Did you know that the temperature at which a sea turtle nest incubates will determine if the hatchlings are male or female?
This is called temperature-dependent sex determination (TSD) and is very common in reptile species, including alligators, land and sea turtles, and some lizards. Research has shown that the middle third of the sea turtle’s incubation period, which typically lasts 55-60 days total, is when sex is determined. The critical temperature for loggerhead sea turtles (Caretta caretta) is around 29°C or 84.2°F. At this temperature, half of the hatchlings produced will be male and half female. Above this threshold, more females are produced, below this threshold, more males. Basically, warmer nests result in more female hatchlings, whereas colder nests result in more male hatchlings. Egg clutches also experience metabolic heating, meaning they create their own heat as they incubate, so are warmer than the sand around them. Scientists are concerned that changing global temperatures will skew the sex ratios of threatened sea turtle populations long-term, causing a shortage of males that could ultimately lead to problems reproducing.
Staff at the Apalachicola NERR on the Florida panhandle have been marking and monitoring nests on Little St. George Island, a remote, undeveloped island owned by the state and managed by ANERR, since the 1990’s. Loggerhead sea turtles are by far the most abundant nesting species, and this beach is part of the smallest genetically distinct subunit of the Northwest Atlantic Ocean Distinct Population Segment, which is the group all Florida nesting loggerheads belong to. The average number of nests on this 15 km long beach is 123, but nesting numbers have been increasing and the 2017 season broke the record for the number of nests recorded at 351! This seemed like the perfect spot to study incubation dynamics so, in 2016, I began looking at incubation temperatures, the amount of metabolic heating going on in an egg clutch, and the overall temperature regime of the beach.
To place temperature loggers in nests, I very carefully move about 50 eggs (approximately half the egg clutch) out of fresh nests less than 12 hours old, place the small logger in the center, replace the eggs in as close approximation as possible to how their mama put them there, and rebury the nest. I also place a control logger one meter away in the sand at the same depth. The difference between these two temperatures is the amount of metabolic heating occurring in the clutch. To measure the beach temperature, in 2017 I created transects from the dune line to just above high water at six locations on the island and buried loggers at 45 cm, which is the standard mid-clutch depth for nests on this beach. Nests were used from the early, middle, and late part of the nesting season.
I now have two years’ worth of nest data and one year of beach data and results show the temperature regimes during the two years were very different…
This research is being conducted under Florida Fish & Wildlife Conservation Commission Marine Turtle Permit #144. This project is funded in part by the Apalachicola NERR and in part by a grant awarded from the Sea Turtle Grants Program. The Sea Turtle Grants Program is funded from proceeds from the sale of the Florida Sea Turtle License Plate. Learn more at www.helpingseaturtles.org.
Be sure to check out part 2 of Megan’s piece later this week!
Megan Lamb is an Environmental Specialist at the Apalachicola NERR and the lead for the NERR’s SWMP nutrient and chlorophyll-a sampling program. She also enjoys studying oyster bar communities, estuarine water quality and anthropogenic effects on estuaries, and sea turtle nesting dynamics. She is also completing her master’s degree in Fisheries and Aquatic Sciences at the University of Florida’s School of Forest Resources and Conservation.