachipterygii (bowfins, sturgeons, paddlefish and gar), and the Actinopterygii. Lungfish and lobe-finned fish (coelacanths), were prevalent in the Devonian Period 340 to 400 million years ago. However, the lungfish consist of only four living genera confined to South America, Africa, and Australia. The first "living fossil" of the coelacanths (Latimeria chalumnae) was discovered in 1938 off the coast of East Africa, and they appear to be confined to deep-sea regions around the Comoros Islands (Thenius & Terofac 1972). Most teleosts and a majority of North American freshwater species belong to the subclass Actinopterygii (ray finned fish). Most teleosts are carnivores, such as northern pike and rainbow trout, or omnivores, such as the common carp. However, they also include a limited number marine and freshwater herbivores, such as the surgeonfish, parrotfish, and grass carp.  A few species, such as the roach (Rutilus rutilus) can switch from a carnivorous to herbivorous diet, dependent on the availability of food, vary their diet from plankton in the summer to fish in the winter, or use algae or bacteria as a source of food.   

bowfin,  blue paddlefish,  shovelnose sturgeon

  Carnivores:  northern pike,  rainbow trout

  Omnivores:  carp

  Herbivores:  blue parrotfish,  ocean surgeonfish


Amphibians:

    Amphibians are believed to have evolved from predacious, lobe-finned Crossopterygii. They presently contain about 4000 species in the three orders; Gymnophionia (worm-like, burrowing amphibians), Caudata (salamanders, newts, and conger eels), and Salientia (frogs and toads), which are often referred to as anurans.  Most amphibians begin life as free-living aquatic larvae that metamorphose into terrestrial adults. Larval amphibians include carnivores, omnivores, and herbivores, but all adult amphibians are carnivores.

  Amphibians:  mudpuppy,  tiger salamander,  leopard frog,  Carolina toad


Reptiles:

    Reptiles appeared in the Paleozoic era and reached their widest distribution and largest body size in the dinosaurs of the Mesozoic.  Class Reptilia is presently comprised of over 6000 species in three orders: Crocodilia, Testudinata, and Squamata. The crocodilians include crocodiles, alligators, caimans, and gavials. The Testudinata or chelonians can be subdivided into turtles (marine), terrapins (fresh water) and tortoises (terrestrial), but are often referred to collectively as turtles.  Squamata consists of snakes, lizards, worm lizards, and the tuatara, a primitive New Zealand reptile. Reptiles inhabit tropical and temperate climates, and include fresh-water, marine, and terrestrial species. Some have adapted to extremely arid environments. Most reptiles are carnivores or omnivores.  Herbivorous reptiles are limited to about 50 species of lizards, 40 species of tortoise, and a few terrapins and turtles. 

Carnivores:  crocodile,  prairie rattlesnake,  sagebrush lizard,  snapping turtle

Omnivores:  Blanding's turtle,  false map turtle

Herbivores:  iguana,  green sea turtle,  Florida red-bellied turtle,  Galapagos tortoise


Birds:

    Storer (1971a) listed the more than 8,000 species of existing birds under 28 orders. Many of the common species are found in the orders Galliformes (cocklike) and Passeriformes (sparrowlike, perching birds). Birds differ from other vertebrates in their cover of feathers and (with the exception of bats) the modification of their forelimbs for flight. They include species that migrate long distances and have adapted to all regions of the earth, including the deserts and arctic (Storer 1971b). The birds consist principally of carnivores such as the eagles, herons and pelicans, omnivores such as the chicken, and species like the honeycreepers that feed exclusively on fruit, seeds, pollen, or nectar. The limitations of flight on gut weight and capacity, have restricted herbivores to ground birds that fly only short distances and flightless ratites.

Carnivores:  great blue heron,  eastern white pelican,  golden eagle

Omnivores & plant concentrate feeders:  leghorn chickens, honeycreeper


    Herbivorous ground birds include a number of grouse, two species of herbivorous partridge, and the hoatzin. The herbivorous grouse consist of 18 species that range between 350 and 6500 g in body weight and inhabit the steppes, mixed forests, and semideserts of North America and Eurasia (Niethammer et al. 1975; Johnsgaard 1983). Forest grouse include capercaillie, ruffed grouse, spruce grouse, and ptarmigan. Spruce grouse consume large quantities of conifer needles, during winter months (Pendergast and Boag 1973). Ptarmigan are distributed the furthest north of all ground birds, in the tundra of North America, Greenland, and Eurasia, where they winter on twigs and buds. Although most partridge are omnivores, the snow partridge and snow cocks are herbivores found in mountains up to 5000 meters above sea level (Raethal 1975). The hoatzin is a 750 g South American folivore, with a limited amount of breast muscle and chicks that are born with claws on their wings (Niethammer 1975).

