ENSO-associated response of field urine osmolality in the insectivorous marsupial Thylamys elegans

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 Niño Southern Oscillation (ENSO). El Niño phase of ENSO determined the initial 2 wet years at the study site, while La Niña 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±405 mOsm/kg in summer versus 2.246±209 mOsm/kg in winter. During dry years in summer, Uosm values were: 3.340±384 mOsm/kg versus 2.481±293 mOsm/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×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.