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and flush their nitrogenous waste primarily as ammonia.

Terrestrial animals, however, because they have less access to water, have

been under selective pressure to

"repackage" their toxic ammonia as less toxic moleculesflush those less toxic compounds with less water

Mammals metabolize ammonia into a molecule called urea:

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As you can see, this molecule contains two atoms of nitrogen. Although it

requires metabolic energy to build, it also is less toxic than ammonia, and

it requires less water to flush from the system. Your urine's main

nitrogenous waste product is urea.

Reptiles (including birds) go one step further, packaging their nitrogenous

waste as uric acid:

...which contains FOUR nitrogen atoms per molecule. It requires more

metabolic energy to make than urea, but it also eliminates more nitrogen

per molecule and, is quite non-toxic, and requires very little water to flush

from the body.

The white, crystalline substance you see in a typical bird or lizard "poop"

is actually urine: a little pool of uric acid crystals in a very small amount of

water.

So there are three general ways that vertebrates rid their bodies of

nitrogenous waste:

ammonia - one nitrogen per molecule; highly toxic; requires lots of

water to flush

urea - two nitrogens per molecule; less toxic; requires less water to

flush

uric acid - four nitrogens per molecule; non-toxic; requires very little

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water to flush

The evolutionary history and natural selective pressure of any given taxon

of vertebrates contributes to which method the animals use. Which

environments would you suspect to exert selective pressure for each of the

three types of excretion above?

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