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9.3.4. Mammals

Class Mammals

Mammals are a class of homeothermic (warm-blooded) vertebrate animals that are characterized by the presence of mammary glands that produce milk with which they feed their young.

Many mammals are terrestrial, although there are also some aquatic ones. There are even mammals that can fly.

Almost all mammals are viviparous, except for the platypus and the echidna, which are oviparous and do not have breasts. They produce a milk that they secrete through the pores of the skin of the abdomen.

The mammalian body is divided into three parts:

  • Head:
    • The neck unites the head with the trunk, and facilitates the movement of the head towards the stimuli that occur in the environment.
    • In the head is the brain, protected by the skull, and most of the sense organs are located:
      • They have ears, which some can point to where the sound is coming from.
      • The eyes allow them to capture images, during the day or at night, depending on the type and number of cells in their retina.
      • The touch is distributed throughout the skin, although it is much more developed in the oral area.
    • The mammalian mouth has lips and teeth.
      • The lips allow the young to suckle from their mother without causing injury.
      • There are some exceptions: armadillos, sloths , and anteaters do n't have teeth.
      • Teeth have different shapes depending on the function they perform:
        • The incisors cut.
        • The canines tear.
        • The premolars tear and grind.
        • The molars grind.
      • The teeth also vary according to the type of feeding of the mammal:
        • Herbivores have highly developed incisor teeth.
        • Carnivores have more developed canines.
        • Omnivores have all teeth of a similar size.
        • Microphages can replace their teeth with baleen, as in whales, which allow them to filter the water and feed on the small organisms it contains.
  • Trunk:
    • It contains the internal organs of the animal, protected by the skeleton.
    • At the end of the trunk, in most mammals, is the tail.
    • Extremities:
      • They are tetrapods, with four legs that help them move, although by evolution they have adapted to other environments:
        • Aquatic mammals, such as seals and whales, have transformed their limbs into fins that make it easier for them to move through the water.

León marino en el Palacio de la Magdalena (Santander)

León marino en el Palacio de La Magdalena (Santader)

Flying mammals, such as bats, have webs between the toes on their forelimbs that turn them into wings that allow them to fly.

Almost all mammals have their bodies covered with hair, which serves to insulate them from cold and heat and keep their body temperature constant. They are homeothermic (or endothermic) beings.

Mammals can have different types of food: carnivores, predators, scavengers, herbivores or omnivores.

All mammals are lung-breathing, including aquatic mammals as well.

Your heart  consists of two atria and two ventricles.

The reproduction of mammals is sexual, with internal fertilization. Only a few, the most primitive ones, are oviparous, but most are viviparous. The most evolved have the placenta, an organ that allows the young to be fed in the mother's womb, so they are born already highly developed, although they have to be cared for by their mothers until they can fend for themselves.

Depending on how the young develop, mammals can be classified into three groups:

Prototheria (monotremes)

  • They are the oldest mammals, they are still oviparous (like reptiles and birds).
  • They have a horny beak, without teeth.
  • His skin is covered by hair.
  • Although they do not have breasts like other mammals, they also produce milk to feed their young. They secrete milk through pores located on both sides of the abdomen. They do not have nipples and their young lap up the milk instead of suckling.
  • The monotreme name means "a single hole", because it has a cloaca, where the digestiveurinary and reproductive systems converge.
  • Examples of monotremes are the platypus (aquatic) and the echidna (terrestrial).

Curiosity: Do you know the echidna?

Surely you know that they are very rare mammals, but do you want to know more?

Did you know that her milk is pink?

Did you know that his penis looks like an arm with four fingers?

Check out this interesting and entertaining twitter thread from Dr. Bioblogo about echidnas.

Marsupials or metatherians

  • They are viviparous animals that are characterized by having a short development in the  maternal uterus, since they do not have a placenta. Later, they complete their development attached to the mammary glands inside the marsupial bag or marsupium. For example, the kangaroo and the koala.

Placental or eutherian

  • They are the most evolved mammals.
  • They are viviparous. The young develop inside the mother's uterus for a long time and are nourished by the placenta, with which it is in contact with the mother. They are born already quite developed.
  • When they are born, the young feed on the milk produced by the mother's mammary glands.
  • There are many different placental orders :
    • Carnivores: like the dog, cat, dog, seal, etc.
    • Insectivoresmoles.
    • Chiroptera: bats.
    • Rodents: mice, shrews.
    • Primates: monkeys and humans.

Mamífera con cría en brazos

Gorila con su cría en brazos

Comparison between monotremes, marsupials and placentals

monotremes marsupials Placentals
They are oviparous They are viviparous They are viviparous
The young develop into eggs The young complete their development in the marsupial bag or marsupium The young develop inside the uterus, nourished by the placenta.
They produce milk, but do not have breasts or nipples. They have mammary glands in the marsupium They have mammary glands
They can be aquatic, like the platypus, or terrestrial, like the echidna They are terrestrial, like the kangaroo and the koala They can be terrestrial, like cats, dogs, and aquatic, like dolphins and whales.


A very important type of mammal, especially for us, is that of humans . Humans are classified within the primates , along with gorillas, chimpanzees, orangutans, etc.

Some of the characteristics of humans are:

  • Bipedalism. Humans are capable of walking upright on their lower limbs.
  • Presence of opposable thumb that allows him to manipulate objects.
  • Great brain development that allows you to communicate through a very complex language.
  • They only have hair on some areas of the body, reducing to fine hair on the rest of the body.