Metabolism and Water Balance of Active and Cocooned African Bullfrogs Pyxicephalus adspersus

The African bullfrog, Pyxicephalus adspersus, becomes dormant (when subjected to desiccating conditions) and may form a cocoon which covers the entire body surface except for the external nares. The resting {aastex} {amsbsy} {amsfonts} {amssymb} {bm} {mathrsfs} {pifont} {stmaryrd} {textcomp} {portland,xspace} {amsmath,amsxtra} {wasysym} {empty} {10}{9}{7}{6} {document}V̇_O₂{document} of Pyxicephalus (0.037 ml O₂ g⁻¹ h⁻¹) is low for Anura but the {aastex} {amsbsy} {amsfonts} {amssymb} {bm} {mathrsfs} {pifont} {stmaryrd} {textcomp} {portland,xspace} {amsmath,amsxtra} {wasysym} {empty} {10}{9}{7}{6} {document}V̇_O₂{document} increased 23-fold during activity to expected anuran values (0.86 ml g⁻¹ h⁻¹). The {aastex} {amsbsy} {amsfonts} {amssymb} {bm} {mathrsfs} {pifont} {stmaryrd} {textcomp} {portland,xspace} {amsmath,amsxtra} {wasysym} {empty} {10}{9}{7}{6} {document}V̇_O₂{document} declined in dormant or cocooned frogs to onefourth of the resting value. The resting RQ of 0.86 did not alter significantly either during the cocooned state or when active. The heart rate of resting frogs was 6.1 beats min⁻¹, and pulmonary ventilation rate was 0.9 breaths min⁻¹. These values increased ninefold and 77-fold, respectively, during activity, but did not significantly decline during periods of dormancy with a cocoon, despite the 75% reduction in {aastex} {amsbsy} {amsfonts} {amssymb} {bm} {mathrsfs} {pifont} {stmaryrd} {textcomp} {portland,xspace} {amsmath,amsxtra} {wasysym} {empty} {10}{9}{7}{6} {document}V̇_O₂{document}. Oxygen pulse consequently declined markedly for cocooned frogs (30 μl kg⁻¹ beat⁻¹) from resting values (101 μl kg⁻¹ beat⁻¹). The rate of evaporative water loss for noncocooned frogs (mean mass = 350 g) was 6.4 mg g⁻¹ h⁻¹ and declined slowly during cocoon formation to 0.22 mg g⁻¹ h⁻¹ or less. Dehydrated, cocooned frogs (mean loss in body mass = 20%) had a higher plasma [urea], [Na⁺], and osmolality compared to hydrated, noncocooned frogs. Normal electrolyte and urea levels, and body mass, were restored after rehydration. Blood hemoglobin content and hematocrit increased during dehydration and cocoon formation, reflecting a loss in plasma volume. This hemoconcentration was reversed by rehydration after removal of the cocoon. Cocooned frogs were markedly acidotic (blood pH = 7.4) compared to resting frogs (pH = 7.9), and the acidosis was associated with a decreased oxygen pulse and greater reliance upon pulmonary ventilation. The high resistance to gaseous diffusion of the cocoon would also be expected to contribute to the acidosis. The rates of rehydration for Pyxicephalus after losing 20% of original mass were similar after removal from cocoons or after rapid dehydration (38 mg H₂O g⁻¹ h⁻¹). These rehydration rates are much lower than those reported for smaller desert frogs. Bladder volumes of hydrated Pyxicephalus averaged 14.1% of body mass (maximum 32%). The adaptive significance of dormancy and cocoon formation, with concomitant reduction in metabolic rate and evaporative water loss, is that the bullfrog can survive the 7–8-mo African dry season by utilizing food and water reserves.