Gas Exchange during Hypoxia and Hypercarbia of Terrestrial Turtles: A Comparison of a Fossorial Species (Gopherus polyphemus) with a Sympatric Nonfossorial Species (Terrapene carolina)

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Abstract
Gopherus polyphemus is a terrestrial and fossorial tortoise that spends extended periods in burrows where the CO₂ concentrations may reach 6%-7% and O₂ may fall to 12%. Terrapene carolina is a terrestrial turtle that is sympatric with the gopher tortoise but is not fossorial. The standard metabolic rates (SMR) of the two species are similar when the data are adjusted for differences in body size. The critical oxygen tension ( ) of the tortoise (10-11 mmHg) is lower than that of the box turtle (26 mmHg). The of both species is unaffected by hypercarbia (6% CO₂). In moderate hypoxia (15% O₂), hypercarbia elicits an increase in of 72% in tortoises after 0.75 h of hypercarbia and 206% in box turtles. After 12-18 h of hypercarbia, of box turtles returns to normal, while it remains elevated by 22% in gopher tortoises. Both species hyperventilate continuously in hypercarbia, with CO₂ loading occurring in 3-5 h in box turtles and 8-10 h in tortoises; during these periods the respiratory exchange ratios (R) initially fall below zero and then recover to normal levels. The responses of box turtles to hypercarbia in terms of relative changes in respiratory parameters are greater and more variable than those of gopher tortoises. Both species hyperventilate in response to hypoxia, with or without concomitant hypercarbia, as indicated by increases in R and . These increases begin at a Po₂ about 20 mmHg above the , suggesting that adjustments of ventilation are an important factor contributing to the low of chelonians.