Hatching Success Rather Than Temperature-Dependent Sex Determination as the Main Driver of Olive Ridley (Lepidochelys olivacea) Nesting Activity in the Pacific Coast of Central America

Abstract

In marine turtles, sex is determined during a precise period during incubation: males are produced at lower temperatures and females at higher temperatures, a phenomenon called temperature-dependent sex determination. Nest temperature depends on many factors, including solar radiation. Albedo is the measure of the proportion of reflected solar radiation, and in terms of sand color, black sand absorbs the most energy, while white sand reflects more solar radiation. Based on this observation, darker sand beaches with higher temperatures should produce more females. As marine turtles show a high degree of philopatry, including natal homing, dark beaches should also produce more female hatchlings that return to nest when mature. When sand color is heterogeneous in a region, we hypothesize that darker beaches would have the most nests. Nevertheless, the high incubation temperature on beaches with a low albedo may result in low hatching success. Using Google Earth images and the SWOT database of nesting olive ridleys (Lepidochelys olivacea) in the Pacific coast of Mexico and Central America, we modeled sand color and nesting activity to test the hypothesis that darker beaches host larger concentrations of females because of feminization on darker beaches and female philopatry. We found the opposite result: the lower hatching success at beaches with a lower albedo could be the main driver of nesting activity heterogeneity for olive ridleys in Central America.

Keywords: Lepidochelys olivacea; albedo; hatching success; olive ridley; sea turtle; temperature-dependent sex determination.

Figure 1

Map of Central America showing the recorded beaches with their estimated darkness (light yellow dots correspond to white sand with a higher albedo and red dots to darker sand with a lower albedo). Green points indicate the position of Holocene volcanos. Blue dots indicate the log10 proportion of olive ridley nests.

Figure 2

Temporal and spatial distribution of the number of olive ridley nests in Central America. Only eight major beaches are named here from among the 90 beaches used in the present study because the number of nests per year for other ones is too small to be visualized in the figure. Beaches: 1: Santuario Playa de Escobilla; 2: Marinera; 3: Morro Ayuta; 4: La Flor, Carazo; 5: Ixtapilla; 6: RVS Río Escalante-Chacocente; 7: Chacocente; 8: Nancite. They represent 96.7% of the nesting of the analyzed beaches.

Figure 3

Relationship between sand darkness and (A) the proportion of olive ridley nests and (B) the proportion of olive ridley nests divided by beach length in Central American beaches. Note that the proportion of nests is log-transformed.

Figure 4

Sex ratio and hatching success at constant incubation temperatures. The thermal reaction norms for (A) sex ratio and (B) hatching success are shown in solid lines. Light gray temperatures in (A) are the range of temperatures that produced a sex ratio from 5% to 95% (transitional range of temperature 5%), and dark gray temperatures are the 95% credible regions for limits of the transitional range of temperature 5%. The temperature that produced 50% of each sex (pivotal temperature) is shown by the interrupted vertical line. In both graphs, the 95% credible regions of the thermal reaction norm are shown with dashed lines. Each point represents a set of eggs from the same origin at a specific constant temperature.