The Palate - Yet Another Difference Between Mammals and Other Vertebrates

You wanna know one of the biggest reason that mammals are special?  We chew.

In fact, only mammals chew. (Pretty much.  It's hard to make absolute statements in biology.)

More on this here.

The gist:  We mammals have evolved warm bloodedness because it helps us move quickly and with sustained energy (reptiles have to bask in the sun, etc. to get warm).  Hot bodies are costly, though--they burn something like 10 times more energy than being cold blooded.  Not only do warm blooded animals have to eat way more (big snakes can go months without eating--try that sometime!), but we have to get every possible calorie out of the food we eat--and fast!  Thus, we chew.  So, what does it take to chew?  Proper teeth, of course.  And...the ability to breath and chew.  Not choking to death is nice, too.

Therefore, the palate was born.  The palate is the bone/flesh separating you nasal passage from your mouth.  I never would have realized this, other than through reading about evolution, but other critters like birds and most reptiles (with the exception of crocodilians who hold their food underwater to drown it without, hopefully, drowning themselves) don't have palates.  They breath through their mouth.

Birdy mouth nasal passages entering mouth (it's the thin slits in the middle of the roof of the mouth):

http://yourownvet.com/?tag=how-to-warm-baby-birds

http://en.wikipedia.org/wiki/Palate

Dinos were mostly mouth breathers.
http://faculty.plattsburgh.edu/thomas.wolosz/turbinates.htm  (And images below.) 

Mouth Breather

Not Mouth Breather

Also Not (they're exceptions to the rule)

Why Pee Is Yellow, Poo Is Brown and Bird Crap Is White

I feel like I struck question gold on this (no urine pun intended).  What's most awesome about the answer to this question is that it involves a whole slew of major organ systems in  your body.  On to the details!

When old blood cells retire the hemoglobin in them gets broken down into bilirubin.  This is no small thing.  You have 30 trillion red blood cells in your body and each of those cells has 270 million hemoglobin molecules in it.  Just to be able to have a stable red blood cell count and compensate for loss you have to make 2.7 million red blood cells a second!!   Bilirubin is also the yellow substance that causes bruises to appear yellow, as well as the yellow pallor of jaundice. 

I thought this was interesting.  It's all about moving oxygen and electrons!

Notice how it basically just loses the iron and gets splayed out.

This worn out hemoglobin gets disposed of in two places. One is our liver.  The liver is a thrifty organ and filters bilirubin out of the blood, concentrating it in the gall bladder as one of the main constituents in bile ('Bile' and the first half of 'bilirubin' come from the same Latin root meaning 'anger', 'wrath', or 'gall'.  'Rubin' means red, so literally it means 'red wrath'.).  Bile is squirted into our G.I. tract and does a great job emulsifying fats for us to process.  You may know someone that has had a gall stone and suffered from the symptom of intense pain after eating fatty foods.  Their body is trying to pump out bile to break down the fats, but is being painfully blocked.  If a severe enough condition exists, gall bladder removal surgery may be prescribed, in which case the patient will have to eat a low fat diet (or deal in other ways) since high fat content without a means of processing the fats can cause serious indigestion and bloating. Assuming proper function, the bilirubin in bile gets further broken down by bacteria in the gut into a brown substance called urobiligen, which is the main reason our feces is brown.

The liver, however, doesn't catch all of the bilirubin floating through the blood stream and the excess gets mopped up by the kidneys to be disposed of.  Before leaving the body it's further broken down into urobilin, which, you guessed it, is yellow.  So, no, it's not urea that makes our urine yellow.  Urea is colorless.

***Be really careful with this photo.  Those colors are for distinguishing them ONLY.  Not true colors.

Review:

Hemoglobin in blood breaks down into bilirubin which is filtered out by A) liver or B) kidney

A) Our liver concentrates it in the gallbladder as bile, which then pumps it into the intestines and is broken down by bacteria into the brown we know our feces as.

B) Our kidney further breaks bilirubin down into urobilin which is yellow and excreted in our urine.

From blood to Liver to Gallbladder to Intestines to Poo.

Blood to Kidneys to Wee.

Red (of blood) to Green (of bile) to Brown (of poo).


Red (of blood) to Yellow (of wee).

So, the next time you use the ole water closet, remember that your pee is yellow and your poo brown because it's broken down blood!!

Amazing ways evolution has made things look like bird sh*t:

Now, bird crap's a whole different story.  When amino acids, the building blocks of proteins, break down (for a number of reasons, like they were digested but not used by the body or just as the wearing out of protein) they can turn into urea directly or its nastier cousin ammonia which is quickly broken down by the kidneys into the safer urea.  Well, that's how it goes if you're a mammal, anyway.   Birds and reptiles do things a  little more complicated for two reasons.  One is to save water.  Urea is water soluble and therefore needs to be flushed out with large quantities of water in order to be disposed of.  This means, as mammals, we're constantly throwing away a relatively large amount of perfectly good water in order to get rid of urea.  Birds and reptiles solve this problem by expending a little more metabolic energy and turning broken down amino acids into uric acid which is relatively insoluble and can be concentrated as the white precipitate paste we see in bird poo (often mixed with brown since they poo/wee out of the same orifice and usually at the same time).

The other more interesting reason is to solve an egg problem.  When a fetus in utero makes waste, the mommy is nice enough to dispose of the babies waste through the placenta.  If you are a species that is isolated in eggs urine trouble! (Get it?  Get it?...)  They have to isolate their waste so it doesn't poison the entire egg.  One of the most effective ways of doing this is to make it mostly insoluble in water so it can't spread - make it into uric acid.  Genius.

It's interesting to note some exceptions.  Some turtles start off making uric acid while  in the egg to get its largely insoluble advantage, but then later in life switch to urea (like us) since it takes less metabolic energy to make (I assume the same is true for egg laying mammals like platypuses and echidnas which do use urea later in life).  The kangaroo rat, even though it's a mammal, is another exception that has evolved to survive in a harsh desert environment by using the water conserving uric acid method of amino acid disposal.

I hope you never think of using the bathroom the same!

Works Sighted [sic]:

http://www.strangequestions.com/question/583/Why-is-urine-yellow.html#ixzz1C69R7NKk
http://www.thenakedscientists.com/HTML/content/latest-questions/question/2213/
http://www.whycenter.com/why-is-urine-yellow/
http://scienceblogs.com/startswithabang/2009/04/putting_the_yellow_in_your_uri.php
http://xenophilius.wordpress.com/2010/08/04/ask-a-scientist-why-is-urine-yellow-is-it-gold-urobilin/
http://www.expasy.ch/cgi-bin/show_image?L5&down
http://sxxz.blogspot.com/2006/07/why-is-urine-yellow.html
http://www.primermagazine.com/2009/field-manual/why-is-bird-poop-white

Pictures from here, here, here, here, here, here, here, here, here, here, here, here, here, here, here, here, here and here.

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.