Frogs seek hypoxic microhabitats that accentuate metabolic depression during dormancy

Many animals occupy microhabitats during dormancy where they may encounter hypoxic conditions (e.g. subterranean burrows). We used the green-striped burrowing frog (Cyclorana alboguttata) to test the hypothesis that animals seek hypoxic microhabitats that accentuate metabolic depression during dormancy. We first measured the partial pressure of oxygen (P_O ) within artificial cavities excavated in wet clay soil, which simulated C. alboguttata underground aestivation chambers, and recorded hypoxic conditions (P_O  as low as 8.9 kPa). Using custom-built tunnels that maintained a longitudinal P_O  gradient (hypoxic to normoxic), we then examined the P_O  preference of C. alboguttata in response to drying habitat conditions. In support of our hypothesis, we found that C. alboguttata chose to spend a greater proportion of time at the hypoxic end of the P_O  gradient compared with the normoxic end. To determine whether hypoxia accentuates metabolic depression in C. alboguttata, we exposed frogs to normoxia (21.0 kPa) or hypoxia (10.5 kPa) for 7 weeks during the transition from an active to an aestivating state. We found that hypoxia exposure accelerated the onset of metabolic depression in C. alboguttata by 2 weeks. Furthermore, we found that frogs exposed to hypoxia exhibited a 66% reduction in O₂ consumption after 7 weeks compared with active frogs in normoxia, whereas frogs exposed to normoxia reduced O₂ consumption by only 51%. Overall, our findings indicate that some animals may seek microhabitats to maximally depress metabolic rate during dormancy, and that microhabitat O₂ availability can have significant implications for energy metabolism.