Re: why is a platapus considered a mamule

Thanks for the interesting question. Even though it has a bill that looks like a duck's, the platypus is considered a mammal because it shares a few characteristics with all other mammals.

All mammals have mammary glands, with which produce milk to feed their offspring. All mammals are warm blooded; their body temperatures are higher than the background temperature, so that they can remain active at night or in the winter. All mammals have hair of some sort (fur, or spines or bristles), which usually serves as insulation to keep retain the mammal's higher body heat, but are sometimes used for defense (like the spines on a porcupine or an echinda), as sense organs (like the whiskers on a cat or a walrus), or for communication (like our eyebrows). Even whales have a few bristles, because whales are mammals too.

So, because the platypus has mammary glands on its stomach, and because it is warm-blooded and has hair, it is a mammal. However, there are many different types of mammals, and we can see the amazing degree to which mammals have changed since the days of the early mammals by comparing the differences between the platypus and most of the other mammals.

The platypus belongs to a group of mammals called the monotremes. This group includes the platypus, and the spiny echinda, a spiny, ant-eating mammal. Both the platypus and the echidna are found only on the continent of Australia. In addition to the characteristics that all mammals have in common, Monotremes share a few other characteristics. First, monotremes lay eggs, like birds and reptiles, and second, all monotremes have a single opening, called a cloaca, through which they excrete bodily wastes, and lay eggs. Birds and reptiles also have cloacae, while the rest of the mammals have a seperate anus, urethra and vagina for solid waste, liquid waste and birth. This tells us that ancient mammals were like birds and reptiles in many ways. One important difference between the platypus and the echidna is that the platypus lays its eggs in mud, while the echidna carries its egg in a pouch on its body. The echidna's offspring hatch directly in the pouch, where they live until they grow spines

Marsupials are a second type of mammal that also raise their offspring in pouches. However, unlike monotremes, marsupials do not lay eggs. Their offspring are born live. The marsupial group includes the koala, the kangaroo, the possum, and many other species. When a young marsupial is born, it is blind and only has front legs. It uses these to climb the inside of its mother's pouch until it can find a mammary gland by smell, where it will begin to drink milk. The young marsupial lives in the pouch, growing larger and developing hind legs, until it can manage the outside world on its own.

Placental mammals are the most common type of mammal. This group includes all the mammals that do not lay eggs and do not raise their young in pouches. This group includes humans, elephants, armadillos, whales, horses, cats, and most of the mammals with which we are familiar. The fetus of a placental mammal grows inside the mother for a long time, deriving nutrients directly from the mother's uterus through a special organ called the placenta. There are a few other consistent differences between placental mammals and the other types (e.g. the types of teeth they have) of mammals as well. Placental mammals have a great advantage over other animals that lay eggs; their placentas allow them to draw on the mother's own resources for their development while she moves around searching for food. Organisms that develop in eggs only have the finite amount of resources available to them.

So, I hope you can see why the platypus is classified as a mammal, and how it differs from the other types of mammals.