Relation of Temperature, Moisture, Salinity, and Slope to Nest Site Selection in Loggerhead Sea Turtles

Nest site selection in reptiles can affect the fitness of the parents through the survival of their offspring because environmental factors influence embryo survivorship, hatchling quality, and sex ratio. In sea turtles, nest site selection is influenced by selective forces that drive nest placement inland and those that drive nest placement seaward. Nests deposited close to the ocean have a greater likelihood of inundation and egg loss to erosion whereas nest placement farther inland results in greater likelihood of desiccation, hatchling misorientation, and predation on nesting females, eggs, and hatchlings. To evaluate the role of microhabitat cues in nest site selection in Loggerhead Sea Turtles (Caretta caretta), we assessed temperature, moisture, salinity (conductivity), and slope along the tracks of 45 female loggerheads during their beach ascent from the ocean to nest sites in the Archie Carr National Wildlife Refuge in Florida on the beach with the greatest density of loggerhead nesting in the Atlantic. Of the four environmental factors evaluated (slope, temperature, moisture, and salinity), slope appears to have the greatest influence on nest site selection, perhaps because it is associated with nest elevation. Our results refute the current hypothesis that an abrupt increase in temperature is used by loggerheads as a cue for excavating a nest. Moisture content and salinity of surface sand are potential cues but may not be reliable for nest site selection because they can vary substantially and rapidly in response to rainfall and changes in the water table. Sea turtles may use multiple cues for nest site selection either in series with a threshold that must be reached for each environmental factor before the turtle initiates nest excavation or integrated as specific patterns of associations.

Fig. 1.

Mean temperatures (C) and slopes (angular degrees) for sites along the nesting crawls of loggerheads at Melbourne Beach, Florida. For sites along crawls, numbers represent meters from nest site; the letters “C” and “N” refer to cloaca and nose positions of nest sites, respectively. Vertical bars represent one standard error, asterisks (*) indicate sites that were significantly different from the adjacent site closer to the water (GLM, repeated contrasts; P < 0.05), plus symbols (+) indicate sites that were significantly different from the cloaca position for temperature and the nose position for slope (GLM, simple contrasts; P < 0.05)

Fig. 2.

Mean moisture content (%) and conductivity (microsiemens) values for sites along the nesting crawls of loggerheads at Melbourne Beach, Florida. For sites along crawls, numbers represent meters from nest site; the letters “C” and “N” refer to cloaca and nose positions of nest sites, respectively. Vertical bars represent one standard error, asterisks (*) indicate sites that were significantly different from the adjacent site closer to the water (GLM, repeated contrasts; P < 0.05), plus symbols (+) indicate sites that were significantly different from the nose position (GLM, simple contrasts; P < 0.05)

Fig. 3.

Factor loading plot from principal component analysis for distance from the dune (dist), temperature (temp), moisture content (moist), conductivity (cond), and slope for track sites (closed circles) and nest sites (open circles) along 45 loggerhead nesting crawls at Melbourne Beach, Florida

Accepted: 23 Jun 1999