...rarely drink water. Instead, they have a highly water-efficient metabolism (their kidneys are at least four times more efficient at retaining water and excreting salt than those of humans) and manufacture water through a metabolic process called oxidative phosphorylation.

One special feature of the kangaroo rat is the animal's efficient kidneys. The kangaroo rat has a longer loop of Henle in the nephrons which permit a greater magnitude of countercurrent multiplication and thus a larger medullary vertical osmotic gradient. As a result, these rodents can produce urine that is concentrated up to an osmolarity of almost 6,000 mosm/liter, which is five times more concentrated than maximally concentrated human urine at 1,200 mosm/liter. Because of this tremendous concentration ability, kangaroo rats never have to drink; the H2O produced metabolically within their cells during oxidation of foodstuff (food plus O2 yields CO2 + H2O + energy) is sufficient for their body. Kangaroo rats lose so little water that they can recover 90% of the loss by using metabolic water gaining the remaining 10% from the small amount of water in their diet. Kangaroo rats lose water mainly by evaporation during gas exchange and gain water mainly from cellular metabolism.

-Wikipedia