Organisms which generally excrete ammonia as nitrogenous wastes is said to be ammonotelic in nature which aquatic animals are generally.

Ammonia is a compound which is highly toxic and is soluble in water as the nitrogen has lone pair of electrons which makes it susceptible to form hydrogen bonds with the oxygen lone pair of water.

Excretion of urea requires a large amount of water which can only be provided by aquatic animals while on the other hand terrestrial animals cannot afford to lose so much of water and get dehydrated so they convert their ammonia to products like urea or uric acid which are comparatively less toxic.

The terrestrial mammals or even amphibians mainly excrete nitrogenous wastes in form of urea and hence are known as ureotelic organisms while reptiles and etc. excrete nitrogenous wastes in form of uric acid and hence are called uricotelic organisms.

Thus this is how the conversion of ammonia to urea or uric acid have been the adaptation that the terrestrial organisms have gone through for conservation of water.