Final answer:
The kangaroo rat has the longest loops of Henle among the given options, as an adaptation to conserve water in its arid habitat.
Step-by-step explanation:
The animals with the relatively longest loops of Henle in their kidneys are those that live in arid environments and need to conserve water. Among the options given—hippos, kangaroo rats, muskrats, and mountain beavers—the kangaroo rat is known for having a very efficient kidney with longer loops of Henle. This adaptation allows the kangaroo rat to produce highly concentrated urine, drastically reducing water loss from the body and enabling it to thrive in dry climates.
Juxtamedullary nephrons, which have longer loops that extend deep into the renal medulla, play a critical role in this process. They are specialized to recover much of the Na+ and water that were filtered by the glomerulus through osmosis in the descending limb and active transport in the ascending limb. Because the kangaroo rat lives in environments where water is scarce, it has evolved to have more of these juxtamedullary nephrons and hence, longer loops of Henle, to efficiently manage its water and electrolyte balance.