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From 1991 to 1994, we assessed physiological responses in field urine osmolality (Uosm) of an insectivorous marsupial (Thylamys elegans) of semiarid Chile faced with a complete cycle of El Nino Southern Oscillation (ENSO). El Nino phase of ENSO determined the initial 2 wet years at the Study site. while La Nina phase determined the 2 subsequent dry years. The marsupial showed marked temporal fluctuations in field Uosm values, with both seasonal and between-year patterns of physiological variability. Indeed, Uosm values during wet years were: 2.719 +/- 405mOsm/kg in summer versus 2.246+209mOsm/kg in winter. During dry years in summer, Uosm values were: 3.340 +/- 384 mOsm/kg versus 2.481+293mOsm/kg during winter. There were significant effects of dry versus wet years and between dry and wet seasons on Uosm values, as well as a statistical interaction year x season. We discuss how the integration of flexible physiological mechanisms enable an insectivorous marsupial to cope with seasonal and yearly water variability in a semidesert, and also the importance of using ENSO climate effects as a proxy for the study of future climate change in natural Communities and its subsequent impact on field physiological performance of desert mammals. (c) 2008 Elsevier Ltd. All rights reserved.