<p>Why mammals defend species-specific body temperatures, often across a range extending from the low 30s to around 40&#xa0;°C, remains a central question in comparative physiology. In this Commentary, it is suggested that mammalian heat tolerance is best understood not through single environmental thresholds or global mean temperature, but through the interaction of microclimate, heat-balance physics, effector coordination and cardiovascular constraint. Using humans as one focal example within a broader comparative framework, it distinguishes endothermy from homeothermy and then consideration as to how different mammalian lineages solve the problem of heat balance through differing combinations of evaporative cooling, behavioural refuge, water conservation, heterothermy and tolerance of transient body temperature elevation. The discussion then addresses mechanistic pathways of heat exchange, emphasising the importance of radiation, airflow and evaporative opportunity within the immediate microclimate, and present a microclimate-linked Arrhenius/Q₁₀ postulate for why a defended body temperature in the high 30&#xa0;°C range is biologically plausible for some mammalian lineages. The central argument is that humans are not thermoregulatory outliers, but one endothermic solution to environmental heat load, and that the question of future survivability depends on whether specific microclimates remain compensable as radiation, humidity, airflow, metabolic demand, and water availability interact.</p>