Mammals: Classification, and Characteristics

Mammals belong to the class Mammalia, which is one of the five main classes of vertebrates. 

Characteristics of Mammals

  1. Mammary glands: Mammals possess specialized mammary glands that produce milk to feed their young offspring.
  2. Hair or fur: Most of them have hair or fur covering their bodies, providing insulation, protection, and sensory functions.
  3. Warm-bloodedness: Their body temperature doesn’t change with the changing temperature of surrounding; have autonomous control over their internal body temperature
  4. Direct birth: They give direct birth to live young, with well-developed offspring.
  5. Efficient respiration: They have a muscle structure called a diaphragm which helps in breathing and ensures an effective exchange of oxygen and carbon dioxide.
  6. Highly developed brains: They often have advanced nervous systems & well-developed brains, enabling highly developed behavioral and cognitive abilities.
  7. Specialized teeth: The different types of teeth are present, adapted for various feeding habits, such as incisors, canines, premolars, and molars.
  8. Four-chambered heart: They have a four-chambered heart, which helps to circulate blood by dividing oxygenated from non-oxygenated blood.
  9. Parental care:  Mammals display varied levels of parental care, with many species displaying caring traits toward their young.
  10. Placenta: They have a placenta that is fully established, which is an intricate organ that joins the growing fetus to the mother’s uterine wall and facilitates gas and nutrition exchange.
  11. Efficient digestive system: Mammals have distinct digestive systems that enable the digestion and absorption of nutrients from various kinds of foods.
  12. Extensive behavioral adaptations: They engage in a variety of activities, such as social interactions, communication, tool usage, and problem-solving skills.
  13. Well-developed senses: Mammals often have highly developed senses, including sight, hearing, smell, taste, and touch that allow them to understand their immediate surroundings and connect with other living things.
  14. Diverse reproductive strategies: Depending on the species, mammals display a range of reproductive behaviors, such as monogamy, polygamy, and promiscuity.
  15. Ecological diversity: Mammals are found in a wide variety of ecological niches, from terrestrial to aquatic settings, and have effectively adapted to a range of ecosystems across the world.


Kingdom:      Animalia (animals)

Phylum:         Chordata (chordates)

Subphylum:             Vertebrata (vertebrates)

Class:              Mammalia (mammals)


The class Mammalia is further divided into several taxonomic ranks. Within the class Mammalia, there are three major groups, called subclasses, based on reproductive characteristics:

1. Subclass: Prototheria (Monotremes)

The most simplest animals in this category are those that produce eggs rather than live offspring. Monotremes possess distinctive reproductive and physiological traits. Echidnas and platypus are two examples of monotremes.

Features of Prototheria

  1. Egg-laying:
  • The only class of mammals that lay eggs are prototheria. 
  • Their reproductive system has characteristics from both reptiles and mammals. 
  • The eggs are leathery and resemble reptile eggs in yolk composition.

2. Cloaca:

  • Cloaca, a single orifice for elimination of waste and reproduction, is a feature of monotremes. 
  • Reptiles, birds, and certain other early vertebrate species share this physical trait.

3. Lack of Nipples:

  • They do not have nipples. 
  • Although they produce milk, it is only released by specialized skin growths known as mammary glands. 

4. Presence of Electroreception:

  • Electroreceptors are found on the snouts of monotremes, most notably the platypus. 
  • These sophisticated sensory organs help them in their hunting by enabling them to recognize electric fields produced by the motions of prey in water.

5. Unique Skeletal Features:

  • Prototherians have unique skeletal characteristics. 
  • For instance, they have a shoulder girdle that is separate from the sternum, which gives their forelimbs additional mobility. 
  • Male monotremes also have a cloacal spur that releases venom during territorial disputes.

Prototheria consists of only one order.

OrderExamples of Species
MonotremataPlatypus, echidnas
Prototheria order and examples

2. Subclass: Metatheria (Marsupials)

Marsupials are characterized by giving birth to relatively undeveloped live young, which then continue their development in an external pouch. Marsupials have a distinct reproductive system and are found mainly in Australia and the Americas. Well-known marsupials include kangaroos, koalas, and opossums.

