Temperature Dependent Sex Determination

Incubation temperatures are also important because the sex of a sea turtle is determined by the average temperature of the nest during the second trimester of embryonic maturation.

This is called temperature-dependent sex determination (TSD) and is common in reptiles. In our area for Loggerhead Sea turtles, the temperature where 50% of the embryos will be male and 50% will be female is around 84.6 degrees Fahrenheit. This is known as the pivotal temperature. A mean temperature of 86.9 degrees Fahrenheit will result in 100% females and a mean temperature of 81.5 degrees Fahrenheit will result in 100% males. Temperatures in between 86.9 and 81.5 will produce a mix of male and female sea turtles. It only takes a small change in temperature of a nest to affect the gender make up of hatchlings (see graph below). As our climate changes and oceans warm, colder northern breeding grounds such as those covered by N.E.S.T. OBX are important to providing sufficient numbers of male hatchlings to sustain and grow overall sea turtle populations.

The above graph shows the estimated makeup of male/female hatchlings in our Northern Outer Banks nests.  We utilized 2nd trimester mean temperature data collected from each of our successfully hatched nests in years 2019-2025 and used the Mrosovsky (1988) model for sex determination.  The graph below shows that numbers of males and females those percentages represent.