Herbivores:  capercaillie grouse,  willow ptarmigan,  hoatzin

    The herbivorous ratites include the rheas, emus, and ostriches, and recently extinct moas.  Rheas weigh up to 25 kg and inhabit the South American steppes and high plains. Emus are Australian birds that reach weights of 55 kg. The largest living birds are the ostriches, which reach weights of 150 kg and  feed on the grass, bushes, and trees of the savannas and deserts of Africa. Evidence of grass and twigs in the crop and fecal specimens of the giant (250 kg) moas of New Zealand indicate that they were herbivores, as well (Sanft et al. 1975).

Herbivorous ratites:  ostrich,  emu,  rhea


Mammals:

    Mammals differ from other vertebrates in their hair-covering, birth at an earlier stage of development, and the fact that they suckle their young.  The more than 4000 species of mammal can be subdivided into the two subclasses Prototheria or monotremes (echidna and platypus) and Theria (marsupial and placental mammals).  They include marine, freshwater, and terrestrial species that inhabit tropical, temperate, desert, and arctic environments.

    Table 1.2 gives the 20 mammalian orders listed by Vaughn (1986) and the diet(s) of inclusive species. Thirteen  orders contain carnivorous species that feed on invertebrates or other vertebrates. Ten orders include species that feed either on plants and animals (omnivores), or plant concentrates such as seeds, fruit, nectar, pollen, or roots.  Eleven orders include herbivores and five of these contain only herbivorous species. The herbivores consist of the largest number of mammalian species, include largest terrestrial mammals, occupy the widest range of habitats, and constitute over 90% of mammalian biomass (Björnhag 1987).

Table 2.2.  Mammalian Orders Listed According to Diet(s) of Included Species
 
(Classification according to Vaughn (1986). Modified from Stevens & Hume 1995)


Mammals - Monotremes, Aardvarks, Scaly Anteaters and Cetaceans:

     The earliest mammals are believed to have been small carnivores, and 13 orders include  species that continue this practice. The monotremes consist of the echidna, a terrestrial anteater, and the aquatic duck-billed platypus, which feeds on mollusks, insects, and worms (Grzimek 1985a).  Both are restricted to Australia and New Guinea. The African and Asian Pholidota (scaly anteaters) and African Tublidentata (aardvarks) also feed on ants or termites.  Cetaceans (dolphins, porpoise, and the toothed and baleen whales) feed on a variety of diets. Dolphins and porpoise feed on fish, killer whales feed on other marine mammals, and sperm whales feed mainly on cephalopods. Baleen whales feed principally on pteropods or small crustaceans. 

Monotremes:  duck-billed platypus,  echidna
  
aardvark,  scaly anteater
 
Cetaceans:  Atlantic bottlenose dolphin,  Atlantic right whale

Mammals - Elephant Shrews, Insectivora, Tree Shrews and Bats:

    The Macroscelidae (elephant shrews) are insectivores indigenous to Africa.  The Insectivora, which includes shrews, moles, tenrecs, and hedgehogs, are mostly insectivores, as the name implies (Fons 1990). However, hedgehogs are omnivores, as are the tree shrews in order Scandentia. The Chiroptera consist of the two suborders Microchiroptera and Megachiroptera.  Microchiroptera  contains 17 families of  insectivorous, piscivorous, sanguivorous (blood-feeding), fruigivorous, or nectivorous bats that are distributed throughout the world. Megachiroptera is comprised of three families of large fruit bats, which inhabit Africa, Asia, and Australia.

