Atmospheric hypoxia increases bone robusticity in the American alligator

Abstract

During vertebrate evolution, atmospheric oxygen (O2) level may have varied from as low as 12% to as high as 30%, but no studies to date have considered the effect of ambient O2 on skeletal plasticity. We incubated eggs and subsequently grew alligator hatchlings under chronic hypoxia (12% O2), normoxia (21% O2) and hyperoxia (30% O2). Animals received monthly injections of fluorescent dyes to determine bone deposition rates. After three months, animals were sacrificed and their femora either sectioned at mid‐diaphysis, or ashed. We found femora of hypoxic alligators to have significantly greater mass‐specific cross‐sectional area (+15%), second moment of area (+20%) and polar moment of inertia (+23%) than those of either normoxic or hyperoxic hatchlings. Mineral content was also significantly higher (+6%) in bones of hypoxic animals. This suggests that exposure to chronic hypoxia, but not hyperoxia, resulted in increased resistance to compressive, bending and torsional stresses on the skeleton. Furthermore, the relationship between body mass growth and periosteal deposition rate was different between treatments groups, with hypoxic animals accruing more bone per unit body mass. We suggest that prevalent atmospheric O2 level need be considered when reconstructing size and growth curves of extinct vertebrates. Funded by the NSF IOB 04445680 and IOS 0922756 to JWH.