Mammals

[4getmenot]

Do all mammals have belly buttons?

[Clanad]

Yes... many cannot be readily seen since they are covered with hair or tend to dissipate as the animal grows older...

[aka estie]

Just a guess, but I doubt that the monotremes (e.g. platypus, echidna) would have bellybuttons since they do not have an umbilical cord. Will have to look that up.....

[4getmenot]

But how do they cut it. Do they just chew through it??

[aka estie]

Yes,many mammalian moms will eat the placenta and the umbilical cord.

[Clanad]

Platypus are the only member of the family Ornithorhynchidae, order Monotremata, and are, as aka estie points out, mammals that lay eggs. They would not have navels but are such an unusual order of mammals that they are often overlooked in discussions of this type. At least that's my story and I'm sticking to it...

[aka estie]

Hiya clanad, I agree that monotremes are an exception, but as a biologist I just couldn't answer yes to this question without mentioning them :)

(P.S. echidnas are definitely also monotremes - I love them!)

[Rabelais]

Only the Eutherians or "true mammals" have a placental development of a type that leads to the formation of umbilicus. So on top of the monotremes, marsupials don't have them either. So to answer the question as asked, no a great many mammals don't have navels, just the Eutherians.

[aka estie]

Oooh, interesting, Rabelais. So the marsupials have a placenta but no umbilicus?

[dancecaller]

Not sure about marsupials, they develop and are born then crawl up into the pouch. OK they're tiny, but surely there is some placental stage.

Seen TV pictures of mum licking path through fur for joey.

[Clanad]

For marsupials, such as kangaroos, wallabies and opposums, the placenta is rather egg like in that it consists of a yolk like structure. The infant remains in this state for only a very short time after fertilization (this in itself is quite an unusual process), is born into the pouch and must make its way to a nipple, where it attaches itself (to small to suck) until its strong enough to emerge...