Thermoregulation is essential yet energetically costly in endotherms, resulting in body temperature fluctuations and biological trade-offs. Advanced biologging technologies allow for continuous, remote monitoring of body temperature in wild animals. We used ruminant implant transmitters (RITs) and GPS collars to examine how desert bighorn sheep (Ovis canadensis nelsoni) thermoregulate and survive in extreme desert conditions. We tracked 43 individuals across six populations in the Mojave Desert of California over a 13-month period—15,342 observation days. RITs logged body temperature and collars recorded ambient temperature—we estimated daily median and variation (heterothermy index, HI) for both. We evaluated seasonal differences between sexes and across populations, and validated collar temperatures with external temperature loggers to assess the effect of ambient temperature on thermoregulation. Desert bighorn body temperature was 38.86°C ± 0.305°C (IQR). Annual HI ranged 0.349—0.617, and individuals with HI > 0.55 had lower survival. Daily HI for both sexes increased with ambient temperature, but male temperatures averaged 0.12°C - 0.21°C cooler than females from July thru October. Our research underscores the critical role of thermoregulation in desert bighorn survival, highlighting sex-specific responses and the impact of ambient temperatures, while demonstrating the potential of biologging to inform wildlife research and conservation.