When she is ready to nest, a female sea turtle hauls herself onto the beach to dig a nest and lay a clutch of eggs in what she deems to be the perfect spot, where the eggs will incubate for up to 70 days (see the full sea turtle life cycle here). When her job is done,  she returns to the sea,  fully entrusting the incubation of her eggs to the sand on the beach. In fact, the nest environment will not only be responsible for developing healthy embryos in her eggs, but it will also determine the sex of the hatchlings that emerge. 

In a freshly laid sea turtle egg, the sex of the embryo has yet to be decided. Unlike in humans (and most species), where sex is determined by genetics, the sex of baby sea turtles is determined by the temperature of the nest during incubation. This fascinating phenomenon is called temperature-dependent sex determination or TSD, and is found among many egg-laying reptiles and fish. For sea turtles, this process is crucial for evolution, population dynamics, and survival.

How does temperature-dependent sex determination work?

As a clutch of sea turtle eggs incubates beneath the sand, the embryos will develop over 45-70 days. The middle third of this range is the “temperature-sensitive period,” during which the embryos will develop gonads (reproductive organs). Warmer temperatures during this time will trigger the release of hormones that will result in the development of ovaries, whereas cooler temperatures will trigger testes.

In other words, a warmer incubation creates more female sea turtle hatchlings, and cooler temperatures more males. A nest can produce fully 100% female hatchlings if average incubation temperature exceeds 31ºC (87.8ºF) during the middle third,  and 100% male hatchlings if temperatures average below 27.7ºC (81.86ºF). The “pivotal temperature,” or temperature at which a perfect 50% female-to-male ratio occurs, is 29ºC (84.2ºF), though this can vary slightly from species to species.

Why temperature-dependent sex determination?

While the adaptive advantage of TSD is still poorly understood, the secret to its success probably lies in its flexibility. By allowing environmental conditions to influence offspring sex, TSD enables populations to adapt to subtle shifts in environmental factors, such as temperature fluctuations. 

TheCharnov-Bull Theory also suggests that the TSD may have evolved because the two sexes survive better at opposite temperature ranges. In the case of sea turtles, this might mean that females fare better in warmer environments and males in cooler ones. An experiment done with freshwater turtles supports this theory, showing that the hatchlings that emerged from 100% female and 100% male nests were consistently fitter than hatchlings of the same sex that hatched from nests with 50% females and males. However, this theory has yet to be tested in sea turtles.

Conservation implications of TSD

With climate change causing global temperatures to rise on beaches everywhere, sex ratios of sea turtle populations are already changing. Warmer temperatures also influence the TSD process, resulting in a higher proportion of female turtles. For example, Raine Island, Australia, one of the largest green turtle nesting beaches in the world, has been producing almost exclusively female sea turtle hatchlings since the 1990s. This dangerously imbalanced sex ratio could have implications for the health of the population. In response, scientists have been exploring ways to cool nests, including spraying them with water and shading them. Learn more about sea turtles and climate change.

Temperature-dependent sex determination is a critical aspect of sea turtle reproductive biology, finely tuned by evolution to adapt to changing environmental conditions. This process has worked well for sea turtles for millions of years, raising intriguing questions about its evolution and advantages. Understanding TSD is essential for protecting these incredible marine creatures from climatic shifts on the horizon.