Blood Osmolality in Mammals Signals Physiological Strain from Climate Change

Ecological investigations have identified blood osmolality as a precise metric for quantifying the physiological duress wild mammals experience under the pressures of a changing global climate. This research establishes a quantifiable internal biomarker for environmental harshness, offering a critical tool for conservation science by underscoring the immediate, measurable biological costs associated with climatic shifts on vulnerable populations.

A pivotal 12-year study focused on the African striped mice, *Rhabdomys pumilio*, within South Africa's Succulent Karoo biome revealed a direct, statistically significant relationship between environmental severity and elevated blood concentration. Researchers tracked these small mammals across seasonal variations, noting that serum osmolality was markedly higher during the arid season, particularly when food availability plummeted, indicating increased dehydration stress. While maximum temperature was the strongest predictor across all seasons, food scarcity explained a greater proportion of the variation within the dry season itself, suggesting heat combined with a lack of water-rich forage directly impacts internal fluid balance.

Specifically, the concentration of the mice's blood increased by an average of 5 mosmol kg⁻¹ during the harsh dry season compared to the temperate moist season, confirming significant dehydration as temperatures soared, sometimes exceeding 40°C. To mitigate this stress, the striped mice reduced their daily energy expenditure by 30% during the food-restricted dry season by curtailing both activity levels and basal metabolism. This physiological adjustment serves as a key mechanism for water conservation when their primary water source—moist food plants—becomes scarce.

The work, involving researchers such as Carsten Schradin and Neville Pillay from the University of the Witwatersrand and collaborators from CNRS in Strasbourg, establishes serum osmolality as a reliable indicator of environmental hardship in mammals adapted to arid zones. The Succulent Karoo Research Station, which has operated for 25 years, focuses on understanding the evolution of these behavioral and physiological mechanisms relevant to predicting species responses to climate change. For small endotherms, rapid water loss from increased thermoregulatory cooling methods can quickly lead to dehydration, a major survival threat.

Conservation managers may leverage these findings by measuring blood concentration in wild animals to gain actionable insights for provisioning during droughts when environmental resources are constrained. This approach provides a mechanistic, bottom-up understanding of climate change impacts, complementing broader population modeling efforts. The study on *Rhabdomys pumilio*, which reported population densities ranging from 36.6 ± 21.2 mice/ha to a peak of 171.1 ± 40.9 mice/ha in December, provides a baseline for assessing how physiological stress might influence population dynamics in this biodiversity hotspot.