How Does Rainfall Affect the Size of Sea Turtle Hatchlings?

As many sea turtle lovers know, temperature determines the sex of hatchlings as they develop in the nest. So, warmer temps result in more females while cooler temps produce more males. But which environmental factors affect hatchling size?

Two research studies from 2002 and 2003 found that for both loggerheads and greens, lower incubation temperatures result in larger hatchlings because their incubation is prolonged. Also, higher sand moisture correlated to bigger hatchlings for both species (1, 2). Additional research discovered that rainfall contributes to both these environmental conditions because it cools the sand (lowering the incubation temp) and increases moisture which is essential for the hatchling’s development (3, 4).

Larger hatchlings have an advantage because they move faster than smaller ones (5). So, bigger hatchlings can avoid predators when they are most vulnerable – on the beach and just offshore.

But which environmental factor – temperature or precipitation – has a bigger impact on the size of loggerhead and green hatchlings?

A 2024 collaboration between Florida Atlantic University (FAU) and University of Tübingen (in Germany) examined this question (6). The researchers analyzed multiple studies conducted at 37 beaches around the world. Correlating all the data, they concluded that rainfall during the nesting season is a better predictor of both loggerhead and green hatchling body size than changes in air temperature.

The study’s co-author and FAU professor Jeanette Wyneken, Ph.D., provided additional insights on their findings saying, “It’s not clear exactly how rain affects hatchling size. One idea is that rain cools the nests, which can change the temperature and impact the sex ratio of the hatchlings. This could lead to differences in hatchling size and shape, similar to how male and female hatchlings of other turtles, like the giant river turtle, have different shell shapes.” (7)

The researchers also called for additional studies on how rainfall impacts hatchling size. As Dr. Wyneken said, “Findings from our study highlight the need for more localized data on how regional weather influences incubation and hatchling development. These data are essential for refining conservation strategies to protect sea turtles amidst global warming.”

Sources:

• (1) Evolutionary Ecology Research journal: “The Effects of Incubation Environment, Sex and Pedigree on the Hatchling Phenotype in a Natural Population of Loggerhead Turtles” by S.E. Reece, A.C. Broderick, B.J. Godley, and S.A. West (2002).
• (2) Journal of the Marine Biological Association of the UK: “Incubation Environment Affects Phenotype of Naturally Incubated Green Turtle Hatchlings” by F. Glen, A.C. Broderick, B.J. Godley, and G.C. Hays (September 2003).
• (3) Inter-Research Science Publisher journal: “Effect of Rainfall on Loggerhead Turtle Nest Temperatures, Sand Temperatures and Hatchling Sex” by A. Lolavar and J. Wyneken (2015).
• (4) Integrative Zoology journal: “Influence of Incubation Temperature on Sea Turtle Hatchling Quality” by D. Booth (January 2017).
• (5) Marine Environmental Research journal: “Green Turtles Nest Survival: Quantifying the Hidden Predation” by L. de Faria, A. Martins, J. Pereira (July 2022).
• (6) BMC Ecology and Evolution journal: “Correlations Between Local Geoclimatic Variables and Hatchling Body Size in the Sea Turtles Caretta caretta and Chelonia mydas” by O. Fernández, P. Parsi-Pour, J. Nyakatura, J. Wyneken, and I. Werneburg (August 2024).
• (7) Florida Atlantic University (FAU) website: “Rain or Shine? How Rainfall Impacts Size of Sea Turtle Hatchlings” by G. Galoustian (August 2024).