Kidneys of desert animals have longer loops of Henle to make the animals' urine as concentrated as possible and limit the amount of water and salt they loose. This helps desert animals live for long periods of time on minimal amounts of water.
The loops of Henle are found in the kidneys of reptiles, birds and mammals; however the loops are generally the longest in mammals that reside in arid environments. The loops of Henle in birds and reptiles are smaller as these animals have developed alternate means of retaining or expelling water and salts.
The two segments of the loops of Henle are the descending loop and the ascending loop. As urine enters the descending loop, the loop's permeable walls absorb water. The urine is extremely concentrated, and very little water remains after it passes through this first segment.
The concentrated urine then continues up through the ascending loop where salts are leeched from the solution. In desert animals, like the kangaroo mouse, both these sections of the loops of Henle are longer, providing greater surface area by which more water and salt can be removed from the urine as it passes through the lops. The loops of Henle found in the kangaroo mouse are so efficient at this process that the animal does not need to drink water because it can filter enough water from the seeds it eats.