The Respiratory Microenvironment within the Burrows of Gopher Tortoises (Gopherus polyphemus)

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Abstract
Concentrations of O2 and CO2 were measured along the lengths and at the ends of occupied burrows of the Gopher Tortoise (Gopherus polyphemus) in Putnam and Leon counties in Florida and Thomas County in Georgia [USA]. Hypoxia and hypercarbia were found at all times of the year, but were most pronounced in burrows in clayey soils. Respiratory conditions were at their worst during the warmest months, but probably represent no significant physiological challenge to animals inhabiting sandy soils (O2 = 20.14%, CO2 = 0.78%). In contrast, burrows in clayey soils, while exhibiting only moderate hypoxia (18.19%), had concentrations of CO2 high enough (2.35%, maximum 6.77%) to be of physiological consequence. Three sources contribute to the hypercarbia in the burrows: CO2 in the surrounding soil; the metabolism of the tortoise; and the metabolism of commensals, particularly those that decompose the feces. Burrows in clayey soils are shorter (3.2 m) than those in sandy soils (4.1 m). We propose that this is due to the significant relationship between the degree of hypercarbia and burrow length that is characteristic of the former, but not of the latter.