Frogs seek hypoxic microhabitats that accentuate metabolic depression during dormancy

Abstract

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 (PO2) within artificial cavities excavated in wet clay soil, which simulated C. alboguttata underground aestivation chambers, and recorded hypoxic conditions (PO2 as low as 8.9 kPa). Using custom-built tunnels that maintained a longitudinal PO2 gradient (hypoxic to normoxic), we then examined the PO2 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 PO2 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.