What Are Mammals?

Mother elephant nursing her baby with nutritious milk.
Mother elephant nursing her baby with nutritious milk.

The term “mammals” was invented in 1758 by a scientist and researcher known as Carl Linnaeus to refer to animals that suckle their offspring using milk produced from an organ known as mammary glands. Mammals nourish their young ones with the nutritious milk. As the baby grows it develops a backbone that supports its body. At some point in life, they grow hair which is used primarily to retain heat. The hair can also be used as camouflage in case of danger. The offspring also develops three hard tissues in their ears which aid in the hearing process. Mammals can be huge, medium, or small in size. Their teeth grow and are replaced once in their lifetime. Their blood is warm, meaning it is not influenced by the surrounding temperature. Examples of mammals include human beings, lions, whales, bats, and monkeys.

​Formation and Categories of Mammals

Mammals have gone through several stages of evolution from the first appearance of their Synapsid ancestors. They have changed form, structure, and outlook constantly through the process of evolution with some becoming extinct in the process. Today, mammals are the only existing synapsids. The first mammal appeared approximately 225 million years ago during the Late Triassic epoch, roughly 40 years after the first therapsids. Therapids went through series of stages before the mammals were fully formed. The oldest known remains are more than 125 million old and were of an underdeveloped mammal (Sinodelphys).

Mammals are classified into three subclasses based on their characteristics and structure. The three subclasses include Prototheria, Metatheria, and Eutheria. Metatheria and Eutheria have been grouped together to form a subclass of known as Theria. Most of the Prototheria mammals are extinct. Monotremata is the known order belonging to the subclass Prototheria. The three subclasses are further divided into 28 orders and 161 families including about 5,000 species.

Where Do Mammals Live?

Interestingly, mammals are found across the globe and can live on land, in water, or both. Those that inhabit the land form the bigger percentage. They obtain their nourishment and all needs on the land. Most of the land mammals cannot survive in water. A small number are partly aquatic and come to land to perform some functions like reproduction or to get food. Some of the known aquatic mammals include seals and whales. They are adapted to an aquatic lifestyle.

​How Do They Reproduce and Develop?

Female mammals bring forth offspring after a mating session. Approximately 3% of females mate with one partner while the rest have multiple partners. In some species, the male will assist in taking care of the newborn and protect them from predators. Some give birth to a single offspring or twins, while others bring forth a litter of young ones. The majority of mammals mates during specific periods of time.

In reference to reproduction, there are three groups of mammals with unique features. These are Monotremes, Metatheria and the Eutheris. The Monotremes, though not common, lay eggs. The Metatheria carry pregnancy for a very short period, thereby giving birth to very fragile young ones which are keenly taken care of by their mothers. The Eutheris carry babies in their womb longer, with the developing young one feeding through the placenta. Once born, the young ones are fed with the protein and fat-rich milk. They are also taught adaptation and behavioral ethics depending on whether they live on land or in the water.

Despite the environmental adaptation and risks involved, some mammals can live up to 70 years with some whale species living up to 200 years. Lifespan differs with small mammals living shorter lives compared to huge mammals.

​Characteristics and Behavior

The behavior of mammals differs from one species to another and the energy levels are dependent on activity patterns. Those in hot areas have a mechanism to keep them cold and reduce perspiration, while those in cold climates must engage in acts that enable their bodies to generate heat. Socially, some mammals live in groups while others live a secluded life except during mating season or when assisting each other to care for a newborn. The majority are active during the day while a few come out at night to hunt, mate, or socialize.

Mammals pass information largely through the hearing mode. Use of smell is used to assist during mating season or to show direction, especially in the cat family. Some produce pungent smell as a defense mechanism against predators. The bat uses echoes to communicate. Vocal codes are used to pass information to young ones or alert others in cases of danger, for example in human beings. Aquatic mammals have hair near their mouth that helps them sense an external object and avoid it. Large mammals can see clearly and far, while nocturnal ones have a well developed hearing ability.

​Feeding Pattern

Mammals do not have a clear cut diet. What they eat is determined by availability. Basically, there are three rigid categories of feeding habits.


Carnivores mainly feed on tissues from other animals or insects. They are known to have very strong teeth and can run very fast on land. This category has only one stomach chamber which produces acid to digest protein. Their small and large intestines are shorter in length. The aquatic mammals feed on smaller sea creatures. An interesting category made up of hyenas and foxes eat the leftovers of other carnivores due to their opportunistic character. Examples of carnivores are lion, cheetah, cats, whales, and dolphins.


Herbivores derive their foods from plants. The green matter provides the energy required by the body. Their mouths do not open widely and digestion begins in the mouth, therefore, they produce a lot of saliva. They can be divided further into ruminants and non-ruminants. Ruminants have more than one stomach and a complex digestion system where food has to be returned to the mouth for further chewing. Examples are cows, goats, llamas, and antelopes. The non-ruminants have simple stomachs and feed on easy to digest plants. All herbivores have elongated intestines to ensure maximum absorption of nutrients.


The anatomy of an omnivore is developed in a way that it can be a herbivore as well as a carnivore, eating both plant matter and other animals. Examples are pigs, human beings, squirrels, and primates.

Interactions Between Mammals and Human Beings

Mammals have various interactions with human beings. Some species are raised and slaughtered as food, with others are kept as working animals, while still others are kept as pets. Some mammal species are used as a means of transport, while others are used for pest control, or for financial gain. The wool of some mammals, such as sheep, goats, and alpacas, are used as the base material in clothing production. The skins of other mammals, such as cattle, provide leather for upholstery, clothing, and accessories.

The intellectual capacity of some mammals has enabled man to achieve a lot. Thus, some of these mammals can be trained and learn to do certain functions that can make life easier for humans. However, some mammals can spread diseases such as rabies, or even destroy crops, while others can cause injury to humans. Some mammals have contributed to the depletion of the ecosystem and hence should be controlled.


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