Hot Chicks, Cool Dudes: Rising Temperatures and Its Impact on Sea Turtles
Sea turtle hatchlings in the Galapagos. Photo credit, Pam Ferris-Olson
Hot chicks and cool dudes. It’s not the latest slogan for Florida but it is apropos. This catchy phrase has been used to describe the effect of climate change on the sex of loggerhead sea turtles hatchlings. Studies show that temperature influences the gender of sea turtle hatchlings. As temperatures warm, it appears that more female hatchlings are born. Hence, the mnemonic hot chicks. As temperatures cool, more males are born. Explaining the mnemonic cool dudes.
Males vs Females: Sex Chromosomes
Most vertebrates including humans possess genetic material or sex chromosomes. These sex chromosomes are passed from parents to their offspring at the time an egg is fertilized. There are two types of sex chromosomes: X and Y. X chromosomes are unique to females. Typically, a female possesses two X chromosomes. Males, however, have both an X and a Y chromosome. Before fertilization, an egg has a single X chromosome. A fertilized egg is typically either an XX or XY. This result is determined by what the sperm delivers at the time of fertilization. A sperm carries either an X chromosome or a Y chromosome. Thus, the type of sperm that fertilizes the egg determines the offspring’s sex. A fertilized egg, in most cases, is either XX (female) or XY (male). In general, the chance that an offspring will be female or male is 50/50.
Boy and Girl Sea Turtles: A Temperature Dependent Story
Most turtles and all species of crocodilians lack sex chromosomes. Sex is determined by the fertilization of an egg but by the ambient temperature in which the egg develops. This phenomenon is know as temperature-dependent sex determination. Research has shown that eggs incubated below a temperature of about 82 degrees Fahrenheit produce males while eggs incubated at temperatures at approximately 89 degrees Fahrenheit produce females. In nests where the temperature fluctuates between these upper and lower limits, a mix of males and females is produced.
Florida is an excellent place to study temperature-dependent sex determination because the state has more loggerhead turtles nesting on its beaches than anywhere else in the world. In Florida, the majority of turtle hatchlings in recent years have been female. The pattern of higher temperatures, above 89 degrees Fahrenheit, has been recorded in Florida during the sea turtle’s May to October nesting season. However, the increasing percentage of female hatchlings is not a phenomenon limited to loggerheads. In a 2018 study of green sea turtles in Australia’s Great Barrier reef a similar result was seen.
Determining the sex of hatchling sea turtles isn’t an easy proposition. Because sea turtles are long-lived and mature slowly, they do not appear sexually dimorphic, showing male and female traits, until almost 25 years old, about the age they become sexually mature. Knowing the sex of turtle hatchlings is important for the management of their populations especially in the face of rapid increases in global temperatures. Because those who monitor sea turtles need a way to accurately assess hatchling sex ratios, researchers at Florida Atlantic University's Charles E. Schmidt College of Science have “developed a first-of-its-kind technique that is minimally invasive and greatly enhances the ability to measure neonate turtle sex ratios at population levels across nesting sites worldwide.” A small amount of blood is taken from a hatchling. This sample is then tested in a laboratory. A hormone known as the anti-müllerian hormone has been reliably found in male turtle hatchlings but is lacking in females. Therefore, the finding of the hormone can be used with a high level of accuracy to identify a hatchling’s sex. These research findings can help “estimate how climate change will affect future generations of hatchlings, and allow for expedited evaluation of management strategies used to help recover imperiled sea turtles and other reptile species with temperature-dependent sex determination.”
This Story Isn’t over Yet
At some point, ambient temperature may increase to a point where the ratio of male to female is no longer an issue. At such temperatures it will too hot for sea turtle eggs to develop. It will bee too hot for hatchlings will survive. However Jake Lasala, a researcher with the Mote Marine Sea Turtle conservation & Research Program in Sarasota, Florida, hasn’t lost hope. He believes that sea turtles have a strong survival instinct. As an example, he noted that “this past summer, a female loggerhead laid eggs far north of Florida on the cooler sands of the Jersey Shore.”
