How do kangaroo rats live off of so little water?

Kangaroo rats stay underground during the day, when temperatures are high and humidity low. Their burrows are much cooler and more humid, so they minimize evaporative water loss. At night, temperature is lower, so there is less evaporation although humidity is not much higher in their desert habitats. They obtain the water they need from their food. They mainly eat seeds, which are high in fat content. When fat is metabolized, water is one of the products.

"Metabolic water refers to water created inside a living organism through their metabolism, by oxidizing energy-containing substances in their food. Metabolism produces about 110 grams of water per 100 grams of fat, 41.3 grams of water per 100g of protein and 55 grams of water per 100g of starch."

http://en.wikipedia.org/wiki/Metabolic_water