Arousal from hibernation increases blood oxygen saturation in 13-lined ground squirrels

Hibernating Ictidomys tridecemlineatus, 13-lined ground squirrels, are considered models of ischaemia-reperfusion tolerance, as both tissues and isolated mitochondria withstand anoxia followed by rapid re-oxygenation in vitro. This tolerance is likely adaptive, protecting against damage during the numerous arousals from torpor throughout the hibernation season. O2 availability is likely low during torpor, but suppressed metabolism lowers O2 demand, potentially mitigating hypoxic stress. During arousal to interbout euthermia (IBE), heart rate, blood pressure and ventilation increase rapidly, suggesting increased O2 availability, but tissue oxygenation has not been measured during arousal or IBE in 13-lined ground squirrels. Using pulse-oximetry collars, we characterized dramatic increases in O2 availability during arousal; carotid artery O2 saturation rose from as low as 35% early in arousal to 87% during IBE. These changes closely followed rising heart rate. Our results demonstrate that hibernating 13-lined ground squirrels survive profound O2 deprivation early in arousal and rapid O2 influx as arousal progresses.