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.