The All-Female Turtle Problem
The sex of a sea turtle is not determined by genetics at the moment of fertilization. Instead, it depends entirely on the temperature of the sand where the egg incubates—a phenomenon called temperature-dependent sex determination (TSD). For most sea turtle species, including the five found in Sri Lanka, there is a critical "important temperature" of around 29°C (84.2°F). Nests incubating below this temperature produce mostly males, while nests incubating above 31°C (87.8°F) will produce almost all females. The middle third of the 45- to 70-day incubation period is the most critical window when temperature directs the development of reproductive organs.
The southern coast of Sri Lanka is a nesting area for species like the Green turtle (Chelonia mydas), the Olive Ridley (Lepidochelys olivacea), and the critically endangered Hawksbill turtle (Eretmochelys imbricata). Early conservation hatcheries, which began appearing in the 1970s and 80s, were established with the best intentions: to protect eggs from poachers and predators. Staff would relocate nests from the open beach to these protected enclosures.
Without understanding TSD, however, these hatcheries inadvertently created a major demographic crisis. The relocated nests were often buried in sun-exposed, open sand pens. This resulted in consistently higher incubation temperatures than in shaded, natural nests. The consequence was a massive skew in the sex ratio, with some hatcheries producing nearly 100% female turtles for years. A population cannot survive without males, and this well-meaning practice was pushing local turtle populations toward a reproductive dead end.
A Balanced Solution
The research and evolving practices at Sri Lankan hatcheries became part of a global effort to correct this imbalance. Conservation scientists began to systematically study the relationship between nest conditions and hatchling sex ratios. The goal was to replicate the thermal diversity of natural nests. A natural beach has a variety of nesting spots—some in open sun, some partially shaded by vegetation, some deeper, and some shallower. This variety naturally produces a mix of males and females.
Modern hatchery protocols, refined through this work, now manage incubation temperatures with scientific precision. Staff use shading cloths to cool nests or relocate them to areas with different sun exposure to ensure a balanced output. They monitor nest temperatures throughout incubation and maintain a density of no more than one nest per square meter to prevent heat from one clutch affecting another. The depth of the relocated nest is carefully measured to match the original nest, as depth significantly influences temperature and gas exchange.
This research did fixed a local problem; it set best practices for ex situ (off-site) turtle conservation that is the standard for hatcheries around the world. By understanding the effect of a few degrees of temperature, conservationists in Induruwa and elsewhere can now ensure that the thousands of hatchlings they release have a viable, balanced future.