There is a great degree of variation among species with TSD when looking at their nesting patterns, behaviors, breeding seasons, and physiology in general. In can be said that each of these however, can potentially have an effect on the sex ratios of offspring.
Laboratory vs. Field Investigations
Much of the research performed on TSD has occured within controlled laboratory conditions in which incubators are kept at constant temperature. However, because of the practical apsects of monitoring temperatures in the wild over a lengthy incubation period, little field study has been done. Field conditions are important to assess due to the daily and weekly fluctuations in environmental temperature. In an experiment conducted on the nests of map turtles, Graptemys geographica,in the field, the sex of hatchlings was demonstrated to be influenced by nest temperature. Researchers found that hatchlings from nests that attained 30 degrees Celsius less than 4 hours per day were much more likely to be male. Those hatchlings from nests that were 30 degrees for 6-8 hours per day were predominantly female (Bull, 1985).
Both the depth of the nest and amount of shade or vegetative cover can influence the temperature and thus the sex ratio of offspring. Scientists have suggested that nesting females may deliberately influence the sex ratio of their offspring and the population by choosing a site that has greater cover. However, it is difficult to hypothesize that nesting females can predict incubation temperature during the differentiation period of offspring (Janzen 1994).
Another factor that may influence sex ratio in turtle hatchlings is metabolic heat produced by developing embryos. One study found that metabolic heating in nests of green sea turtles (Chelonia mydas), determined by both the number of eggs and number of developing embryos in a nest, had the potential to raise nest temperature by as much as 2.5 degrees Celsius, leading to an estimated 30% increase in the proportion of females produced (Broderick, Godley, and Hays, 2001).
The wide habitat ranges over which many species reside similarly presents a wide range of temperatures over which turtles of the same species may mate and lay their eggs. However, studies have demonstrated that within species, the climatic temperature range greatly exceeds the variation in threshold temperatures from warmer and cooler locations. This might lead to the conclusion that southern populations of a species would have female biased sex ratios, but this was demonstrated not to be the case (Bull, Vogt, and McCoy, 1982).
The potential implications of a drastic climate change such as global warming on species that employ TSD are indeed serious. A modest increase in global temperature as small as 2 degrees Celsius could drastically skew the sex ratios of species with TSD. Thus far, long term studies on local populations of painted turtles (Chrysemys picta) have not demonstrated a significant change in offspring sex ratio, but the possibility certainly exists (Janzen 1994). In fact, rapid global climate change has been suggested as a factor in the mass extinction of dinosaurs, presuming that they too employed TSD (Standora and Spotila, 1985).