Abstract
For species with temperature-dependent sex determination (TSD) there is the fear that rising temperatures may lead to single-sex populations and population extinction. We show that for sea turtles, a major group exhibiting TSD, these concerns are currently unfounded but may become important under extreme climate warming scenarios. We show how highly female-biased sex ratios in developing eggs translate into much more balanced operational sex ratios so that adult male numbers in populations around the world are unlikely to be limiting. Rather than reducing population viability, female-biased offspring sex ratios may, to some extent, help population growth by increasing the number of breeding females and hence egg production. For rookeries across the world (n = 75 sites for seven species), we show that extreme female-biased hatchling sex ratios do not compromise population size and are the norm, with a tendency for populations to maximize the number of female hatchlings. Only at extremely high incubation temperature does high mortality within developing clutches threaten sea turtles. Our work shows how TSD itself is a robust strategy up to a point, but eventually high mortality and female-only hatchling production will cause extinction if incubation conditions warm considerably in the future.
Highlights
Understanding the drivers of variations in offspring sex ratios is a longstanding challenge regardless of whether species exhibit genotypic or environmental sex determination (GSD or ESD, respectively) [1,2,3,4,5]
In groups with temperaturedependent sex determination (TSD, which is one form of ESD), such as reptiles, extreme sex-ratio biases caused by high incubation temperatures are thought to be an important extinction driver [7,8], and there is concern that this escalating extinction risk is associated with climate warming [9,10,11]
Using empirical data collected across seven species, we develop a model for how the offspring sex ratio and egg survival interact to influence the number of surviving male and female offspring at different incubation temperatures and how these numbers translate into operational sex ratio (OSR)
Summary
Understanding the drivers of variations in offspring sex ratios is a longstanding challenge regardless of whether species exhibit genotypic or environmental sex determination (GSD or ESD, respectively) [1,2,3,4,5]. We used data from published research to describe the relationships [12,17] between incubation temperature and (i) the primary sex ratio, i.e. the sex ratio in developing embryos assuming no differential mortality of eggs with incubation temperature, and (ii) hatching success [17], i.e. the proportion of eggs surviving to produce hatchlings These studies compiled published data from across species of sea turtles. To locate additional articles that might not have been identified by the initial search, we checked the reference lists of relevant papers based on the pre-defined terminology In these studies, the sex ratios of hatchlings were either measured directly, which involves sacrificing hatchlings and the histological examination of the gonads, and/or were predicted by in situ measurements of sand temperatures on the nesting beaches combined with established relationships between sand temperature and hatchling sex ratio. We only used population size estimates for the specific sites for the same year as the hatchling sex-ratio measurement was made, or if this sex-ratio had been measured over several years, we used a corresponding range of years to calculate the mean number of nests per year
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