Mammals: General Characters and Its Classification

 

Mammals: General Characters and Its Classification

Definition of Mammals:

An animal of the type that gives birth to live babies, not eggs, and feeds its young on milk from its own body.

Or

Mammals are vertebrates with hair, mammary glands used to suckle young with milk, a presence of pinna, and a lower jaw made up of a single pair of bones that articulates in a unique way with the skull.

Mammals are a diverse group, but all mammals have unique characters are,

• ·         Produce milk to feed their young
• ·         Have hair or fur
• ·         Have a unique jaw structure
• ·         Are warm-blooded

Mammal groups

Mammals are divided into three groups –

• 1.      Monotremes,
• 2.      Marsupials
• 3.      Placentals,

All of which have fur, produce milk and are warm-blooded.

• ·    Monotremes are the platypus and echidnas and the females lay soft-shelled eggs.
• ·    Marsupials give birth to small, poorly developed young and most female marsupials, such as kangaroos, wallabies and the Koala, have pouches.
• ·   Placental mammals, like humans, whales, rodents and bats, differ from monotremes and marsupials in that they generally give birth to well-developed young.

Mammals can be defined as ‘highly percipient and mobile animals, with large brains, spiral cochlea, warm blood, left aortic arch, and water-proof, usually hairy skin, whose young are born alive, and are nourished by milk. 

Kingdom: Animalia

Phylum: Chordata

Superclass: Tetrapoda

Clade: Amniota

Class: Mammalia

General Characters of Mammals:

1. Body of mammals is covered by epidermal hair.

2. Integumentary glands are — sweat (sudoriferous), sebaceous (oil), scent (odoriferous) glands.

3. Mammary glands are present to supply milk for the nourishment of suckling young.

4. External fleshy pinna is present in mammals.

5. Eyes with upper and lower eyelids and often with eyelashes.

6. Nictitating membrane is translucent and hairless; it is vestigial in higher mammals.

7. A muscular diaphragm is present in between the thoracic and abdominal cavities.

8. Endo-thermal homoeotherm animals.

9. RBCs are non-nucleated, biconcave and usually circular in form.

10. The four-chambered heart is highly powerful.

11. Only left aortic arch is present in the arterial system.

12. Cerebral hemispheres are very large and highly convoluted.

13. Cerebellum is large, complex and solid in mammals.

14. There is a single urinary bladder in mammals.

15. Testes remain in scrotal sacs.

16. Small eggs are devoid of yolk. Fertilization is internal.

17. Mammals are viviparous animals.

18. The skull has double occipital condyles. Quadrate absent.

19. A bony palate is formed by the union of premaxillae, maxillae and palatines that separates the

nasal passage from the buccal cavity.

20. The lower jaw is composed of a pair of bones — the dentaries.

21. Vertebrae are acoelous type.

22. Ribs are double-headed — capitulum and tuberculum.

23. The teeth are heterodont, thecodont and diphyodont type.

24. Molars are tribosphenic (three-cusped).

25. Paired forelimbs and hind limbs are present in mammals.

26. The digits of the limbs are provided with either claw or nail or hoof.

27. Cranial nerves twelve pairs.

28. Kidneys are metanephric type

THERE ARE THREE SUBCLASSES OF MAMMALS

These are:

1. Prototheria – egg laying animals, several species found only in Australia,(eg. the platypus and

echidna)

2. Metatheria – marsupial animals found in Australia and South America, and (includes the marsupials)

3. Eutheria that includes majority of contemporary species and is distributed in all continents of our

planet. (Includes humans and other placental mammals)

Detailed description

Subclass — Prototheria (Greek: protos = first, therion = beast):

Infraclass Ornithodelphia – monotreme mammals

Prototherians are considered to be the most primitive mammals which are only restricted in Australia

and its neighboring islands (Tasmania New Guinea). Besides egg-laying habit, they have several

reptilian characters including a cloaca. They lay eggs containing ample amount of yolk. Subclass

prototheria includes one order Monotremata e.g., Omithorhynchus, Tachyglossus- (Echidna).

