There are striking parallels in physiological traits between all newborn mammals and adults of species capable of hibernation. Tolerance to low environmental oxygen (i.e. hypoxia) is one of these. As hypoxia tolerance is not present in adults of most non-hibernating species, we hypothesized that differences in hypoxia tolerance in adult hibernators and non-hibernators reflect developmental changes in the way oxygen demand and supply are matched. To test this hypothesis, we exposed newborn and adult hibernators (the big brown bat; Eptesicus fuscus), adult daily heterotherms (the Argentine brown bat; Eptesicus furinalis), and adult non-hibernators (the little yellow-shouldered bat; Sturnira lilium) to progressive reductions in inspired levels of oxygen (21, 12, 9, and 7% O2) and measured their metabolic, thermoregulatory, and ventilatory responses. Severe hypoxia (7% O2) led to a profound depression in oxygen demand in newborn (57 ± 6%) and adult (43 ± 12%) hibernators, and adult daily heterotherms (40 ± 14%), independent of decreases in body temperature (<2 °C). Unlike hibernators and daily heterotherms, adult non-hibernators did not reduce oxygen demand or body temperature in hypoxia; instead they increased oxygen supply through a significant increase in ventilation (105 ± 21%). While non-hibernating bats match oxygen supply to demand in hypoxia by a brisk ventilatory response, newborn and adult hibernators, and adult daily heterotherms do so by suppressing metabolism. Our results suggest that bats employ divergent strategies to tolerate hypoxia, and that hibernating bats may in fact just be big babies.