Herbivore thermoregulation, movement behaviour and survival during a severe drought in Namibia

Climate change is expected to increase the frequency and severity of droughts in dryland ecosystems, yet the mechanisms determining whether large herbivores survive these events remain poorly understood. We investigated how an extreme drought influenced movement, water use, body temperature and survival of two water-dependent grazing herbivores, plains zebra (Equus quagga) and blue wildebeest (Connochaetes taurinus), in Etosha National Park, Namibia. GPS collars and abdominal temperature loggers were deployed on free-ranging animals during the most severe drought on record. We quantified changes in movement, drinking behaviour, thermoregulation and survival across seasons classified as cool drought, hot drought and rainy season recovery periods. Both species exhibited substantial behavioural plasticity as drought intensified, including shifts in diel activity, shorter median intervals between visits to perennial water sources and highly variable drinking schedules among individuals. Zebra increased daily travel during the hot drought, whereas wildebeest reduced daily travel and shifted activity toward cooler periods of the day, reflecting contrasting strategies for balancing resource acquisition with thermoregulation. Despite these behavioural adjustments, both species exhibited their greatest body temperature variability during both the cool and hot drought periods, indicating that these behavioural responses were insufficient to fully buffer the physiological consequences of the drought. Following rainfall, increased vegetation greenness was associated with higher minimum body temperatures and reduced daily temperature amplitudes, consistent with physiological recovery. Nevertheless, drought-related mortality reached 27% in zebra and 15% in wildebeest, differences that were not statistically significant. Although this drought was considerably more severe than those reported from other African protected areas, grazer mortality was comparable to or lower than that documented during less severe droughts, suggesting that behavioural flexibility together with the large, heterogeneous landscape of Etosha partially buffered drought impacts. Our findings demonstrate that behavioural responses can delay, but not fully prevent, the breakdown of physiological homeostasis during extreme drought. Integrating movement, physiology and survival provides a mechanistic framework for identifying populations approaching their resilience limits as climate change increases the frequency of extreme drought events.