Order Monotremata - monotremes; egg-laying (oviparous); "one hole" - spiny anteater (=echidna), duckbilled platypus

General Characters:

1. The females lay eggs.

2. The testes are abdominal.

3. The cloaca receives the openings of urinary bladder, vas deferens and ureters.

4. Ribs possess single head.

5. The mammary glands lack teats.

6. External pinna absent.

7. In childhood, teeth are present but adults lack teeth.

The characteristics of Prototheria are given below:

1. External ear is absent.

2. They are united by having, for at least some period of their life cycle, a notochord, a hollow

dorsal nerve cord, pharyngeal slits, an endostyle, and a post-anal tail.

3. Cephalochordates have a notochord and a nerve cord and a very simple circulatory system.

4. Attempts to work out the evolutionary relationships of the chordates have produced several

hypotheses.

5. All of the earliest chordate fossils have been found in the Early Cambrian Changing fauna, and

include two species that are regarded as fish.

6. There is famous shale called Middle Cambrian Burgess Shale of British Columbia, which has

yielded pikaia fossils.

7. The segmental arrangement of the mooneyes facilitates the lateral movements used during

swimming.

8. Corpous callosum is absent.

9. Testes are abdominal.

10. Egg lying mammals, placenta is absent

Affinities of Prototheria

Members of the subclass Prototheria lay eggs like most non-mammalian vertebrates. However, they

feed their newborn with mammary gland secretions like all other mammals. They lack nipples, but the

skin over their mammary glands exudes milk for their babies. The Prototheria are also referred to

as monotremes which literally mean that they have one opening for excretion and reproduction. This is

similar to birds and reptiles. The Prototheria are also similar to reptiles in some aspects of their

skeletons. Notably, their legs are on the sides of their bodies rather than underneath them. This results

in a reptile-like gait. There are only three surviving rare species groups of Prototheria. These are the

Australian platypus and 2 echidna (spiny anteater) species of Australia and New Guinea.

Subclass Theria

Infraclass Metatheria – (Marsupialia) – marsupial mammals (pouched mammals) (many species found

in Australia, also quite a few in South and Central America)

Metatheria:

The characteristics of Metatheria are given below:

1. Marsupium is present in females.

2. Mammary glands are developed.

3. Corcoid and Interelative are absent.

4. Epiphyses on vertebrae present.

5. Corapus callosum is poorly developed.

6. Vagina and uterus are double.

7. Placenta is absent.

8. Viviparous.

9. Marsypial bone is present.

Females have a marsupium or brood-pouch for rearing young ones. Infraclass metatheria includes one

Order Marsupialia. Mammals of this order are called marsupials or pouched mammals, e.g., Macropus,

Didelphis (Opossum) and Phascolarctos (Koala).

Subclass — Theria (Greek: therion = beast):

General Characters:

1. Female members of this subclass do not lay eggs but give birth to young ones.

2. Mammary glands are provided with nipples or teats.

3. Pinna or external ear is present.

4. The ureters open directly into the urinary bladder.

5. At the end of the digestive tract an anus is present.

6. Teeth are present throughout the life period.

7. Testes are situated in the scrotum.

8. Ribs possess two heads for articulation with vertebrae.

Infraclass — Metatheria (Greek: Meta = next to):

General Characters:

1. The youngs are born in an immature condition and undergo further development in the marsupium of

females.

2. Mammary gland with teats opens into the marsupium.

3. Epipubic bone of the pelvic girdle protects the marsupial sac.

4. Placenta is chorioviteline type.

Order — Marsupialia (Latin: Marsupium = a sac):

General Characters:

1. Body in covered with soft fur.

2. Pinna is well developed.

3. Most of the female members possess marsupium.

4. Tail is well-developed and helps in balancing.

5. The second and third toes of the hind-limb are slender and remain enclosed in a sheath of skin,

i.e., syndactylous digits. The fourth toe is largest. All digits are clawed.

6. Forelimbs are shorter than the hind limbs.

7. Dental formula is i = 5/4, c = 1/1, p = 3/3, m= 4/4.

8. Caudal vertebrae are with chevron bone.

9. Atlas is incomplete and is provided with cartilage in the ventral incomplete side.

Examples:

Macropus (Kangaroo), Didelphis (Opossum), Thylacinus (Tasmanian wolf – the mystery marsupials),

Myrmecobius (Banded ant-eater), Nottoryctes (Marsupial mole), Petaurus (Flying opossum),

Phascolarctos (Koala bear), Vombatus (Wombat), etc.

