The costs of an aquatic life are likely to be strongly influenced by an animal's buoyancy, which can be measured as specific gravity (SG). The shells of chelonians are negatively buoyant, and the tendency to sink is opposed by air in the lungs. Turtles can therefore regulate buoyancy by adjusting residual lung volume, but must do so within the constraint of a relatively fixed body volume set by the rigid shell. Emydids have been reported to effect changes in lung volume with reciprocal changes in the volume of water stored in the urinary bladder and, perhaps, for rapid short-term adjustments, the cloacal bursae. We examined buoyancy control in two species, the eastern painted turtle Chrysemys picta (Emydidae), which has well-developed cloacal bursae, and the common musk turtle Sternotherus odoratus (Kinosternidae), which lacks bursae. We measured SG in captive turtles, then manipulated SG by attaching either weights or floats, and measured SG again at 4 h and 24 h after attachment. Initial SG was consistently and significantly higher in bottom-walking Sternotherus (1.03) than in more free-swimming Chrysemys (1.01). Both species compensated nearly completely for weights up to 6% of body mass by decreasing weight in water (reflecting higher lung volumes); compensation in both species was mostly complete at 4 h. Both species also compensated for attached floats (buoyant forces equivalent to up to 4% of body mass in Sternotherus and at least 7% in Chrysemys) by decreasing lung volume. Lung volume changes (gauged by underwater weight) were not matched by changes in stored water volume (gauged by body mass in air) in Sternotherus and only partially matched in Chrysemys. Changes in lung volume did not affect surfacing frequency. Small differences in buoyancy regulation between species reflect differences in natural history and, presumably, morphology, but neither the mechanisms of chronic lung-volume maintenance nor the function of cloacal bursae are clear.
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Vol. 64 • No. 2