Temperature‐Dependent Sex Determination in Sea Turtles in the Context of Climate Change: Uncovering the Adaptive Significance

Abstract

The adaptive significance of temperature-dependent sex determination (TSD) in reptiles remains unknown decades after TSD was first identified in this group. Concurrently, there is growing concern about the effect that rising temperatures may have on species with TSD, potentially producing extremely biased sex ratios or offspring of only one sex. The current state-of the-art in TSD research on sea turtles is reviewed here and, against current paradigm, it is proposed that TSD provides an advantage under warming climates. By means of coadaptation between early survival and sex ratios, sea turtles are able to maintain populations. When offspring survival declines at high temperatures, the sex that increases future fecundity (females) is produced, increasing resilience to climate warming. TSD could have helped reptiles to survive mass extinctions in the past via this model. Flaws in research on sex determination in sea turtles are also identified and it is suggested that the development of new techniques will revolutionize the field.

Reconstructing past thermal conditions in beach microclimates

“…For sea turtles, there are specific reasons why understanding past nest conditions is important. Sea turtles exhibit temperature‐dependent sex determination (TSD), with females being produced from eggs incubating at high incubation temperatures (Bentley et al, 2020; Standora & Spotila, 1985; Santidrián Tomillo & Spotila, 2020). Hence, there is a concern that warming temperatures as a part of climate change may raise temperatures sufficiently to produce single‐sex (female‐only) hatchling cohorts and so ultimately population extinctions.…”

“…For instance, the largest green sea turtle rookery in the world was recently shown to be extremely female‐biased (Jensen et al, 2018) and for our study site in Cape Verde, primary sex ratios were estimated to be >70% female (Laloë et al, 2014). An important question is whether these skewed sex ratios are a normal feature for sea turtles and have persisted for a long time, possibly being adaptive by allowing increased population growth rates (Santidrián Tomillo & Spotila, 2020), or whether these skews are a maladaptive consequence of climate warming. Reconstructing the last century or more of sand temperatures on nesting beaches across the globe will help answer this question.…”

A review of the effects of incubation conditions on hatchling phenotypes in non-squamate reptiles

“…feminisation increasing population growth rates until collapsing due to a lack of males) (Hays et al 2017;Laloë et al 2017;Santidrián Tomillo et al 2021;Schwanz and Georges 2021). For instance, in sea turtles, differences in breeding periodicity between the sexes can balance operational sex ratios despite biased adult sex ratios (Hays et al 2010(Hays et al , 2014Santidrián Tomillo and Spotila 2020). Despite the importance of sex ratios, reductions in hatching success may have a larger effect on population viability.…”

Effects of global warming on species with temperature‐dependent sex determination: Bridging the gap between empirical research and management

“…As such, we should consider the potential for elements of the thermal response curve (e.g. the pivotal temperature and transitional range of temperatures) to vary from traditional values—either from plastic responses or as a result of adaptive evolution (Santidrián Tomillo & Spotila, 2020 ).…”

“…Given these constraints are related to sexual reproduction, independently of TSD, and given that temperature affects a wide range of traits such as size, developmental rate or embryonic mortality, Rollinson ( 2019 ) proposes that males should be produced at the temperatures that maximize the fitness potential of these traits, while females should be produced at the suboptimal extremes. As mortality increases with high temperatures, the Mighty Males hypothesis can be easily applied to type II (female–male–female) and type Ia (male–female) TSD species (Santidrián Tomillo & Spotila, 2020 ). On the other hand, it may not apply to type Ib (female–male) TSD species, unless evolutionary mechanisms exist that can significantly push back the upper temperature limits for mortality.…”