Insectivora:  masked shrew,  hedgehog

Chiroptera:  pallid bat,  Rodriguez fruit bat

Mammals - Carnivora:

    The Carnivora, Marsupialia, Edentata, Rodentia, and Primates include carnivorous, omnivorous, and herbivorous species. The Carnivora are comprised of the 10 families of Canidae (dogs, wolves, foxes, jackals), Felidae (cats), Viverridae (civets, genets, mongooses), Hyaenidae (hyenas, aardwolf), Procyonidae (raccoons, pandas, ring-tail cats), Ursidae (bears), Muskelidae (minks, skunks, badgers, weasels, wolverines, otters), Phocidae (earless seals), Otariidae (eared seals), and Odobenidae (walruses).  Most Carnivora feed on other vertebrates, but aardwolves feed almost exclusively on one genus of termite (Anderson et al. 1992), bears and raccoons are omnivores, and the pandas are herbivores.   

Carnivores:  red wolf,  African lion,  aardwolf

Omnivore:  grizzly bear; Herbivore: giant panda

Mammals - Marsupials:

    The marsupials consist of 242 species of mammals listed under the suborder Metatheria and order Marspupialia. However, the various families show as wide a variation in their diet and digestive system as the orders of eutherian mammals (Hume 1999).  Carnivores are found in four Australian and two American families. The Australian families are the Dasyuridae, Myrmecobiidae, Notoryctidae, and the Thalacinidae. The Dasyuridae contain 49 species that range from tiny shrew-like planigales to the 10 kg Tasmanian devil. Each of the other families contain only one species respectively: the insectivorous numbat, marsupial mole, and the Tasmanian tiger, which is probably extinct.  The American families are the Caenolestidae (shrew-possums) and some Didelphidae (opossums).  The omnivorous species include some Didelphidae (opossums), the Peramelidae, (bandicoots and bilbies), the arboreal striped, Leadbeater, eastern and mountain pigmy possums, and the yellow and sugar gliders.

Carnivores:  Tasmanian devil,  eastern quoll

Omnivores:  short-nosed bandicoot,  Virginia opossum

    The marsupial herbivores consist of the Vombatidae (wombats), Phascolarctidae (koalas), Phalangeridae (cuscuses, brushtail, and scaly-tailed possum), Pseudocheiridae (greater glider, and ringtail possum), ) Potoridae (rat-kangaroos), and Macropodidae (kangaroos and wallabies). Wombats are relatively small (20-50 kg) grazers that inhabit the forest and semiarid regions of Australia (Grzimek 1975b). Koalas, cuscuses, and the herbivorous possums and gliders are arboreal folivores. Rat kangaroos are small (1-3 kg) browsers or selective feeders. The kangaroos and wallabies are browsers or grazers that vary in size from the 1 kg hare-kangaroos to 70 kg kangaroos (Grzimek & Heinemann 1975). The small and many intermediate-sized kangaroos and wallabies are browsers, but the larger species are grazers that are distributed in a wide range of habitats including semi-arid environments.
 
Herbivores:  red kangaroo,  hairy-nosed wombat,  koala

Mammals - Edentates:

    Edentates are a relatively primitive group of New World mammals that includes three families and 31 species (Moeller 1990). The Myrmecophagidae are ant eaters and the Dasypodidae (armadillos) feed on a variety of animals and some plant material. The Bradypodidae (sloths) are small (4-9 kg) arboreal inhabitants of Central and South American forests that feed on leaves, young shoots, blossoms, and fruit. Although the name of this order indicates an absence of dentition, all but the ant anteaters have teeth in one form or another.

Edentates:  giant anteater,  nine-banded armadillo,  Hoffman's sloth

Mammals - Rodents:

    The rodents represent the most successful group of  mammals, as measured by the number of families, species, individuals, or breadth of ecological distribution.  They consist of a few carnivores and numerous omnivorous and herbivorous species. The omnivores include many species of rats, mice and Richardson's ground squirrel, which feed on seeds, fruits and carrion. The herbivorous rodents range from 6 g voles to 49 kg capybara and inhabit all types of environment, including the desert species of jirds and the Arctic lemmings.

Rodents:  Norway rat,  pine vole,  ground squirrel,  capybara

Mammals - Primates:

    The primates consist of 230 species that can be divided into two suborders; Stepshirhini (lower primates) and Haplorhini (higher primates).  The Strpshirhini consist of the lemurs, indri, sifakas, aye-aye, pottos, lorises, and bush babies, and include insectivores, species that feed on fruit, leaves, and plant exudates, and a few herbivores.  The higher primates consist of six families; Cercopithecidae (Old World monkeys), Cebidae (capuchins and most other New World monkeys), Callitrichidae (marmosets and tamarins), Hylobatidae (gibbons), and Hominidae (great apes and humans). Old World monkeys inhabit Africa and Asia, and include many omnivores such as the baboon, and a few herbivorous species. The herbivorous Colobiidae monkeys  consist of four genera of langur, snub-nosed, and proboscis monkeys in Southeast Asia and one genus of African colobus monkeys.

