Final answer:
Mammals do not excrete ammonia because it is toxic; they convert it into urea through the urea cycle, making them ureotelic animals. Aquatic animals excrete ammonia directly, but terrestrial animals convert it into less toxic compounds like urea or uric acid to avoid toxicity.
Step-by-step explanation:
Mammals are not ammonotelic animals because they do not excrete ammonia directly. Instead, they must convert ammonia into a less toxic substance called urea through the urea cycle. This conversion is necessary because ammonia is extremely toxic and its accumulation in the body would be fatal.
In contrast, aquatic animals can afford to be ammonotelic because they can dilute and excrete ammonia directly into their surrounding water. Terrestrial animals have developed mechanisms to eliminate toxic ammonia from their systems by converting it either into urea or into uric acid, depending on the species. Mammals, being ureotelic animals, produce urea, while reptiles and many terrestrial invertebrates produce uric acid.
The process of deamination of amino acids produces ammonia, and this ammonia must be detoxified due to its toxicity. The evolution of different excretory systems reflects adaptations to specific habitats. For example, terrestrial arthropods, birds, and reptiles convert ammonia to uric acid, which is less soluble in water and conserves water for the organism.