The sex of a turtle is determined by the temperature of the nest during incubation (Janzen 1994).  This process is called temperature-dependent sex determination (TSD).  For the turtles in this study, nests that incubate at higher temperatures tend to produce female hatchlings and lower temperatures result in males.  The sex ratio of a nest can be affected by as little as a 2° C change in temperature (Kolbe 2002). In one study conducted in Nebraska on snapping turtles(Chelydra serpentina) documented a dramatic difference in the sex ratio when the temperature of the nest was changed from 25.8° C to 23.9° C (Kolbe 2002).  Similar trends were found in Illinois during a study on painted turtles (Chrysemys picta) ; a temperature change from 26.6° C to 24.5° C caused the nest to shift from 0 to 100 percent male hatchlings (Kolbe 2002).  This sensitivity to temperature change indicates that any change on the surface, including roads, fences, and vegetation, can cause dramatic differences in a nest’s sex ratio.
The humidity of the nest is another environmental influence that can alter the physical traits of the hatchlings.  The shell of a turtle egg is semi-permeable and therefore allows water to move through it.  When the egg is in a more humid environment, it absorbs a large amount of water from the surrounding soil.  However, in a drier nest, the egg will not absorb as much water and may even lose some of its mass.  A result of this permeability is the eggs in moist nests will be have more mass and will yield larger hatchlings (Packard et al. 1991), which have a higher probability of living to sexual maturity (Cagle et al. 1993).  The eggs from a more humid nest also have a longer incubation time then the eggs from a drier nest (Packard et al. 1991).  The humidity of the nest does not, however, have any influence on the gender distribution of the hatchlings (Packard et al. 1991).