Shape and size

Mammals come in variety of shapes and sizes. They range from the extremely small size like from the 30–40mm Bumblebee Bat to the gigantic and the largest animal on Earth 33-meter blue whale.

Milk and mammary gland

All female mammals nurse their young with milk which comes out from special glands called mammary glands. Mammals are the only animals that have the capacity to produce milk. All female mammals have mammary glands, and this is where they produce milk for their young ones.

The monotremes branched early from other mammals and do not have the nipples seen in most mammals, but they do have mammary glands.

Nervous system

All mammalian brains possess a neocortex, a brain region that is unique to mammals. Neocortex is the outer layer of the cerebral hemispheres, and made up of six layers, labelled I to VI (with VI being the innermost and I being the outermost).

Skeletal system

The majority of mammals have seven cervical vertebrae (bones in the neck); this includes bats, giraffes, whales, and humans. The few exceptions include the manatee and the two-toed sloth, which have only six cervical vertebrae, and the three-toed sloth with nine cervical vertebrae.

Hair and Fur

All mammals have body hair and are the only animals that posses true hair. Some have coarse hair while others have a thick fur coat. The hair or fur helps mammals maintain a constant body temperature..