Final answer:
Urea is less toxic than uric acid because it is more water-soluble and thus more easily excreted by the kidneys. Terrestrial animals like birds and reptiles excrete the less soluble uric acid. The kidneys are essential in this excretion process.
Step-by-step explanation:
The assertion that urea is less toxic than uric acid is indeed true. Urea is more water-soluble than uric acid, making it less toxic and easier for the body to excrete. Urea is synthesized in the liver from toxic ammonia, which is a byproduct of protein metabolism, and then it is excreted by the kidneys. This conversion to urea is critical for mammals, including humans, as it allows for the safe elimination of nitrogenous wastes.
Terrestrial animals, such as birds and reptiles, convert ammonia to uric acid, which is less soluble and excreted as a paste or powder to conserve water - an adaptation essential for surviving in dry environments. The kidneys play a crucial role in this process through a network of blood vessels and tubules that filter blood, reabsorb water and electrolytes, and concentrate wastes like urea and uric acid into urine.
In contrast, if these terrestrial organisms evolved to spend most of their time in water, their excretory systems may adapt to more efficiently excrete nitrogenous wastes such as ammonia directly into the water, similar to aquatic animals.