Re: Synapsids weren't reptiles?

[Erik Boehm <erikboehm07@yahoo.com>]

> some mammals excrete uric acid too,
> like
> > kangaroo rats?
> > 
> > I think all do, actually, but not in a concentrated
> form
> > like crown
> > sauropsids. Actually, I'm not really clear on
> this, so
> > someone more
> > knowledgeable should feel free to step in. And
> don't
> > turtles differ
> > somehow from other crown sauropsids in this respect?
>  
> ++++++++++++++++++++++++++++++++++++++++++
> 
> Unless I missed a recent discovery, Kangaroo rats excrete
> urea just like all other mammals. 

>From wikipedia:
"In humans and higher primates, uric acid is the final oxidation product of 
purine catabolism. In most other mammals, the enzyme uricase further oxidizes 
uric acid to allantoin.[2] The loss of uricase in higher primates parallels the 
similar loss of the ability to synthesize ascorbic acid.[3] Both urate and 
ascorbate are strong reducing agents (electron donors) and potent antioxidants. 
In humans, about half the antioxidant capacity of plasma comes from uric acid.

Uric acid is also the end product of nitrogen catabolism in birds and reptiles. 
In such species, it is excreted in feces as a dry mass. While this compound is 
produced through a complex and energetically costly metabolic pathway (in 
comparison to other nitrogenated wastes such as urea or ammonia), its 
elimination minimizes water loss. It is therefore commonly found in the 
excretions of animalsâsuch as the kangaroo ratâthat live in very dry 
environments. The Dalmatian dog has a defect in uric acid uptake by liver, 
resulting in decreased conversion to allantoin, so this breed excretes uric 
acid, and not allantoin, in the urine."

I know wikipedia is far from the best source, and merely states uric acid is 
found in the urine- not that it is excreted in concentrated form.

"The kangaroo rat has a longer loop of Henle in the nephrons which permit a 
greater magnitude of countercurrent multiplication and thus a larger medullary 
vertical osmotic gradient. As a result, these rodents can produce urine that is 
concentrated up to an osmolarity of almost 6,000 mosm/liter, which is five 
times more concentrated than maximally concentrated human urine at 1,200 
mosm/liter."

I thought the point of uric acid, was that it could be concentrated more than 
urea. How does the Kangaroo rat keep the nitrogen soluble if it doesn't have 
significant uric acid? How does this compare to typical bird excrement?

How do whales get water anyway? I seem to remember an attempt to get kangaroo 
rats to drink saltwater (they wouldn't) under the theory they should be able to 
use it. Is that perhaps what is going on in whales?