Ability of Mammalian Kidney
The ability of mammalian kidney to concentrate urine is closely related to the length of the loop of Henle. Animals that produce the most highly concentrated urine (e.g., sand rat) have very long loops. Those that have a limited ability to concentrate the urine (e.g., beaver) have short loops. On the other hand, mammals with an intermediate ability to concentrate the urine (e.g., man, rabbit) have kidneys with both short and long loops.
Figure: Comparison of the kidneys of three rodents from different environments
Since the loop of Henle is a U-shaped tube embedded in the medulla, longer loops would mean thicker medulla. The relative thickness of the renal cortex and medulla in mammals is therefore, correlated with the urine concentration in the animals. In forms which produce low maximum urine concentration, the medulla is much thinner and smaller than in species that produce a highly concentrated urine. This is best appreciated from a comparison of the medullary thickness in rodents from aquatic, general terrestrial and desert environment.