Features of Metatheria

  1. Pouch Development:
  • The reproductive system of marsupials, which involves the growth of their offspring in an external pouch, is one of their most distinguishing characteristics. 
  • The undeveloped young, known as joeys, burrow into the mother’s pouch after a brief gestation period where they continue to grow and get nutrition from specialized mammary glands.

2. Underdeveloped Newborns:

  • Marsupials give birth to young that are comparatively underdeveloped. 
  • These infants arrive in the world with undeveloped organs and restricted movement. 
  • In the pouch, where they are sheltered and cared for by the mother, they finish developing and carry on nursing.

3. Placenta Functionality:

  • Despite having a placenta, marsupials’ placentas are less developed than those of eutherian mammals (placentals). 
  • In marsupials, the placenta mostly produces hormones and exchanges a little amount of nutrients to promote early embryonic development.

4. Diverse Ecological Adaptations:

  • The ecological niches to which marsupials have adapted are numerous. 
  • They may be found in a variety of environments throughout Australia, the Americas, and some regions of Southeast Asia. 
  • Herbivorous, carnivorous, arboreal, and terrestrial species are included in this varied group, demonstrating how well-adapted they are to various settings.

Orders and Examples of Metatheria

OrderExamples of Species
DidelphimorphiaVirginia opossum
PaucituberculataShrew opossums
MicrobiotheriaMonito del monte
DasyuromorphiaTasmanian devil, quolls
PeramelemorphiaBandicoots, bilbies
NotoryctemorphiaMarsupial moles
DiprotodontiaKangaroos, wallabies, koalas
PhalangeriformesPossums, gliders
MacropodiformesWallabies, kangaroos
BurramyidaePygmy possums
PetauridaeSugar gliders
Metatheria order and examples

3. Subclass: Eutheria (Placentals)

Placental mammals are the most varied and successful category of mammals. They deliver birth to full grown live young that are fed by a placenta throughout pregnancy.  Eutherians are found all over the world and contain a diverse variety of animals, from mice and bats to primates and whales.

Features of Eutheria

  1. Placental Development:
  • The placenta, a specialized organ that provides for nutrition and waste exchange between the mother’s body and the developing baby, is well-developed in Eutherian animals. 
  • During pregnancy, the placenta supplies sustenance, oxygen, and waste removal, allowing the fetus to grow fully before delivery.

2. Viviparity (Live Birth):

  • Eutherians give birth to fully mature live offspring rather than laying eggs or carrying undeveloped young in a pouch. 
  • Because of the prolonged gestation time permitted by the placenta, the children are born in a more advanced stage than monotremes and marsupials.

3. Nourishment via Mammary Glands:

  • Eutherian animals have well-developed mammary glands, allowing them to produce milk for their babies. 
  • The milk offers critical nutrients and antibodies, helping the survival and development of the young.

4. Diverse Adaptations:

  • Eutherians have a remarkable spectrum of adaptations and occupy a variety of ecological niches. 
  • They have evolved to flourish in a variety of situations, including terrestrial, aquatic, and airborne habitats. 
  • Primates, carnivores, ungulates, rodents, and cetaceans (whales and dolphins) are all examples of Eutherians.

5. Global Distribution:

  • Except for a few distant and severe settings, eutherian animals have a worldwide distribution and can be found in practically every ecosystem on the planet. 
  • They have colonized different continents effectively, demonstrating adaptability and ecological success in varied habitats.

The subclass Eutheria can be further divided into various orders based on their evolutionary relationships and shared characteristics. 

OrderExamples of Species
PrimatesHumans, monkeys, apes
CarnivoraLions, tigers, dogs, cats
RodentiaMice, rats, squirrels
CetaceaWhales, dolphins
ArtiodactylaCows, deer, pigs
PerissodactylaHorses, rhinos, zebras
LagomorphaRabbits, hares
SoricomorphaShrews, moles
PilosaSloths, anteaters
CarnivoraBears, seals, otters
Eutheria order and examples

Learn more:

Connecting links of evolution

Binod G C

I'm Binod G C (MSc), a PhD candidate in cell and molecular biology who works as a biology educator and enjoys scientific blogging. My proclivity for blogging is intended to make notes and study materials more accessible to students.

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