Infraclass — Eutheria (Greek: eu = true):

General Characters:

1. The young are born as miniature adult and go through a considerable period of prenatal growth.

2. A highly-organised allantoic placenta attaches firmly with the uterine wall during developmental

period.

3. Brain is highly-developed, cerebral hemispheres have well-developed neo- pallial region. The

two hemispheres are connected by corpus callosum.

4. The anal and urinogenital apertures are separate.

5. The tympanic bone is ring-like and forms a tympanic bulla.

6. Dental formula is i = 3/3, c = 1/1, pm = 4/4, m = 3/3. In some forms there are modifications in

dental formula, and in some cases teeth are absent.

7. Epipubic bone in the pelvis is absent.

Order — Insectivora (Latin: insecta= in sects; voro = to eat):

General Characters:

1. Body covered with hair. Some members possess dorsal spines which are modified hair.

2. Nocturnal animals with long snout are insectivorous.

3. Skull is constricted in the middle.

4. The zygomatic arch and bony palate are incomplete.

5. Teeth have sharp molar cusps.

6. Each limb possesses five digits with claws.

7. Locomotion is plantigrade type.

8. Caecum in the intestine is small or absent.

9. Scrotum is absent and the testes are internally situated.

10. Uterus is bicornuate type.

11. Mammary glands are many and are distributed all along the two milk-lines on the ventral

surface.

Example:

Talpa (Mole), Tupaia (Tree- shrew), Erinaceus (Hedgehog), Sorex (Shrew), Desmana (Water mole),

Chrysochloris (Golden mole), Neomys (Water shrew).

Order — Chiroptera (Latin: cheir = Hand; pteron = wing):

General Characters:

1. The forelimbs are modified to form wings.

2. The bones of the digits of the forelimbs are elongated except pollex. These bones support a

membrane that runs between forelimbs and hind limbs. This membrane is called as patagium.

3. A short tail is often included in the inter-femoral membrane.

4. The wings are having direct arteriovenous connections.

5. Pollex is small, free from the wing and bears claw.

6. The hind limbs are weak, having five clawed-digits.

7. Pinna is well-developed.

8. These are nocturnal animals. They are able to fly and catch prey in the dark with the help of their

special radar system. This capacity is called echolocation.

9. The sutures of the skull is obliterated.

10. The ribs are flat and fused with the vertebrae to become rigid during flight.

11. The hind limbs are rotated, so the knee is directed backward.

12. The testes are abdominal in position.

13. Only one young is born at a time.

14. The sternum is provided with a flat keel for the attachment of pectoral muscle.

Examples:

Pteropus (Fruit bat) (Fig. 1.129A and C), Rhinolophus (Horseshoe bats), Desmodus (Vampire bats),

Vespertilio (European bats)

Order — Primates (Latin: primus = first; Primate = One first in rank):

General Characters:

1. Body is covered with thick hair except the palm, sole and some parts of the face.

2. Neck is short and mobile.

3. Forelimbs are shorter than hind limbs.

4. Pentadactyl limbs possess digits with flat nail.

5. The pollex or thumb, hallux or first toe are smaller than other digits and are opposable.

6. Locomotion is plantigrade type.

7. A tail is present.

8. Mammal are two and thoracic in position.

9. Testes lie in scrotal sac.

10. Highly developed brain possesses much convoluted cerebral hemispheres.

11. The eyes are directed forward and the vision is binocular and stereoscopic.

12. Teeth show reduction in number.

13. The skull is more inclined to the vertebral axis.

14. Zygomatic arch is complete.

15. Foramen magnum faces downward.

Example:

Homo sapiens (Man), Gorilla, Presbytis (Langur), Macaca (Rhesus monkey, Macaque), Papio (Babon),

Hylobates (Gibbon), Pongo (Orangutan), Pan (Chimpanzee), Callithrix (= Hapale) (Marmoset), Loris

