Mammals

[mendenhall]

which two mammals that grow hair only after they are born

[Rabelais]

Well most marsupials are born hairless as are rats and mice. I think quite a lot of bats are born hairless too.

[Peter Pedant]

Hi

I thought of rats and mice,but waht about shrews and dormice,\in fact anything small rabbits and hares?

unfortunately marsupials are not mammals, see above they erm are vertebrate marsupials

a duck billed platypus is in a section all by itself (ovoviviparous)

[Rabelais]

Unfortunately marsupials are not mammals? See above what? What idiot put them all in the class Mammalia then? Hold on while I quickly go off and rewrite the textbooks of the world with your interesting new info. Oh and I'll also have to shift the platypus from Mammalia to to this new ovoviviparous group (incidently the are oviparous not ovovivparous as they don't hatch the eggs inside their bodies) And just for you information, even though rabbit kittens are born naked, leverets are born fully furred.

[New Forester]

Rabelais -  quite right about leverets, and marsupials.

 

However, there is a lot of argument about the relationship between monotremes and other mammals (though the consensus does seem to be that they are pretty close)

 

One way to look at it is that land vertebrates are divided into many groups, often of roughly equal "rank".  For example, tortoises and turtles make a good group, lizards and snakes another, and birds, two groups of dinosaurs, the pterosaurs and crocodiles yet another.

 

Some of these groups happen to be warm and furry and we call them mammals.  One is warm and feathered, and are called birds.  The remaining (extant) groups are all cold-blooded and scaly, and we dump them in a category called reptiles.  However, crocodiles, for example, are in fact far closer to birds than they are to tortoises or snakes, and are no nearer to plesiosaurs than they are to mammals.

 

This all leads to some anomalies.  If (some) dinosaurs were feathered and warm-blooded, were they reptiles, or birds?  Pterosaurs were close to dinosaurs and crocodiles, but were furry and probably warm-blooded.  Surely that makes them mammals?

 

You can count monotremes as mammals, but in a way it's just as sensible to see them as warm(ish)-blooded furry reptiles.  But then we are that too, really.

[Rabelais]

You missed the most important factor from which we derive the term mammal. This plants them squarely in the group even if their lactation is primative. While monotreme don't have nipples, they produce caesin and lactose secretion. That's all you need to be a mammal whether you are prototherial, metatherial or therial

[New Forester]

Rabelais -- What you are really saying is that all species of mammals produce milk, and this is indeed often used as a definition of them.  You're right, I didn't mention it.

 

However, I think it's really a description rather than a definition.   We often use it as a definition because it so happens that all known mammals do share this characteristic.  But imagine if we found that an otherwise "obvious" mammal did not have milk, or had milk but not caesin and lactose?  Would it stop being a mammal?  On the other hand, what if we found that pigeons, say, did produce milk.  Would they suddenly become mammals?  (They do, by the way).

 

Historically, form and behaviour have steadily given way to more fundamental classification factors, as we have understood biology better.  Whales were once "obviously" fish, and snakes were regarded as worms.  Understanding the structure of these animals clarified their true nature -- and we are now able to use ever more fundamental things, such as DNA sequences.

 

I think you either have to go by fundamental things, like structure and DNA sequences, or be aware that you are talking of things (like fur, feathers, warmth, shape -- and perhaps indeed milk) which may be adaptive features, and could arise in all sorts of different ways on different evolutionary occasions.

[New Forester]

Peter -  Did you intend to include a link?  You say "see above"....