Florida Researcher Uses Data Loggers to Predict Sea Turtle Hatchling Gender

Outdoor, Water
Temperature data loggers were used to determine whether removal of nearby vegetation would affect sea turtle hatchling sex ratios.

Over the last decade, Rookery Bay National Estuarine Research Reserve (RBNERR) and the Conservancy of Southwest Florida collaborated to deploy temperature data loggers in loggerhead nests to monitor incubation temperatures on Keewaydin Island, Collier County, Florida. A total of 449 HOBO temperature data loggers have been deployed in loggerhead sea turtle nests on Keewaydin Island over the duration of the study. Sea turtles have temperature-dependent sex determination (TSD) in which sand temperature influences the sex of the hatchlings. Incubation temperatures can be used to predict hatchling sex. HOBO® temperature loggers are launched to record hourly incubation temperature and vacuum-sealed in plastic with desiccant prior to deployment. As the sea turtle lays her clutch, loggers are opportunistically deployed in the egg chamber. After the nest hatches, data loggers are recovered and downloaded using Onset software depending on the type of logger. Data are exported to Excel for data analysis.


Figure 1

Figure 1. Average and standard deviation of loggerhead sea turtle incubation temperatures on Keewaydin Island during 2001 – 2009. The dashed line represents the pivotal temperature of 29˚C in which equal ratios of male and female hatchlings are produced. Temperatures greater than 32˚C produce females and temperatures less than 28˚C produce male hatchlings.



The study was initiated to examine whether removing exotic Australian pine (Casuarina equisetifolia) trees and consequently shade from Keewaydin Island would affect hatchling sex ratios. Additionally, the study has recorded nest temperatures during active tropical storm seasons and documented the impact of precipitation and tidal inundation on nest temperatures. The study has continued an effort to examine long-term temperature trends and the role environmental factors (rain, air temperature, storm events, climate change) play in determining hatchling sex. A HOBO® rain gauge data logger was deployed on Keewaydin Island for the first time this summer to record rainfall events and air temperature for the duration of the sea turtle nesting season.



Results of the study have indicated that removing the Australian pines did not affect hatchling sex ratios. Nests on Keewaydin Island are producing predominately mixed ratio and male biased clutches based on incubation temperature predictions (Figure 1) and confirmed by histology (conducted by the University of Alabama at Birmingham). The fact that male hatchlings are being produced regularly on Keewaydin Island makes it an essential nesting ground. Nests on the east coast of Florida are producing mainly female biased clutches so Keewaydin is supplying the South Florida Nesting Subpopulation with much needed males. This is one of the longest and most intensive incubation temperature studies of its kind and is contributing important data to the professional community. The data are also being used to educate the public on the anthropogenic and environmental factors that influence sea turtle nests.


Figure 2

Figure 2. Incubation temperatures recorded in Nest #47 on Keewaydin Island. The sudden temperature decrease and then increase on 8/13/04 indicates that the nest was inundated by a high tide and then washed out and the data logger was left lying on the beach.


The study has had its share of excitement, such as finding that male hatchlings are being produced regularly, as well as disappointing setbacks, such as losing over half the sea turtle nests (along with temperature loggers) during storm events in 2001 and 2004. While we were preparing for the 2010 sea turtle nesting season we had a discovery that surprised us. A HOBO® temperature data logger was found on the beach and we wrongly assumed the logger was from a 2009 nest. Miraculously, the logger was still in good shape and the data were downloaded. Based on the temperature data, the nest was apparently inundated on August 13, 2004, which is the day that Hurricane Charley impacted Collier County. The logger was reburied on the beach and continued recording data until May 2005. The logger was recovered six years after deployment, still functional and providing invaluable data!