(Slender loris), Lemur, etc

Order — Rodentia (Latin: rodere = to gnaw):

General Characters:

1. Body is covered with soft hair.

2. Eyes are small but pinna is well-developed.

3. Limbs are provided with blunt claws.

4. Forelimbs are smaller than the hind limbs.

5. Locomotion is plantigrade type.

6. Jaw muscles are well developed for gnawing. Intestine and caecum are large.

7. Testes are inguinal.

8. Prolifically reproducing animals. Females possess abdominal teats.

9. Single pair of large, chisel-shaped incisors are present both in upper and lower jaws.

10. Canine and anterior premolars are absent forming a diastema between incisors and cheek teeth.

11. There are two premolars in the upper jaw and one in lower jaw.

Example:

Sciurus (Squirrel), Petaurista (Flying squirrel), Rattus (Rat), Mus (House mouse), Hystrix (Porcupine),

Cavia (Guinea- pig) (Fig. 1.130), Bathyergus (Mole-rat), Cricetus (Hamster), Mesocricetus (Golden

hamster), Microtus (Vole), Dipus (Jerboa), Castor (Beaver), Geomys (Pocket gopher), Dipodomus

(Kangaroo rat), Citellus (Ground squirrel), Marmota (Moarmot, woodchuck).

Order — Cetacea (Latin: cetas = whale):

General Characters:

1. The large, torpedo shaped body devoid of hair.

2. The skin is smooth and skin glands are absent.

3. The nictitating membrane of eye, pinna of ear and nail of the digits are absent.

4. Forelimbs are modified to form flippers, hind limbs are absent.

5. The nasal openings are asymmetrical, located far back on the upper surface of the head and can

be closed by valves.

6. The tail terminates in a horizontal fin, called fluke. It is a neomorphic structure.

7. Dorsal fin is fleshy; it is also a neomorphic structure.

8. A thick subcutaneous fat layer, called blubber, is present.

9. The lungs are highly elastic and extensible.

10. Brain is highly developed but olfactory lobe is reduced.

11. Two mammary glands are located in the inguinal area.

12. Single, large, well-formed young is born at a time.

13. The cranium is dorsoventrally flattened and the facial part is elongated.

14. Cervical vertebrae are fused to form a bony mass.

15. Sacral vertebrae are absent.

16. Caudal vertebrae are with chevron bones.

17. Ribs lack heads.

18. Number of digits are either reduced to four or increased to more than five (hyperdactyly).

Numbers of phalanges are more than the usual number in second and third digits

(hyperphalangy).

Example:

Platanista (Ganges dolphin), Physeter (Sperm whale), Delphinus (Dolphin), Phocaena (Porpoise),

Balaenoptera (Blue whale), Balaena (Right whale).

Order — Carnivora (Latin: carno = flesh; voro = to eat):

General characters:

1. Pentadactyle limbs, with digits ending in sharp claws and claws may be retractile.

2. Locomotion is either digitigrade or plantigrade type.

3. Intelligence in the form of mental alertness and coordinated actions is evident.

4. Brain is highly developed.

5. Intestine is short and caecum is small or absent.

6. Testes are present in scrotal sac.

7. Mammary are abdominal in position.

8. Placenta is deciduate and zonary.

9. Uterus is bicornuate in shape.

10. Skull is short. Sagittal and lambdoidal crests are well-developed.

11. Zygomatic arch is strongly built.

12. Each jaw possesses three pairs of incisors. Canines are large, sharp and pointed.

13. The last upper premolar and first lower molar transformed into blades, and act as incisors — they

are jointly known as carnassial teeth.

14. The atlas is large and is provided with wing like lateral processes.

15. Thoracolumbar vertebrae are 20 to 21 in number.

16. Sternum is long narrow and made up of 8 to 9 sternibrae. Sternal ribs are not calcified.

Examples:

Canis (Wolves, Dogs, Jackals etc.), Vulpes (Fox), Ursus (Bear), Procyon (Raccon), Ailurus (Panda),

Ailuropoda (Giant Panda), Viverra (Civet), Herpestes (Mongoose), Hyaena (Hyena), Felis (Cats, Pumas,

Leopards, Lions, Tigers, Jaguars), Eumetopias (Sea lion), Odobenus (Walrus), Phoca (Seal), Mephitis

(Shunk), etc.

