A morphologist, a modeler, and an endocrinologist consider sea turtle sex ratios in a changing climate. Some wine was involved

Abstract

Because the sex of sea turtles is determined by temperature during embryonic development, many populations are vulnerable to increased bias in primary sex ratios as global temperatures rise. Higher temperatures produce more females, and some populations are already showing years with all-female offspring production. But because sea turtles take decades to mature and have long adult lifespans, these primary sex ratio biases can take years to impact adult sex ratios, and the males from cohorts that are produced during cooler years may compensate for the sex ratio bias if they can breed more frequently and with multiple females. To date, little is known about male sea turtle reproductive behavior, making predictions of sex ratio skew impacts highly speculative. We used data from southern Florida loggerhead sea turtle nests to parameterize a simple population model to explore the effects of an increase in the proportion of female hatchlings over time on population trends, effective population size (Ne), and quasi-extinction probability. We also tested the effects of increasing the frequency of relatively high male production years to simulate potential mitigation strategies. While heuristic rather than predictive, our results expectedly show a rise in nest counts due to the increase in females over time, followed by population decline as males become limiting. Population collapse due to increased female bias will take many decades to occur, but sex ratio skew can have large impacts on Ne, and thus increase the potential for inbreeding. An increase in the frequency of male production years, even just one additional “good male year” per decade, can help mitigate these outcomes if the rate of feminization is not too rapid. Male breeding frequency and mating success are critical drivers of the results and must be prioritized for research.

Adaptation of sea turtles to climate warming: Will phenological responses be sufficient to counteract changes in reproductive output?

“…Feminization of populations may lead to demographic collapses (Mitchell & Janzen, 2010), although some evidence suggests that a shorter period between breeding bouts in males and promiscuous breeding behavior may help balance operational sex ratios in warmer climates (Hays et al, 2023). The long-term consequences of skewed primary and adult sex ratios on population dynamics and the proportion of males required to sustain populations need to be fully understood for more robust assessments of the impacts of climate change on sea turtles (Boyle et al, 2014(Boyle et al, , 2016Heppell et al, 2022). Similarly, for studies that aim to predict future hatchling production, such as this one, lack of data on the relationship between constant temperatures and hatching success remains problematic as well as lack of knowledge of how to integrate varying temperatures into constant equivalent temperatures and a lack of understanding on metabolic heating and its contribution to hatching success (Gammon et al, 2020(Gammon et al, , 2021.…”

Estimating Changing Marshland Habitat and Conservation Potential for Diamondback Terrapins (Malaclemys terrapin) in New Jersey under Climate Change

“…The percentage of nesting areas able to produce males could be reduced to 0.3% under the 2100, 17% sea level rise scenario with a 7.75 • F [4.3 • C] temperature increase. For loggerhead sea turtles that lay approximately 100 eggs at a time [39] and have similar nesting substrates, it would require males to mate five times annually at a 95% to 5% female sex ratio to maintain a stable population [40]. Comparatively little is known about the mating frequency of male diamondback terrapins; however, the ability of males to sustain the population in combination with the loss of nesting area implies a dire scenario.…”

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

“…Because the percentage of female hatchlings in sea turtles increases at high temperatures and global air temperatures are rising, there is some concern about the potential over-feminization of populations (Jensen et al 2018;Tanner et al 2019). Highly skewed sex ratios from climate warming could reduce genetic diversity, the effective population size and increase the potential for inbreeding (Heppell et al 2022;Maurer et al 2021;Lockley and Eizaguirre 2021). On the other hand, high temperatures also increase egg failure and hatchling mortality, which are therefore threatened by climate change (Santidrián Tomillo et al 2009;Valverde et al 2010).…”