Sea turtles are among the most charismatic of megafauna. They have fans everywhere, and more than their share of outreach programs designed to make more fans. Even locals who don’t much see the appeal can tell you that turtle ecotourism is big business.
And it’s not hard to see why. Sea turtles are one of conservationists’ big success stories. Almost extinct before anyone even noticed, their numbers plummeted in the face of conservation efforts, recovering at what seemed the last possible minute thanks to heroic efforts and one important discovery about their physiology that came not a moment too soon. The story of the Kemp’s ridley sea turtle, at one time the closest to the brink, is a story of science at work in real time.
At the beginning of the 20th century, when Kemp first put his name on the species, female Kemp’s ridleys flocked to beaches by the tens of thousands, laying their eggs all at once in a phenomenon known as arribada, or arrival. But humans caught on to this, and thanks to a brisk trade in turtle eggs, not to mention the help of other predators, the arribadas had disappeared by the time the Endangered Species Act was passed.
In 1977, Mexican and American wildlife agencies began a collaboration to protect nesting habitat at Rancho Nuevo, a park near the border that was the last known nesting site for Kemp’s ridleys. The conservationists knew that if they left nests where they were laid, they’d lose eggs to raccoons, foxes, and opportunistic humans. So when beach patrols found eggs, they dug them up and removed them to either nest areas near a base camp and inside of a fence, or Styrofoam boxes filled with sand. Around hatching time, in a move that must have looked very strange to anybody not in on the reasoning, researchers allowed the hatchlings to crawl down the beach into the ocean, then scooped them up with nets to raise in captivity and release after about a year. This was called head-starting, and the thinking behind the nets was that females were more likely to return and lay eggs on beaches they had successfully imprinted on.
Researchers didn’t know, until head-started females returned to lay their eggs, how many females and how many males they were raising. It’s hard to tell the sex of hatchling sea turtles; they keep the relevant organs inside a shell, and at such a young age their blood hormone levels are not very high. After they disappear into the ocean, it’s some years until females reach maturity and return to lay eggs, and many hatchlings don’t make it so long. Sea turtles are difficult to research in general: there aren’t many of them, they live most of their life cycle well out of the reach of land-bound humans, and if you want to study their hatching, you had better be prepared for a swashbuckling adventure from a pirate novel, racing to find buried treasure deposited on the beach at night before a poacher snatches it up. For the most part, researchers incubating eggs for release into the wild had to rely on the findings of studies of captive populations (like the Cayman Turtle Farm), because to count sex ratios of their own hatchlings would mean sacrificing precious live turtles.
Researchers knew, from a paper published in 1979, that sex determination in turtles (as with other reptiles) depends on temperature. Instead of being genetically determined at conception, as sex is for humans, birds, and other chromosome-dependent species, sex among turtles is determined by the temperature at which the egg is kept.
To say that a trait as fundamental as an organism’s sex can come from outside factors like the temperature sounds crazy. It cuts to the heart of the genes-versus-environment question in development (the question being, “which is more important?” and the answer being, “it appears to depend on what trait you measure”). Fortunately for turtle researchers, the concept of temperature-dependent sex determination had been floating around evolutionary biology circles for some two decades, in such strange and wonderful organisms as the nematode, and the orchid, and evolutionary theories on the benefits of temperature-dependent sex determination (or TSD) abounded. Unfortunately, the pivotal temperature, at which an even number of males and females develop, differed from species to species, and for sea turtles like the Kemp’s ridley, no one knew the key temperature.
It was only after about six years of head-starting that researchers could even begin to determine the pivotal temperature; they needed to wait for adult females to return, then determine using some careful statistics just how many males and females they had released some years before. In 1988, Donna Shaver, a ranger at Padre Island National Seashore, published the results of about ten years’ worth of study on head-started hatchlings’ sex ratios, identifying the pivotal temperatures. Over 30.8 C (roughly 87 degrees F), all of the hatchlings turned out female; under 29.0 C (87 F) they all turned out male. This was a terrifyingly tiny temperature range; it was lucky that artificial hatching and head-starting efforts had produced any males at all. This publication roughly coincided with the lowest Kemp’s population (in 1985, just 518 wild females nested), and knowing how to produce a natural sex ratio was key to conservation and repopulation efforts.
Nearly thirty years later, we still don’t know exactly what gene or network of genes in turtles determines sex in response to temperature. There may even be different pivotal temperatures for different subpopulations within the species that nest on different beaches. Although the head-starting program was shut down amidst debates about its cost-effectiveness just ten years after it began, the practice of moving nests to more protected areas until they hatch is still widespread. And nowadays, when nests are re-located, volunteers take care to move the eggs quickly, to avoid placing them on hot sand or in direct sun, keeping conditions as natural as possible. While the Kemp’s ridley is still on the critically endangered list, it survives; we have the work of conservationists to thank for that.
For one more thought on temperature dependent sex determination: maybe it killed the dinosaurs. I ask you.