Order — Perissodactyla (Greek: perissos = odd; daktylos = finger):

General Characters:

1. These are large, herbivorous mammals having streamlined body.

2. The neck and facial parts are elongated.

3. Tail is with long tuft of hair.

4. The powerful limbs are suitable for fast movement.

5. The lower part of the limbs became elongated and the upper segments shortened.

6. One distal carpal, the capitate (magnum), become enlarged and interlocked with the proximal

carpals.

7. Of the five digits, the first and fifth digits are lost. The second and fourth digits remain as splints.

The middle or third digit is stout and is provided with hoof.

8. Stomach is simple and undivided.

9. Digestion of cellulose takes place by symbionts in the caecum and large intestine.

10. Brain is relatively small and macrosmatic type. Olfactory lobe is highly developed.

11. Skull is elongated.

12. The incisors are three in each quadrant of the jaws. The incisors having pit on the free surface.

13. The canine may be reduced or absent and there is often a diastema.

14. The molars have developed an elaborate grinding surface with the formation of a longitudinal

ectoloph along the outer edge of the upper molar and parallel transverse ridges, the protoloph and

metaloph.

15. Ulna and fibula are reduced.

16. The femur is provided with a prominent process on the other surface of the shaft. The process is

called third trochanter.

17. Astragalus has a double-keeled pulley-shaped surface for articulation with tibia.

18. The third metatarsal of the hind limb is long and erect and is known as cannon bone.

19. Uterus is bicornuate. Placenta is of diffuse epitheliochorial type, with a large allantoic sac.

20. The yolk sac grows to a large size and forms a yolk sac placenta during the early part of the

development.

Example:

Tapirus (Tapir), Rhinoceros (Rhinos — Indian and Javan), Dicerorhinus (Horses, Asses, Zebras).

Order — Artiodactyla (Greek: Artios = even; daktylos = finger):

General Characters:

1. These are even toed ungulates and latest mammalian herbivores.

2. Neck is elongated but tail is small.

3. Usually possess a pair of epidermal horns. Horns may be hollow or solid and are located on the

frontal bone of the skull.

4. The characteristic of the limbs is the equal development of third and fourth digits, with reduction

of the rest.

5. Gait is digitigrade type. Hoofs have developed on the toes.

6. The long metapodials have become united to make the cannon bone.

7. The presence of two digits has led to the retention of two bones in the distal row of carpals, the

hamate (unciform) and fused magnum-trapezoid. These articulate in interlocking fashion with

the three proximal carpals.

8. In the hind foot the two cuneiforms are fused to provide thrust upon the third digit, while the

fourth sends its thrusts to the cuboid and the latter is fused with the navicular.

9. The eyes are large with horizontal pupil. Pinna is large with an acute sense of hearing.

10. Tongue is long, mobile, prehensile and pointed.

11. The upper incisors are lost, which crop up by means of the lower incisors biting against the

hardened gum of the pre- maxilla.

12. The canine may form tusks.

13. Molars are of hypsodont and solenodont (moon-tooth) condition.

14. Stomach is complicated and divided into several chambers.

15. Intestine is short and a short caecum is present.

16. Mammae are abdominal or inguinal in position and may be more than one pair.

17. Brain is moderately developed. The olfactory organ and related parts of the brain are welldeveloped.

18. The uterus is bicornuate type.

19. Placenta of pig is of diffused epitheliochorial type. In ruminants there is a cotyledonary placenta,

but the contact between maternal and foetal tissues is never very close (Syndesmochorial) and

the allantois is usually large.

Examples:

Sus (Pig), Hippopotamus (Hippo), Camelus (Camel, dromedary — Asia), Moschus (Musk deer), Ceruus

(Red deer), Dama (Fallow deer), Rangifer (Rein deer), Giraffa (Giraffe), Gazella (Gazelles), Bos (Cattle,

Yak), Bison (Buffalo), Capra (Goat), Ovis (Sheep).