By Marc Girondot

Ever since the discovery of temperature-sensitive sex determination (TSD), research has been hindered by scientists’ inability to estimate hatchling sex ratios using nonlethal methods, because the sacrifice of embryos is inconceivable in species that we wish to protect. But numerous methods have been attempted. Typically, average incubation temperature is measured in nests, then compared to a profile obtained from a nest at constant temperature; sometimes that comparison is conducted over just one-third of the incubation period. Other methods use the duration of incubation as an integrating proxy. Scientists sometimes validate those methods by sacrificing a small number of embryos, but often no validation is done. 

Although it is tempting to presume that average temperatures or incubation duration can be used as sex ratio proxies, it has been shown clearly that this is not the case; average nest temperature is not a reliable indicator for hatchling sex ratio; neither is the incubation period. Indeed, after 50 years of work on the subject, we still do not have a simple method for estimating sex ratios from a series of nest temperatures. 

The most precise method to estimate sex ratios is to (1) obtain a TSD pattern from incubation at constant temperatures, (2) model embryo growth to define the precise thermosensitive period of sex determination using embryonic stages (not incubation duration), and (3) apply the constant temperature equivalent with the TSD pattern using a weighting scheme to correct for the fact that some temperatures have a greater capacity than others to sexualize the gonad. Such a model is difficult to apply; moreover, it should be calibrated for each specific nesting site to ensure accuracy. This approach is not simple field-based science by any means.

Recent promising results could do much to advance TSD research. It is possible using a blood test called ELISA (enzyme-linked immunosorbent assay)—or DNA methylation—to nonlethally determine the sex of hatchling turtles. The practicality of such techniques, however, remains to be determined. Meanwhile, it is important to continue to gather accurate data about nest temperatures globally only when those data will contribute to research goals that adhere to the strictest standards of analysis—as well as to monitor hatchling sex ratios whenever possible while minimizing the loss of animals. It is good to remember that marine turtles have numerous ways to control their own nest temperature by shifting phenology. Management is not always the best solution, and shading in hatcheries or cooling nests by watering will never be a practical solution to manage the tens of millions of nests laid annually. Sea turtles have survived global temperature peaks and valleys for millennia, and it is likely they will continue to do so. 

Author Affiliation

Marc Girondot, Conservation des Populations et des Communautés group, París-Saclay University, France

This article originally appeared in SWOT Report, vol. 19 (2024). Download this article as a PDF.