    Most Colobiidae occupy rain forests or swamps, but the largest langur, Presbytis entellus schistceus, inhabits conifer and rhododendron forests at elevations up to 4000 m, which can be deeply covered with snow (Feider and Thenius 1975)  Most New world monkeys are omnivores.  Gibbons are Southeast Asian and East Indian arboreal folivores that feed occasionally on insects and small vertebrates. The great apes consist of the orangutans, which inhabit Borneo and Sumatra, and the chimpanzees and gorillas of Africa.  Chimpanzees, like humans, are omnivores, but orangutans and gorillas are chiefly folivorous herbivores.

  Carnivore:  bushbaby

  Omnivore:  baboon

  Herbivore:  colobus monkey

  Herbivore:  orangutan

Mammals - Dermoptera:

    The Dermoptera, which are often referred to as flying lemurs, consist of two species of small arboreal gliders that feed on fruit, buds, flowers, and leaves, and are classified in a separate order.
          
Mammals - Artiodactyls:

    Simpson (1945) divided the Artiodactyla into the suborders Suiformes, Tylopoda, and Ruminantia.  Suiformes consists of the families Suidae (swine), Tayassuidae (javalines or peccaries), and Hippopotamidae (hippos). The Tylopoda consist of the family Camelidae, which includes the Old World (dromedary and bactrian camels) and New World (alpacas, llamas, guanacos, vicugnas) camelids. The Ruminantia  consist of the family Tragulidae (chevrotains or mouse deer), and the more advanced pecora in families Cervidae, Giraffidae, Antilocapridae, and Bovidae.  The pecora include the sheep, goats, deer, moose, elk, giraffes, antelope, cattle, bison, buffalo, yaks, and muskoxen, and many species have adapted to tropical, arctic, desert, or high altitude regions where few other large mammals can survive. The swine are omnivores and the peccaries, chevrotains, and smallest ruminants include seeds and fruit in their diet. However, the hippos, camelids, and most of the advanced ruminants are
strict herbivores.

  Suiformes:  warthog,  collared peccary,  hippopotamus


Tylopoda:  bactrian camel,  alpaca

Pecora:  nubian ibex,  African buffalo,  giraffe,  muskox

Mammals - Lagomorphs and Perissodactyls:

    All species in the remaining mammalian orders are herbivores.  Lagomorphs consist of 11 genera of rabbits and hares, and one genus of pika that weigh from 0.4 to 2.0 kg (Angermann et al. 1972). They are distributed throughout  the world, including its deserts, arctic tundra, and altitudes up to 6000 m. The perissodactyls consist of  the three families of tapir, equine, and rhinoceros species. Tapirs and black rhinos are browsers.  Equids and white rhinos are grazers.  Some equids, such as the African and Asiatic asses inhabit semiarid environments. 

Lagomorphs:  pica, rabbit, hare

  Perissodactyls: rhinoceros,  tapir,  zebra

Mammals - Proboscideans, Sirenians and Hyrocoids:

    Despite marked differences in their appearance, the proboscideans (elephants), sirenians (manatees and dugongs) and hyracoideans (hyraxes) are related to one another, and to the artiodactyls, perissodactyls, and cetaceans under the general heading of ungulates (hoofed animals). Elephants inhabit the forests and steppes of Africa and Asia, grazing on grasses and browsing on shrubs and trees.  Manatees inhabit rivers, river mouths, and fresh water lakes, feeding on fresh water plants, seaweed and marine algae. Dugongs inhabit coastal seas and feed principally on four species of sea grasses (Kurt et al. 1972). The hyracoids consist of one family and three genera that range from 2.5 to 3.5 kg in body weight (Rahm and Thenius 1972). They inhabit the dry climates of Syria, Sinai, and Arabia, and more temperate climates of North and South Africa. The rock hyrax inhabit forests, steppes, and mountainous plains up to 3700 m above sea level. Sahara hyrax can survive practically without drinking water.

Proboscideans:  African elephant; Sirenians:  dugong; Hyracoids:  rock hyrax

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