Thermal conductivity of sand and its effect on the temperature of loggerhead sea turtle (Caretta caretta) nests

The conductivity of sand at a depth of 30-50 cm was measured at 15 sites on the beach at Captiva Island in south-west Florida which is used by nesting loggerhead turtles (Caretta caretta). The mean daily temperature of the sand was correlated with conductivity at the same depth measured the same day (r=0.611). When day to day variation was removed the correlation between nest temperature and conductivity increased to 0.694. The sand was highly variable in its grain structure. The dominant variability (80.6%) was redescribed by the first two principal components of a Principal Components Analysis (PCA). These two components were influenced mostly by percentages of large (>1 mm) and small (<500 mu m) grains respectively. Conductivity was strongly correlated with the grain structure of the sand. The first three principal components describing sand grain structure, explained 84.1% of the variation in conductivity. Moisture content of the sand (always <5%) was not an important factor. Sites dominated by larger grains generally had poorer conductivity and were cooler. Comparisons of eight nests to seven adjacent random sites revealed no strong evidence for directional selection in nest placement relative to sand conductivity. The variance in conductivities recorded at nests was also not significantly different from the variance at random sites.