Mammals are warm-blooded animals with fur that nourish their young with milk. There are several other characteristics that distinguish different mammal species from each other, but all of them have those three basic traits. The most common mammal species is the placental mammals. There are also marsupials and monotremes, which are rare.
While there are less than 5,000 placental species, these animals are the most common and diverse mammal species. They have four superorders that contain 20 orders. The largest include rats, bats, dogs and whales. The most common mammals in this group are humans, domesticated pets, livestock and rodents.
The distinguishing characteristic of placental mammals is their ability to nourish their young inside their bodies using a placental sack, which provides oxygen, food and everything else the embryo needs. Placental mammals are thus born at a more advanced stage of development, which means they are more quickly able to fend for themselves and don’t require as much protection in the beginning as marsupials do. As a general rule, placentals are considered to be the most advanced mammal species and are the most recent to have evolved.
Marsupials are separated from other mammals by the way they give birth and nourish their young. A marsupial infant is born in a very early stage of development, and it will crawl to find its mother's nipple, which is usually in a pouch. The pouch offers protection so that the undeveloped animal can mature safely.
There are many different kinds of marsupials filling many ecological niches. Some of these include predators like the Tasmanian devil, grazing animals like kangaroos and small foraging creatures like the bilby. Marsupials are mostly found in Australia, Tasmania and New Guinea. The only marsupial found in North America is the opossum, which is generally known for its willingness to eat nearly anything and an ability to play dead when threatened.
There are only three species of monotremes—the platypus and the two varieties of spiny ant eater, which are also known as the echidnas. Monotremes are usually characterized by very primitive characteristics that still show some traces of their reptilian origin, including the fact that they give birth by laying eggs. The platypus is especially well-known for several primitive characteristics, including the duck-like bill, a reptilian gate when walking and a venomous spike, which the males uses as a defensive mechanism.