Temperature Dependent Sex Determination (TSD) is a form of environmental sex determination where the temperature experienced by an egg during incubation dictates the sex of the developing embryo. This mechanism stands in contrast to Genetic Sex Determination (GSD), the system found in mammals and birds, where sex is fixed at conception by the inheritance of sex chromosomes. TSD is a widespread phenomenon among oviparous vertebrates, notably in all crocodilians, most turtles, and some species of lizards and fish. The prevalence of TSD in reptiles establishes the incubation environment as the ultimate predictor of an individual’s sex.
The Biological Mechanism of Sex Determination
Temperature does not influence the embryo’s sex throughout the entire incubation period but only during a highly specific window known as the Critical Period, or thermosensitive period. This phase typically occurs during the middle third of embryonic development, when the gonads are bipotential and poised to differentiate into either testes or ovaries. The temperature maintained during this short time frame acts as a switch, triggering a cascade of molecular events that determine the final sex.
The temperature signal is translated into a hormonal cue via the enzyme aromatase, which governs sex differentiation. Aromatase is responsible for converting androgen hormones, specifically testosterone, into estrogen hormones. High incubation temperatures, which often produce females in many species, stimulate an increase in the expression and activity of the aromatase enzyme within the developing gonad. This high activity results in a surge of estrogen, which subsequently drives the bipotential gonad toward ovarian differentiation.
Conversely, male development proceeds under temperatures that suppress the expression of the aromatase enzyme. Low aromatase activity leads to low estrogen levels, which allows the default developmental pathway—testicular differentiation—to occur. The temperature that produces an equal 50:50 ratio of males to females is defined as the Pivotal Temperature. A shift of just 1–2 degrees Celsius away from this point can dramatically skew the sex ratio toward one gender.
Diverse Patterns in the Animal Kingdom
Reptiles exhibit three distinct patterns of temperature-dependent sex determination, categorized by the relationship between temperature and the resulting sex ratio. The most common pattern, Type Ia, is characterized by females being produced at high temperatures and males at low temperatures (MF pattern). This pattern is observed in the majority of turtle species, including most sea turtles and the Red-eared slider turtle (Trachemys scripta).
The second pattern, Type Ib, is the reverse, where females are produced at low temperatures and males at high temperatures (FM pattern). This pattern is less common in reptiles but is found in the Tuatara, a reptile species endemic to New Zealand. Both Type Ia and Type Ib feature a single transition zone where the sex ratio shifts from predominantly male to predominantly female, or vice versa, around the pivotal temperature.
The third, more complex pattern is Type II, which produces females at both thermal extremes—low and high temperatures—with males developing only at intermediate temperatures (FMF pattern). This pattern is found in crocodilians, such as the American alligator (Alligator mississippiensis), and some lizard species, including the Leopard gecko (Eublepharis macularius). The FMF pattern requires two transition zones, making the range for male production a narrow band between the low and high female-producing temperatures.
Adaptive Value of Temperature Dependency
The persistence of TSD over evolutionary time suggests that it provides an adaptive advantage over GSD, a concept formalized in the Charnov-Bull model. This model posits that TSD is favored when the environmental factor determining sex—temperature—differentially influences the fitness of male versus female offspring. TSD ensures that the sex produced under a specific temperature is the one that will achieve the highest reproductive success in adulthood.
For species with the Type Ia pattern, the high-temperature conditions that produce females may also lead to larger hatchlings. Larger females often have higher fecundity and reproductive output. TSD acts as a mechanism to match the sex with the highest potential fitness trait conferred by the incubation environment. TSD can also generate seasonal shifts in sex ratio, where early-season nesting produces one sex, and later nesting produces the other, matching the sexes to the time of year that is optimal for their survival and growth.
Conservation Crisis and Climate Change
The mechanism of TSD, which evolved to match offspring sex with optimal environmental conditions, now presents a vulnerability in the face of global warming. Rising global temperatures are causing nesting beaches to exceed the pivotal temperature with increasing frequency, leading to skewed sex ratios in many TSD-dependent populations. This phenomenon is most pronounced in sea turtles, which exhibit the Type Ia pattern, where warmer sands produce nearly all female hatchlings.
In nesting sites, such as the northern Great Barrier Reef, researchers have documented local green sea turtle populations (Chelonia mydas) where juvenile and sub-adult individuals are over 99% female. This shortage of males reduces the effective breeding size of the population and limits genetic diversity, jeopardizing the long-term viability of the species.
Conservation efforts have focused on mitigating thermal conditions of nesting sites to restore a balanced sex ratio. Strategies include the physical shading of nests using artificial structures or natural vegetation to lower sand temperatures. Translocation of at-risk egg clutches to cooler beaches or into climate-controlled incubators is also used. Novel techniques, such as the artificial irrigation of nests with water, are being tested to cool the sand through evaporative effects.