100 RP and the GLS areas, and within the different monitored islets of the GLS as they exhibit heterogeneous habitat characteristics (Bourgogne et al. 2024a). This is consistent with available literature documenting the influence of habitat on nest temperature, especially in regards to the color of beaches substrates (Hays et al. 2003; Laloë et al. 2014), exposure to extreme weather events events (Fuentes and Abbs 2010; Lolavar and Wyneken 2015; Laloë et al. 2020) and vegetation type and cover (Kamel 2013). The cooler nest temperature reported in the GLS area suggested the production of a wellbalanced primary sex ratio, contrasting with the constantly warmer and highly female-skewed RP beach. This result is consistent with that of Read et al. (2013), also pointing out the important risk of population feminization in the RP beach due to high nest temperatures, with an estimated 99% proportion of females produced in the nests of the RP beach. The pattern of a more balanced sex ratio reported here for GLS over the three considered seasons should however be tempered in regards to the moderate number of monitored nests (n = 12). This smaller sample size is a consequence of logistical constraints. Indeed, the remoteness and highly scattered nature of the coral-islets nesting habitat requires intensive field efforts to detect fresh nests or females laying eggs (Bourgogne et al. 2024c). Hence, the deployment of new loggers was not guaranteed at every field trip. This difficulty to attend nesting events has also required us to adapt our temperature logger deployment protocol. Whereas Read et al. (2013) and Chu et al. (2008) have placed loggers in the middle of the monitored nests, we could not run the risk of impacting the embryogenesis by digging the eggs out. This is the reason why all loggers have been deployed right above the egg chamber. It has been demonstrated that temperatures in the center of the egg mass were significantly higher than those at the top of the egg chamber (average 0.4°C; Hanson et al. 1998). Therefore, our protocol leans towards conservative thermal predictions, which might suggest that the actual sex ratios are in fact even more female-skewed. Nonetheless, the remarkable magnitude and consistency of this temperature difference across years suggest confidence in the contrasting patterns described here between RP and GLS, as well as its ecological consequences. We note that several sources pointed out that the simple temperature factor or the incubation period are not an accurate proxy for primary sex ratio (Georges et al. 1994; Fuentes et al. 2017, 2024; Girondot et al. 2018). Indeed, it does not reflect on the clutch size (number of eggs), the dynamic of embryonic development (and hence the precise temporal window in which the gonad differentiation step occurs), the thermal tolerance (allowing to implement the embryo survival rate at measured temperatures), nor the sex ratio reaction norm (describing the relationship between male proportion and temperature when held constant during incubation). Nevertheless, the uncertainty resulting from the use of these simplified models is
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