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Re: Uricotelism



Just because you're uricotelic doesn't necessarily mean you can't produce
dilute and copious urine. Urates precipitate out when (amongst other things)
water is resorbed from the urine, so if the animal is hydrated and not
conserving water then a lot of liquid can be lost with the urates. You see
similar effects in estuarine crocodiles (Crocodylus porosus) from freshwater
areas versus saltwater areas. In the former, urine is often voluminous and
dilute, in the latter urine is sparse, concentrated, and hence contains a
lot of precipitates.

Perhaps the Postosuchus illustrated in WwD was particularly hydrated? It was
in freshwater habitat after all.

Mammals are presumably not uricotelic because their nephrons can resorb
water within the kidneys, so you end up with hyper-osmostic urine if water
conservation is a priority. Reptile kidneys can't do this, producing
hypo-osmotic urine from which water must be resorbed later if it needs to be
conserved. Being uricotelic enables this to happen while still getting rid
of concentrated urate wastes. I'm sure there's a lot more to it than that
however.

Adam Britton



----- Original Message -----
From: "Patrick Mellor" <patrickmellor@hotmail.com>
To: <dinosaur@usc.edu>
Sent: Sunday, March 04, 2001 4:26 AM
Subject: Uricotelism


How could Postosuchus have sprayed mammal like urine, when all living
archosaurs (and most diapsids) are uricotelic? Also, I have a question that
I think is interesting: If living synapsids (mammals) are not uricotelic but
evolved from basal ammniotes along with diapsids, why would they have ever
lost something as advantageous in terms of water retention as uricotelism?