When population-advantageous primary sex ratios are female-biased: changing concepts to facilitate climate change management in sea turtles

Abstract

Sea turtles have temperature-dependent sex determination. Because females are produced at high temperatures, increasing global temperature may lead to population feminization. Primary sex ratios (PSR) of sea turtle hatchlings are naturally female-biased, but this translates into a more balanced operational sex ratio because male turtles reproduce more often than females. As a consequence, a balanced PSR and the temperature that produces it (pivotal temperature) are of limited use to guide climate mitigation management because an equal PSR may be demographically suboptimal. Here, I define population-advantageous primary sex ratios (PA-PSR) as the PSR that will tend to be in equilibrium in a population and that will result in balanced operational sex ratios; I then estimate PA-PSR for different reproductive frequencies (years elapsed between reproductive seasons) of adult female and male turtles. I also define population equilibrium temperature (PET) as the temperature that would result in the equilibrium PSR of hatchlings (i.e., PA-PSR). These concepts may help assess the influence of rising temperatures on populations, as they can better indicate if PSRs depart from those at equilibrium. I compared PA-PSR and beach PSR for two populations of sea turtles for which male and female remigration intervals were known and found that a mild or no feminization over the PA-PSR may be occurring. Because PSR varies inter-annually, and hatchlings coming from beaches of different thermal conditions could recruit to the same population, it is critical to estimate beach PSR at the right temporal and spatial scales. Climate mitigation strategies based on these concepts could provide better management guidance for conservation practitioners. Similar approaches could be considered for other female-biased species with temperature-dependent sex determination.

Short‐term resilience to climate‐induced temperature increases for equatorial sea turtle populations

“…Certainly, large numbers of males do not need to be produced every nesting season. Rather, since sea turtles are long‐lived and breed multiple times, it is likely that just one big male‐producing year each decade might be enough to sustain a population at risk of extreme feminisation (Hays et al, 2022; Santidrián Tomillo, 2022; Santidrián Tomillo et al, 2014). In this regard, we might consider male production to be akin to the ‘boom‐bust’ population dynamics commonly seen in fisheries biology where a combination of biotic and abiotic factors culminates into the occasional (e.g.…”

“…where the theoretical hatchling sex ratio is 1:1) is typically around 29°C (Mrosovsky & Pieau, 1991; Wibbels, 2003). At nesting sites around the world, female‐biased hatchling sex ratios tend to dominate (Hays et al, 2014) and extreme feminisation is rare (Santidrián Tomillo, 2022). Despite this, there are grave concerns that climate warming will cause extreme feminisation for more populations and ultimately population decreases due to a lack of males (Hays et al, 2023; Schoeman et al, 2014).…”

Climate warming and sea turtle sex ratios across the globe

“…First, having more females may increase the per capita reproductive output of populations thereby allowing increased growth when females are in excess (Mazaris et al., 2017). Second, there is now strong theoretical (Santidrián Tomillo, 2022) and empirical (Hays et al., 2010) evidence that adult male turtles breed more frequently than females and hence female‐skewed hatchling sex ratios may be required in order to produce balanced operational (breeding) sex ratios.…”

Understanding the role of environmental temperature on sex determination through comparative studies in reptiles and amphibians

“…TSD serves as a typical example of phenotypic plasticity, and the distinct reaction norms of TSD are expected to reflect genetic variations underlying the thermal sensitivity of TSD. Intraspecific variations in reaction norms have been investigated in several testudines and squamates (Bentley et al, 2020; Doody et al, 2006; Ewert et al, 2005; Janzen, 1992; Rhen et al, 1998; Santidrián Tomillo, 2022). These studies revealed substantial geographic variations of reaction norms within species.…”