Dear Straight Dope:
After coming out of a store and finding our car splattered with bird doo-doo, my wife and I engaged in a heated and absurd debate. I bet her a nickel that birds urinate and she swears this is not so. So who gets a nickel?
At the Straight Dope we like to broaden our readers’ horizons, Alex. Today we’re going to introduce you to the concept of Too Much Information.
Anyway, you win the nickel. Birds urinate, it’s just that their urine is semisolid rather than liquid.
Birds excrete their nitrogenous wastes, derived mostly from the breakdown of proteins, in the form of uric acid rather than urea as mammals do. Unlike urea, uric acid is almost insoluble in water, and is excreted in the form of crystals that form a semisolid white paste. Not needing to store liquid wastes, birds lack a bladder. Instead urine passes from the ureters into the cloaca, a common chamber for the passage of digestive and urinary wastes, as well as for reproductive products. A bird dropping usually contains both white uric acid crystals, and a concentrated mass of digestive wastes such as insect cuticle or seeds.
Most aquatic vertebrates excrete nitrogenous wastes in the form of ammonia, which is highly toxic but very soluble and easily gotten rid of if water is in ample supply. Uric acid excretion may have first developed in the first vertebrates to evolve shelled, fully terrestrial eggs. Such eggs must retain the waste products produced by embryonic metabolism within the shell until hatching. Toxic, soluble ammonia would soon poison a developing embryo, while non-toxic, insoluble uric acid can simply be stored inside the shell as long as necessary. In developing live birth, mammals may have switched to back to a more soluble compound, urea, so that embryonic waste products could be diffused into the blood stream of the mother and thus excreted.
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