Mammals: characteristics, classification and examples

Mammals are animals that have two outstanding characteristics: the presence of hair in the body and the presence of mammary glands, which produce milk.

Mammals are vertebrate animals, belonging to the Mammalia class, which stand out for their hair and milk production. Currently there are more than 5,300 known species of mammals, the human being being being one of them.

Main characteristics of mammals

Mammals have two characteristics that distinguish them from other groups of animals. These are:

• Presence of hair. All mammals have hair on their body, this being an important characteristic, next to the fat layer below the skin, for the heating of the animal's body. However, hair not only has this function, it is also related to camouflage and the perception of sensations.
• Presence of mammary glands, i.e. milk-producing glands, which is why mammals receive this name. All females of all species of mammals have the capacity to produce milk and to breastfeed their young. A curious fact is that although they have mammary glands, not all mammals have nipples and milk is ejected through the skin in some species.

Besides these characteristics, others can be observed in this group, but they are not class exclusivities, such as:

• Closed circulation, double circulation and heart with four cavities
• Lung respiration, i.e. all mammals have lungs. This characteristic is observed even in aquatic mammals, such as the whale, which needs to go to the surface to get oxygen. In aquatic mammals a relatively large capacity to store oxygen is observed.
• Urinary system formed by kidneys, ureters, urinary bladder and urethra.
• Nervous system with well developed brain.
• Digestive system formed by a food channel and several accessory glands. An interesting fact is that mammals have teeth adapted to process different types of food. Carnivorous mammals, for example, have well-developed canines to help cut the flesh of their prey. In other vertebrates, the dentition is usually less specialised.
• Presence of separate sexes, i.e. males and females of each species are observed.
• Internal fertilisation.
• Endorthermia. Mammals are endothermic animals, i.e. they keep the body temperature constant.

Classification of mammals

Mammals can be classified into three groups: monotremates, marsupials and euteriums. See more characteristics of each one:

Monotremados: these are animals that have as a remarkable characteristic the ability to lay eggs. Despite this peculiarity, they also feed their young initially with milk, which is sucked directly from their mother's skin, since these animals do not have nipples. Monotremates are found in Australia and New Guinea, echidna and platypus being their representatives.

The platypus is a monotremado.

Marsupials: they are animals that stand out because of the way their puppies develop. In this group of mammals, it is observed that the embryo develops inside the uterus, but is born very early and completes its development inside the marsupial, a kind of pouch. Prawns and kangaroos are examples of marsupials.

The kangaroo is a marsupial.

Euterium: these are mammals that stand out because of the presence of complex placentas. The placenta is an organ that appears during gestation and has, among others, the function of ensuring the exchange of nutrients between the mother and the cub. Unlike marsupials, the development of these mammals is completed completely inside the uterus. Human beings, dogs, cats, monkeys, whales, oxen, deer, zebras, armadillos and elephants are examples of euterium.

The monkey is an euterium.

Examples of mammalian animals

We know that there are over 5,300 species of mammals, and many of these are known to us. Here are some examples:

• gorillas
• boars
• bobcats
• hyenas
• yaks
• platypuses
• pikas
• mules
• Humans
• wolverines
• leopards
• rats
• pumas
• tree kangaroos
• kangaroos
• pigs
• echidna
• tigers
• bonobos
• donkeys
• porpoises
• fishing cats
• hedgehogs
• bears
• dogs
• buffalo
• orangutans
• bison
• dingoes
• lynxes
• beavers
• moose
• sheep
• hippos
• elephants
• antelope
• foxes
• gibbons
• horses
• wolves
• goat
• weasels
• whales
• hogs
• lions
• shrews
• cheetahs
• sea lions
• jaguars
• coyotes
• anteaters
• cows
• badgers
• monkeys
• rabbits
• mice
• hares
• hyraxes
• rhinos
• sea otters
• otters
• dolphins
• squirrels
• moles
• cats
• bats
• koalas
• seals
• ocelots
• meerkats
• deer
• naked mole rats
• chimpanzees

Summary on mammals

• There are over 5,300 species of mammals.
• Mammals are animals that have hair and mammary glands.
• Mammals have closed circulation; heart with four cavities; urinary system with kidneys and bladder; digestive system formed by a food channel and associated glands; developed encephalus; and breathe through lungs.
• Mammals are classified into three groups: monotremates, marsupials, and euteriums.
• Ornithorrins and echidna are examples of monotremates.
• Prawns and kangaroos are examples of marsupials.
• Human beings, dogs and horses are examples of euterials.