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Deserts are relatively dry and dehydrating places, by definition, but desert animals are composed of 65-75% water, just like non-desert animals. Do desert animals maintain water balance and body hydration level by conserving water better (decreasing output, as compared with non-desert species) or by getting more water each day (increasing input), or both? These questions can be evaluated with measurements of daily water flux rates (in ml/day) as determined with isotopically labeled water through free-ranging animals that are maintaining constant body masses. Inspection of the many field measurements now available reveals great variation (10 million-fold among terrestrial vertebrates alone), due largely to differences in body mass, but also due to taxonomic affinity (endothermy vs. ectothermy), season, diet and age, as well as to variation in habitat aridity. Fortunately, water flux rates can be normalized for much of this variation by dividing them by field metabolic rates, to yield units of ml water used per U energy metabolized (termed Water Economy Index, WEI). The WEI values of desert-dwelling eutherian mammals are significantly lower than in non-desert eutherians, and (surprisingly) desert endotherms have lower WEIs than desert ectotherms. Year-round WEI measurements on desert kangaroo rats, which have the lowest index yet measured, revealed that they often obtained more water than was available just from dry seeds, not by drinking, but by eating young green vegetation when available in spring, and especially by hydrating dry seeds before consumption by storing them in humid burrows for a while. (c) 2004 Elsevier B.V All rights reserved.