Evolution of life forms is an amazing story of survival. Through millions of years, different life forms have had to face changing climatic and environmental conditions. Some survived, whereas others perished. While we all originated from the same life form, some deviated from the main group and developed separate characteristics according to the climate or geographic conditions of the place they lived in. Hence, the similarities and differences in characters of different organisms. It is on the basis of these characteristics that we have been able to classify living organisms into different groups. One of the groups are the mammals. So what are the unique characteristics that set mammals apart from other organisms? What are the Characteristics of Mammals? Unique Characteristics Like I already said, animals of certain groups share some common characters. Then what are the unique mammal characteristics that only mammals possess? They are the following:

• The females of the class Mammalia have mammary glands that produce milk with which females feed their young ones. In fact, the class got its name from the Latin word Mamma which means breast.
• All mammals bear hair on their body at some point in their lives.
• There are three bones in the middle ear of every mammal. They are the malleus, incus, and stapes which are more commonly known as the hammer, anvil and stirrups respectively. These bones help mammals in hearing by transferring sound waves from the eardrum to the inner ear.

General Characteristics The other general characteristics are given below.

• While the jaws of all other vertebrates are made of more than one bone on one side, the jaws of mammals are made of a single bone on each side.
• Mammals have a unique heart. Although mammals have a four chambered heart like birds, the main artery turns left, as it leaves the heart. In birds it turns to the right, whereas, in all other vertebrates, there are more than one artery that originate from the heart.
• All mammals have two sets of teeth in their lifetime, that is their teeth are replaced only once.
• All mammals have a sheet of muscles and tendons, known as the diaphragm that separates the thoracic cavity from the abdominal cavity.
• One of the physical characteristics is that mammals have a backbone, that is they are vertebrates.
• They are warm blooded animals. It means that their body temperature does not fluctuate with that of the environment.

Mammals Categories The group of mammals has over 5000 species that are found in different sizes and are spread all over the world. This is a diverse group and according to the environment, the individuals of this group have developed different characteristics. While the smallest mammals like shrew and bats weigh as little as 3 grams, on the other hand, the largest mammal on earth, the blue whale, weighs over 160 metric tons. Based on the variety of features that they display, mammals are further divided into the following categories.

• Prototheria: Although they are mammals, Prototherians lay eggs. This group includes the platypus and three varieties of the spiny anteaters.
• Metatheria: These are mammals that have pouch in which they carry their babies, like the kangaroos. They are also known as marsupials. Although even marsupials have placenta, it is not as well-developed as it is in placental mammals. Hence, the animal that is born is not completely developed. It completes its development inside its mother's pouch.
• Eutheria: These are the placental mammals. A rather diverse group, it includes armadillos, bats, elephants, shrews, whales, and of course, human beings. One of the most identifying placental characteristics is that when inside the uterus, the fetus derives nutrition from the mother through the placenta. Unlike marsupials, the placenta plays an important a role in fetal development in the Eutherian mammals.

As mammals are not just confined to land or within a small geographical region, they have a wide variation in their characteristics. Among the mammals there are those that live in the oceans. They are called the marine mammals. In fact, marine mammals once lived on land. However, during the evolution of mammals they got more suited to life in the seas and oceans. They have the unique characteristics of mammals, but they have modified themselves as per life in water. For example, they do have lungs, but they have to come out of water periodically to get fresh air. Seals and walruses have a furry coat. But what about dolphins and whales? Early in their life these animals do have hair around their mouth that they lose soon after birth. The follicles can be seen in the adults where the hair once grew. Other than these, marine characteristics include a thick layer of fat under their skin that provides insulation when in water. The cetaceans, a group of marine mammals, that live in water their entire lives, have streamlined bodies. Marine mammals have more blood than land mammals, as compared to their body sizes and can store extra blood in muscles which enables them to stay underwater for long. So that was the story of mammals, a saga of evolution and survival. As they adapted to lives in diverse conditions, they developed characteristics that best suited their niche.