As I understand it, birds don't urinate, urea and other water soluble stuff are pushed out with the faeces. I'm told that many reptiles do the same thing.

Mammals, on the other hand, have all kinds of extra plumbing to push urea etc. out through (or near) the reproductive organs.

The two different excretory systems must have split quite a while ago (how far back? Do we know anything about excretion in synapsid reptiles?). So here's the question: there must have been quite a strong selection pressure to cause the evolution of all that extra plumbing that isn't/wasn't present for reptiles and birds. Do we have any idea what it was? In short, why do we pee?

Thank you!

G'Day James:
In short, urinating is a balance between getting rid of waste and keeping hydrated. Birds and reptiles ‘urinate’ in a way to conserve their water. Unlike mammals, which excrete waste fluid (e.g., urea in water) as urine and solid waste as faeces, birds and reptiles mix both liquid waste and solid waste into the semi-solid paste (the ‘whitish part is like uric acid - nitrogen containing waste formed from the breakdown of proteins - uric acid is less soluble than urea in water) you see as droppings. Many snakes, crocodiles, birds and most lizards lack a urinary bladder. I see that there are many google entries saying that ostriches and rhea have a bladder - and secrete their urine and faeces separately. I’m not so sure that these birds have an actual ‘bladder’ - maybe it is a dilated pouch of the ureter (? subtle anatomical distinction) from where urine is discharged e.g., see … -p0173.pdf

By the way, we concentrate our urine by the action of the hormone vasopressin (or antidiuretic hormone) on the kidney - vasopressin enhances water permeability of the renal collecting ducts. The avian kidney is structurally part-mammalian and part-‘reptilian’. Birds have a closely related hormone to vasopressin called vasotocin which also appears to alter kidney water permeability, although in comparison to mammals the avian kidney can apparently only weakly concentrate the urine. In contrast, the reptilian kidney has a different kind of structure than the mammalian equivalent and is not able to concentrate urine, so cannot respond to dehydration by concentrating urine as we do!!

For more info on urine formation/excretion see: … ary_System

A link to some basic info on vertebrate kidneys is: … dneys.html

Re the vertebrate urinary bladder, Google "Osmoregulatory and other uses" and download article by Bentley PJ (1979) for a brief comment on bladder evolution (including a reference to Charles Darwin's interest in tortoise bladders!!).

Last edited by Steve Lolait (1st Jul 2010 14:27:13)