Sex Ratio of Sea Turtles: Seasonal Changes
Sexual differentiation in sea turtles, as in a number of reptiles, depends on the ambient temperature during incubation of the eggs (1-3). Therefore the sex ratio of offspring should differ at different times of year. This is especially likely to happen in species of sea turtles that lay several clutches over an extended nesting season. This idea has been discussed (2, 3), but conclusive data are lacking. We now report that seasonal changes occur in the sex ratio of loggerhead turtles (Caretta caretta) nesting in the southeastern United States. The effects are large and have implications for conservation programs and for the study of sea turtle demography.
Hatchling loggerhead turtles were collected from 1979 to 1982 from various barrier islands in South Carolina and Georgia. The nesting beaches frequented by loggerhead turtles in these regions are predominantly primary dune, either devoid of cover or sparsely covered with sea oats (Uniola paniculata). The lack of dense vegetation and the associated shade, along with the relative openness and homogeneity of the barrier island beaches, reduces the importance of spatial variables. From each clutch sampled, ten hatchlings were taken at random (4). Sex was determined histologically (5).
Sex ratio ranged from 10 percent female or less during the cooler ends of the season to 80 percent female in the middle of the summer (Fig. 1). Although variability occurred among clutches, none was less than 40 percent female between 12 June and 14 July, and most were 75 percent female or more. Not all the data came from the same year. When the results for 1982, the year with most available data, are considered separately, the seasonal trends are essentially the same. Also, some nests had been transplanted to protected sites soon after laying. However, there is no evidence that these nests had markedly different sex ratios (Fig. 1). This is not surprising because the areas selected for reburying the eggs were similar to those selected by nesting turtles.
Because seasonal frequency of nesting is known for these beaches, the overall sex ratio for the whole season may be estimated by combining the sex ratio and nesting frequency data (Fig. 1, A and C). When all the sex ratio data are combined with the average nesting frequency data for 6 years (1977 to 1982) for Sand and South Islands, 56.3 percent of the hatchlings are female (6). When similar calculations are made from the 1982 data only, 48.2 percent of the hatchlings are female. Because relative nesting frequencies...