Freshwater turtles as a group are more resistant to
anoxia than other vertebrates, but some species, such as painted turtles, for reasons not fully understood, can remain anoxic at winter temperatures far longer than others. Because buffering of
lactic acid by the shell of the painted turtle is crucial to its long-term anoxic survival, we have tested the hypothesis that previously described differences in
anoxia tolerance of five species of North American freshwater turtles may be explained at least in part by differences in their shell composition and buffering capacity. All species tested have large mineralized shells. Shell comparisons included 1) total shell CO2 concentration, 2) volume of titrated
acid required to hold incubating shell
powder at pH 7.0 for 3 h (an indication of
buffer release from shell), and 3)
lactate concentration of shell samples incubated to equilibrium in a standard
lactate solution. For each measurement, the more
anoxia-tolerant species (painted turtle, Chrysemys picta; snapping turtle, Chelydra serpentina) had higher values than the less
anoxia-tolerant species (
musk turtle, Sternotherus odoratus; map turtle, Graptemys geographica; red-eared slider, Trachemys scripta). We suggest that greater concentrations of accessible CO2 (as
carbonate or
bicarbonate) in the more tolerant species enable these species, when acidotic, to release more
buffer into the extracellular fluid and to take up more
lactic acid into their shells. We conclude that the interspecific differences in shell composition and buffering can contribute to, but cannot explain fully, the variations observed in
anoxia tolerance among freshwater turtles.