Why is a platypus a mammal?

Question

Platypie lay eggs, but their considered mammals? Their also is a fish called shiner perch that has live birth but is still a fish?

Answer 1

The characters that specifically define a mammal, as opposed to any other group of vertebrates, are as follows:

1 - Female mammals produce milk from modified sweat glands to feed their young.  These are known as mammary glands and are what give the group their name.  Platypus females don't a breast and nipple like some other mammals, but they produce milk in a milk patch on their belly.

2 - The possession of hair.  Even baby whales have a few hairs.  No other living group has hair of the same origin as mammals.  Presumably some of the fossil reptile groups do though, so it's not really that great as a defining character.

3 - The lower jaw of mammals is a single bone on either side.  

4 - Related to that, mammals have an inner ear that is made up of three bones: the stapes, incus and malleus.  These bones were once part of the lower jaw, but became involved in hearing.  In fossil forms, this is THE definitive character, and once you have a critter with this trait, it is labeled a mammal.

5 - The main artery leaving the heart curves to the left to become the aortic arch.  In birds, the artery curves to the right.  Both birds and mammals are warm blooded (which is why that's NOT a defining mammalian trait), and all of the ectothermic groups have more than one blood vessel leaving the heart.

6 - Mammals have a muscular diaphragm that separates the thorasic and abdomenal cavities.  It is related to warm-bloodedness, and improves efficiency of breathing.  In crocodilians an enlarged liver serves a similar function.

Those are the traits that define a mammal, and the platypus has all of them, so it is defined as a mammal, no matter whether it lays eggs or bears live young.

Answer 2

Simply because they are warm blooded. Even though they lay eggs they are not classified as anything other than mammal. It is ridiculous that people think otherwise. It drives me bonkers! But again they are mammals also because they nurse their offspring and because they are hot blooded.

Answer 3

they are warm blooded, have fur and feed their offspring with milk. A lot of fish give birth live (guppies) but they have scales, are cold blooded and have no lungs.

Answer 4

the one truly defining characteristic of a mammal is a fused lower jaw bone, which is why a platypus is a mammal.

all the other mammilian traits (with the exception of the mammary gland) can be found else where in nature: warm blood in sharks, fur not found on all mammals etc.

Answer 5

Them having hair, being warm-blooded (endothermic), or laying eggs has nothing to do with it.  The "main" reason Mammals are classified as Mammals is that they "nurse their young".

Some Mammals have no hair, like the Pangolin which has scales like a reptile, or Porpoises and Whales which have very tiny hairs.  Some Mammals lay eggs like the Platypus.  Some raise their young in pouches like Kangaroos or Koalas. Birds are warm-blooded too.  But, ALL Mammals nurse their young, even the whales, the Kangaroos, the Pangolins,  and the Platypuses.  

Answer 6

They have hair/fur, are warm-blooded, and nurse their young.  

Answer 7

Haysoos2 has your most detailed and correct answer on what defines a mammal.  I just want to add a note on eggs VS. live birth.  These are not and have never been group classifiers.  Mammals, in fact, have three sorts of birth.  

The ancestral way is egg laying.  All early mammals laid eggs.  Monotreme mammals like the modern echidna and platypus kept this trait when their family branch split off of the mammal family tree.  It is the oldest style of mammal birth.

The next newest branch to split off were the marsupial mammals.  They give live birth, but do not have a placenta.  Marsupial babies are born extremely premature and spend most of their development time outside of the mother's body, attached to a teat.  

At this point it is worth noting that babies are not recognized by the mother's immune system.  Babies that develop too long inside the mother will eventually be attacked and killed by the mother's immune system.  This is the typical immune response.  For an animal to develop for a long time inside its mother safely, it needs to be invisible to the mother's immune system or it will be treated like the parasite it technically is.

The most recent style of birth involves a placenta (placental mammals proved really competitive and have thus, dominated the other mammal types).  This organ attaches to the interior of the mother's uterus and feeds off the mother's body, passing the nutrients to the baby.  Placental mammals are the only large group of animals that have their offspring develop for a long time inside the mother's body. Individual species from sharks to snakes also give live birth, but how they developed this trait is unknown.  We do know how placental mammals managed to evolve a style of birth that would normally put their offspring at risk of being destroyed by the mother's immune system: a retrovirus.  

Retrovirus ERV3 is lodged in the DNA of all placental mammals.  Normally, viruses go about their business of invading cells and hijacking their inner working to produce more viruses.  Many viruses slip by the immune defense by sending out a chemical signal that makes them invisible to the immune system.  Lots of viruses, over time, have gotten stuck in their hosts DNA.  ERV3 is one, and millions of years ago, it was co-opted by its former host to make placental mammal style birth possible.

Answer 8

The platypus is warm blooded, covered in hair, and has mammal glands to give milk to it's young.

So you see species are not defined and categorized by their young, alone, and so a fish that has live young, is still a fish in every other sense :)

Answer 9

mammals have 6 specific characteristics:

1 All mammals have some sort of hair

2 All female mammals have mammary glands

3 All mammals have sweat glands

4 All mammals are vertebrates

5 All mammals have many different types of teeth

6 All mammals have fat producing glands

Not sure about the other one but I am sure it has it list also.

Answer 10

Everything said so far is utter nonsense, I am sorry people if that seems a bit harsh, but it is true, do any of you actually use the facility to check what the definition of a mammal is ? ( you know, the little box at the side of this writing area, ? the one that says " Do a little research" ? and I include the asker of this question in this !....

A mammal is simply one that suckles it young ! it is what we derive the term mammary glands from, it means that the adult female has mammaries ( b*****s, to you people, ) and will feed its live young this way...

Answer 11

They have fur and nurse their young but to be more specific they are monotremes along with the echidna.