Only Mammals Chew

Great stand up comedians have a gift at pointing out the obvious in everyday life and making everyone in the audience say in their mind, "That's so true!  I've never thought about that, but it's so true!"  This is one of those things.  Only mammals chew.

Try to think of an exception.  

I mean really chew.  Not tear up, bite through, or rip apart, but really gnaw, masticate, mash up.

How interesting.

Why?

Doesn't seem to be an easy answer, but some of it has to do with our endothermia - warm bloodedness.  To sustain warmbloodedness, which keeps enzymes and proteins at their optimal temperature for physiological fine tuning, we need constant energy inputs of high quality foods.  In contrast, reptiles, like snakes, can go months in a semi-dormant phase that doesn't require the constant fueling of the 'fire' of warmbloodedness.  Mammals can't.  A simple way of putting it is that reptiles can wait for their chemical digestion to break down their food, but mammals are in too much of a hurry (with exceptions).  We need it now and mechanical break down offers that.

Herbivorous mammal teeth:

Carnivorous mammal teeth (take note of molars, which semi-only occur in mammals):

Herbivorous reptile teeth:

Carnivorous reptile teeth:

There are two exceptions that I should mention, though.

Gizzards:

Pretty much all birds, a few fish, a good number of insects 'chew' using a muscular sack in their gut called a gizzard.  This can use grit/pebbles or in the case of larger dinosaurs even up to stone size rocks to crush up their food.  There are even some insects and mollusks that use chitinous 'teeth' plates in their gut to 'chew'.

Some reptiles do chew.  Most notably dinosaurs like hadrosaurus.  Chewing in dinosaurs is one of the reasons it's been speculated that they might have been warm blooded.

Also, there was one crocodilian that was recently unearthed that seemed to have chewed as well:

http://www.scientificamerican.com/blog/post.cfm?id=crocodile-relative-might-have-chewe-2010-08-04

Why this is neat:

It highlights how species are integrated systems.  Who would have thought that a tooth innovation might have started the whole process and paved the way for a metabolism innovation.