Water conservation in the South American desert mouse opossum, Thylamys pusilla (Didelphimorphia, Didelphidae)

The aim of this paper was to analyze the renal performance of the insectivo rous desert mouse opossum, Thylamys pusilla, a marsupial that inhabits temp erate drylands of the Neotropics. We measured the urine concentrating abili ty of this marsupial in the field and under three different experimental co nditions in the laboratory. (1) Ad libitum treatment: animals were fed ad l ib. on apples, sausages and cat food; (2) restriction treatment: foods high in nitrogen were supplied ad lib., while only apple, a food with high wate r to nitrogen load, was restricted, and (3) fasting: 2 days of food depriva tion. Osmolarity values recorded in the field were similar to those obtaine d under the restriction treatment. The maximum urinary concentration found (9015 mOsm/l) is the highest reported for a marsupial and more in keeping w ith similarly sized desert rodents. Similar urine concentrations in T. pusi lla and desert rodents suggest that xeric environments, rather than phyloge ny, play a major role in water conservation. The kidney of T. pusilla posse sses a long papilla and shows high renal indices (M/C: 9.37; RMT: 11.76). O ur results question the idea that urine concentration is not high in desert -dwelling insectivores. Although their dietary water intake is potentially high, this may not always be sufficient to eliminate the nitrogenous waste resulting from insectivory. Thus, T. pusilla uses its ability to conserve w ater to overcome periods of food shortage and high nitrogen load. (C) 2001 Elsevier Science Inc. All rights reserved.