Coping with the heat: Thermoregulation and welfare of the plains vizcacha under acute thermal challenge (Rodentia: Chinchillidae).

Environmental temperature is a key determinant of animal welfare, and rising global temperatures increase the risk of heat stress. While thermal tolerance is critical for predicting wildlife vulnerability to climate change, physiological responses in South American wild rodents remain understudied. Here, we assessed heat stress responses in the plains vizcacha (Lagostomus maximus), a semi-fossorial rodent with key ecological functions, to establish upper critical Temperature-Humidity Index (THI) thresholds. Within an observed THI range of 18.5 to 30.7, non-invasive longitudinal monitoring and terminal metabolic indicators were utilized to identify critical shifts in thermoregulatory strategy. Fourteen adults were randomly assigned to either a Control (n = 6) or Heat Stress group (n = 8). All animals were housed at 22 ± 1 °C for 7 days, while Heat-Stress animals experienced a daily 8-h thermal wave peaking at 32 °C. At thermal peak, nasal surface temperature (Tnose) was significantly higher in Heat Stress animals (37.6 ± 0.09 °C) compared to Controls (35.1 ± 0.08 °C; P < 0.001) but consistently returned to baseline after exposure. Tnose plotted against THI revealed a marked slope increase above THI = 25, indicating the transition from comfort to mild stress, with overt signs of exhaustion emerging above THI = 30. Despite this, no cumulative effects were detected: hematocrit, cortisol, kidney function, body weight gain, and total activity remained unchanged. Heat exposure did, however, shift resting behavior toward recumbency and modestly increased feeding, consistent with energy allocation to thermoregulation. Based on these findings and within the observed range, we propose species-specific thresholds for vizcachas: comfort (<25), mild stress (25-30), and moderate stress (30-30.7). This profile, early sensitivity but resilience to moderate heat, likely reflects adaptation to their wide geographic distribution, though long-term resilience under intensifying climate conditions remains uncertain.