Here's why:

* Urea is the primary nitrogenous waste product of mammals. It's formed in the liver from ammonia, which is produced during the breakdown of proteins.

* Other animals may excrete different waste products:

* Fish excrete ammonia directly.

* Birds and reptiles excrete uric acid.

So, while other animals might produce some urea, mammals are the ones that primarily excrete it as their primary waste product.