Does Temperature Decide a Turtle's Sex? Hot Sand, More Females

How temperature-dependent sex determination works

In mammals and birds, sex is fixed at fertilization by sex chromosomes. Most turtles have no sex chromosomes. Instead, every embryo starts out able to develop either ovaries or testes, and the temperature it experiences while developing tips the balance. Scientists call this temperature-dependent sex determination, or TSD.

The key stage is the middle third of incubation, known as the thermosensitive period. Temperatures during these few weeks affect enzymes and hormones in the developing embryo. Warmer conditions favor the production of estrogen, which steers the gonads toward becoming ovaries; cooler conditions lead to testes. The exact genetic switch is still being studied.

Each species has a pivotal temperature, at which a nest produces roughly equal numbers of males and females. Around it is a narrow transitional range, often only about 1–4 °C (2–7 °F) wide. Above that range nearly all hatchlings are female, and below it nearly all are male.

Sea turtles: hot sand, more females

All seven species of sea turtle have TSD, and all follow the same pattern: cool nests make males and warm nests make females. A handy way to remember it is “hot chicks and cool dudes.” For the green sea turtle and the loggerhead sea turtle, the pivotal temperature is close to 29 °C (84 °F), with small differences between populations.

Because the window is so narrow, small details matter. Eggs in the center of a clutch, which is warmed by the developing embryos, tend to be warmer than those at the edges. Nests in pale sand stay cooler than nests in dark volcanic sand, nests shaded by vegetation stay cooler than those in open sun, and heavy rain can cool a nest for days. Nests laid early or late in the season are often cooler and produce more males.

Temperature also affects survival and timing. Warmer nests hatch faster, roughly 45–55 days in hot sand compared with 60–70 days or more in cool sand. But if a nest stays above about 33–35 °C (91–95 °F) for long, many embryos die or hatchlings emerge weak.

Other turtles: not all follow the same rule

Most freshwater turtles and most tortoises also have TSD, but the pattern is not always the same. Many species, like sea turtles, produce males at cool temperatures and females at warm ones.

The common snapping turtle is a good example of a second pattern. In snappers, females are produced at both low and high temperatures, while males come mainly from a band of moderate temperatures in between, roughly the low to mid-20s in degrees Celsius (low to upper 70s °F). Alligators and some other crocodilians have a similar pattern.

Some turtles do not use temperature at all. North American softshell turtles and the wood turtle, among others, have sex chromosomes, so their sex is decided genetically, just as in mammals. Scientists think genetic sex determination evolved from TSD several times in different turtle groups.

The northern Great Barrier Reef study

The clearest warning sign comes from Australia. In 2018, a team led by Michael Jensen of NOAA, working with Australian scientists, published a study in the journal Current Biology on green turtles of the Great Barrier Reef. Because hatchlings and young turtles look the same on the outside, the researchers used genetic markers to tell which nesting region each turtle came from, and hormone tests and other methods to work out its sex.

The results were striking. Among turtles from the northern Great Barrier Reef population, which nests mainly on Raine Island and is one of the largest green turtle populations in the world, about 99.1% of juveniles, 99.8% of subadults and 86.8% of adults were female. Turtles from the cooler southern Great Barrier Reef were about 65–69% female.

Linking these results with sand temperature records, the researchers concluded that the northern beaches had been producing almost only females for at least two decades. They warned that, if warming continues, the population could face a shortage of males, and that hotter sand could eventually reduce hatching success.

Why it matters, and what is being done

A female-biased population is not automatically in trouble. Male sea turtles can mate with several females and often return to breed every year, while females usually nest only every two to four years. That means a population can produce plenty of eggs even if most hatchlings are female, at least for a while. Some scientists also point out that a moderate female bias may help a population grow in the short term.

The worry is what happens if the bias becomes extreme and lasts for decades, combined with more nests failing in very hot sand. Turtles have survived past climate changes, partly by shifting nesting seasons and beaches, but today’s warming is happening fast, and many nesting beaches are hemmed in by coastal development.

Conservation teams are testing ways to keep nests cooler on vulnerable beaches, such as shading nests, sprinkling them with water and moving some clutches to cooler parts of the beach. Protecting natural vegetation behind beaches also helps by providing shade. Most researchers agree that the most important long-term step is reducing the pace of climate warming itself, while continuing to protect nesting beaches and reduce other threats so that populations stay large enough to adapt.

Myths and facts

Myth: you can tell a baby sea turtle’s sex by its size or color. Hatchlings of both sexes look identical; sex can only be determined internally until males mature and grow long tails.

Myth: the mother decides the sex of her eggs. She does not choose the sex directly, but she does influence it by choosing where to nest, how deep to dig and in which part of the season to lay.

Fact: a single nest can contain both sexes. Because temperatures vary within a clutch and over the incubation period, many nests near the pivotal temperature produce a mix of males and females.