Analyzing the Effects of Egg Incubation Temperatures on Sex Ratios of Hatchlings

Kemp's ridley sea turtle eggs.

NPS Photo.

The international program to restore and enhance the Kemp’s ridley sea turtle population and establish a secondary nesting colony at Padre Island National Seashore began in 1978 (Shaver 1987, Shaver and Calliouet 2015). Sea turtle eggs collected in Rancho Nuevo, Mexico, were packed in Padre Island sand in expanded polystyrene foam (e.g., Styrofoam) boxes and shipped to the incubation facility at Padre Island National Seashore. Incubating sea turtle eggs in foam boxes typically produced clutches with more male embryos. The implications of such a phenomenon are profound in a project designed to increase the numbers of endangered species, therefore, a study was undertaken to determine the ideal incubation temperature to produce clutches that were predominately female (Shaver et al. 1988, Shaver and Calliouet, 2015). Researchers at the park found that Kemp’s ridleys possess a pattern of sex determination in which cooler temperatures produce more males and warmer temperatures produce more females (Shaver et al. 1988). Probes were placed with incubating eggs to record temperatures and methods were used to attempt to increase temperatures at the incubation facility. The data suggest that the temperature range that produces a mixed sex ratio extends from approximately 29 to 32.5°C (Shaver et al. 1988, Shaver 1989, 2005, LeBlanc 2012). The results provide a basis for predicting Kemp’s ridley sex ratios based on incubation temperatures. These findings could provide a method for accurately estimating sex ratios produced in the Kemp’s Ridley Recovery Program at Rancho Nuevo and in Texas, in order to help manage the recovery of this species.

In conjunction with incubation studies, a study of beach temperature profiles was undertaken during the summer of 1986 to examine temperatures at which Kemp’s ridley sea turtle eggs would incubate if laid and left in situ on the beach at Padre Island National Seashore (Shaver et al. 1988, Shaver 2005). Researchers found that clutches undergoing incubation early in the nesting season at Padre Island or Rancho Nuevo should produce primarily males, later portions of the season primarily females, and the middle of the season a mixture of both males and females (Shaver et al. 1988, Shaver 1989, 2005). Beach temperatures varied slightly with latitude and were warmest in the south, at Rancho Nuevo, and coolest in the north at Padre Island National Seashore (Shaver 2005). As beach temperatures rise due to global climate change, sea turtles whose hatchling sex determination is temperature dependent, may be adversely affected (Bevans et al. 2019). One hypothetical response of sea turtles to near-future elevated temperatures is a shift in nesting distribution to maintain suitable thermal conditions. Recent beach temperature profile studies indicate beaches are warming (Bevans et al 2019). Northern beaches in Texas and Mexico, such as La Pesca, South Padre Island, and Padre Island National Seashore, could provide cooler incubation temperatures and suitable nesting habitat under a predicted near-future climate warming scenario than beaches to the south, where the majority of Kemp’s ridley nesting currently occurs (Bevans et al. 2019). Therefore, conservation strategies to address rapid anthropogenic environmental warming throughout the nesting range of Kemp’s ridley sea turtles should prioritize protection of these northern beaches. These studies highlight the importance of Padre Island National Seashore as a nesting habitat for the continued and future recovery of the critically endangered Kemp’s ridley sea turtle.

 

Literature Cited:

Bevan, E.M., T. Wibbels, D.J. Shaver, J.S. Walker, F. Illescas, J. Montano, J. Ortiz, J.J. Pena, L. Sarti, B.M.Z. Najera, and P. Burchfield. 2019. Comparison of beach temperatures in the nesting range of the Kemp’s ridley sea turtle in the Gulf of Mexico, Mexico and USA. Endangered Species Research 40:31–40.

LeBlanc, A.M., T. Wibbels, D.J. Shaver, and. J. S. Walker. 2012. Temperature-dependent sex determination in the Kemp’s ridley: Effects of incubation temperatures on sex ratios. Endangered Species Research 19:123–128.

Shaver, D.J. 1987. Padre Island Kemp's Ridley Sea Turtle Project Update. Park Science 7(4):8–9.

Shaver, D.J. 1989. Results from eleven years of incubating Kemp's ridley sea turtle eggs at PAIS. In: Proceedings of the 9th Annual Workshop on Sea Turtle Conservation and Biology, Feb. 7–11, 1989, Jekyll Island, Georgia, p. 163–165. S.A. Eckert, K.L. Eckert and T.H. Richardson (compilers). NOAA Technical Memorandum NMFS-SEFC-232.

Shaver, D.J. 2005. Analysis of the Kemp’s ridley imprinting and headstart project at PAIS, Texas, 1978–88, with subsequent Kemp’s ridley nesting and stranding records on the Texas coast. Chelonian Conservation and Biology 4(4):846–859.

Shaver, D.J., D.W. Owens, A.H. Chaney, C.W. Caillouet Jr., P.M. Burchfield, and R. Marquez M. 1988. Styrofoam box and beach temperatures in relation to incubation and sex ratios of Kemp's ridley sea turtles. In: Proceedings of the 8th Annual Workshop on Sea Turtle Conservation and Biology, Feb. 24–26, 1988, Fort Fisher, North Carolina, p. 103–108. B. Schroeder (compiler). NOAA Technical Memorandum NMFS-SEFC-214.

Shaver, D.J., and C.W. Caillouet, Jr. 2015. Reintroduction of Kemp’s ridley (Lepidochelys kempii) sea turtle to PAIS, Texas and its connection to head-starting. Herpetological Conservation and Biology 10(Symposium):378–435.

 

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