Here’s a very brief overview of the kidneys of animals (other than humans). All vertebrates have kidneys and the functional unit of the kidney in all species is the nephron. Freshwater fish–like the zebrafish–have a kidney which is kind of like a long tube. In zebrafish embryos (as discussed previously in another post) the kidney starts out as a structure called the pronephric duct, which consists of a single nephron, and as the fish becomes an adult develops into the adult mesonephros which contains multiple nephrons. The function of the freshwater fish kidney is predominantly to create a dilute urine that osmolar homeostasis.
The frog kidney must be able to adapt to both freshwater and land conditions–thus the frog kidney must be able to create both a dilute and a concentrated urine, depending on the environment to which the frog is exposed.
Snakes live in a dry environment and therefore their kidneys must retain water. Snakes metabolize nitrogenous wastes into uric acid, which can be excreted in an insoluble form using very small amounts of water.
The bird kidney also eliminates waste via uric acid. The whitish pasty component of birdshit
is in fact a uric acid paste that represents the renal waste product, which is usually mixed in with the brown stuff (the feces from the GI tract).
Mouse kidneys are similar to human kidneys in that they use soluble urea, rather than insoluble uric acid, as the waste product.