Evaporative cooling and vasodilation mediate thermoregulation in naked mole-rats during normoxia but not hypoxia

Naked mole-rats are among the most hypoxia-tolerant mammals but have a poor thermoregulatory capacity due to their lack of insulating fur and fat, and small body size. In acute hypoxia, naked mole-rat body temperature (T_b) decreases to ambient temperature (T_a) but the mechanisms that underlie this thermoregulatory response are unknown. We hypothesized 1) that naked mole-rat blood vessels vasodilate during hypoxia to shunt heat toward the body surface and/or 2) that they augment heat loss through evaporative cooling. Using open-flow respirometry (indirect calorimetry) we explored metabolic and thermoregulatory strategies of naked mole-rats exposed to hypoxia (7% O₂ for 1 h) at two relative humidities (RH; 50 or 100% water saturation), and in two T_a's (25 and 30 °C), alone, and following treatment with the vasoconstrictor angiotensin II (ANGII). We found that T_b and metabolic rate decreased in hypoxia across all treatment groups but that neither RH nor ANGII effected either variable in hypoxia. Conversely, both T_b and metabolic rate were reduced in 100% RH or by ANGII treatment in normoxia at 25 °C, and therefore the absolute change in both variables with the onset of hypoxia was reduced when vasodilation or evaporative cooling were prevented. We conclude that naked mole-rats employ evaporative cooling and vasodilation to thermoregulate in normoxia and in 25 °C but that neither mechanism is involved in thermoregulatory changes during acute hypoxia. These findings suggest that NMRs may employ passive strategies such as reducing thermogenesis to reduce T_b in hypoxia, which would support metabolic rate suppression.