Figure 8. Aquatic overwintering eliminates hypoxic ventilatory depression characteristic of the mature central breathing control system .

A, integrated CN V neurograms from semi‐terrestrial (top) and aquatic overwintered (bottom) bullfrogs. Upon transition to hypoxic aCSF (0% O₂) (and presumably tissue anoxia) burst activity recorded through CN V underwent complete inhibition in semi‐terrestrial bullfrogs. In contrast to the normal adult response to hypoxia, brainstems from aquatic overwintered bullfrogs did not stop bursting during hypoxia. B–D, mean data and individual lung burst frequency responses to hypoxia. Sample sizes are included in each figure. In each figure, the grey lines connect baseline burst frequency to the response of that individual after 15 min of hypoxia. There was a main effect of hypoxia (P = 0.0031; two‐way ANOVA) because of decreases in burst frequency in semi‐terrestrial bullfrogs (B; P < 0.05; Holm–Sidak's multiple comparisons test). Both groups of aquatic overwintered bullfrogs did not change burst frequency during hypoxia (C and D; P > 0.05 Holm–Sidak's multiple comparisons test). E, the response to hypoxia as a percentage of baseline. There was a significant interaction between acclimation group and hypoxia (P = 0.0037; two‐way ANOVA). Hypoxia depressed lung bursting in semi‐terrestrial bullfrogs (P < 0.001; Holm–Sidak's multiple comparisons test), but not in both group of aquatic overwintered bullfrogs. Bursting during hypoxia decreased more in semi‐terrestrial bullfrogs compared to both overwintered groups. ^* P < 0.05, ^*** P < 0.001 for within group comparisons. ^† P < 0.05, ^†† P < 0.01 for between group comparisons. Error bars represent SEM.