In species with temperature-dependent sex determination (TSD), global climate change may result in a strong sex ratio bias that could lead to extinction. The relationship between sex ratio and egg incubation at constant temperature in TSD species is characterized by two parameters: the pivotal temperature (P) and the transitional range of temperature that produces both sexes (TRT). Here, we show that the proportion of nests producing both sexes is positively correlated to the width of the TRT by a correlative approach from sex ratio data collected in the literature and by simulations of TSD using a mechanistic model. From our analyses, we predict that species with a larger TRT should be more likely to evolve in response to new thermal conditions, thus putting them at lower risk to global change.

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Theoretical evolutionary ecology assumes the existence of fitness variability in natural populations. As realistic measures of fitness are usually difficult to perform directly, integrating fitness indices are proposed in all taxa. In sauropsids, locomotor performances have been proposed as a good integrating index of fitness in natural populations. Concerning aquatic turtles, a performance trait that may be important for the survival of juveniles is the righting response of individuals when they are placed on their carapace. In the present study, we examined the righting response in juveniles of the red-eared slider turtle, Trachemys scripta elegans . We tested two different measures of the righting response for 170 juveniles from constant incubation temperature and for 86 juveniles from three sinusoidal fluctuating incubation temperatures that are considered as more representative of natural conditions in the nest. We compared the effects of offspring sex, maternal identity, juvenile growth rate, and juvenile survival (i.e. individual characteristics), as well as the nutritional status of juveniles (i.e. experimental conditions), on the two different measures of righting response and for each thermal incubation treatments of the eggs (i.e. experimental treatments). We observed that the effects of the individual characteristics were markedly different between the two measures of the righting response and between experimental treatments. These results highlight the importance of the choice of the measure and of the experimental conditions and treatments in the study of a phenotypic trait. Results obtained for only one performance measure under constant laboratory conditions must therefore be extrapolated to the field with caution. Our results also show that the righting response presents individual variability that is probably heritable and is indirectly correlated with survival. These findings support the validity of the righting response as a good candidate for a fitness index in aquatic turtles.

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ABSTRACT1. Massive importation of slider turtles (Trachemys scripta elegans) as a pet in France over the past few decades has been followed by the release of many of these turtles into natural environments. T. scripta elegans is now widely distributed in France.2. This paper reports on the successful reproduction of this species in France, with confirmed production of both sexes from nests incubated in the wild. These results indicate that the turtle T. scripta elegans can reproduce in the wild and that its long-term persistence is possible in France.3. The potential impact of this invasive species in natural ecosystems warrants future study. Meanwhile, authorities are strongly encouraged to educate the public to the potential danger of exotic species introduction for local ecosystems and to capture and remove this species from wetlands in southern Europe.

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Summary 1.In temperature-dependent sex determination (TSD) species, offspring sex-ratio is dependent on the environmental temperature. For oviparous sauropsid species, temperature within the nest influences gonadal sexual differentiation during a small window of embryogenesis called the thermosensitive period (TSP). 2. The absence of sexual dimorphic characteristic in juveniles of TSD species coupled with the lack of a non-invasive method to determine sex is a great obstacle to studies estimating sex-ratios under natural conditions. Some authors have proposed proxies of sex-ratio obtained through correlative approaches. They commonly extrapolate the empirical profile of sex-ratios as a function of constant incubation temperature established for several species in the laboratory to a field context. However, most of these proxies have been refuted by studies realized under field conditions, and consequently they cannot be used to predict sex-ratio under natural conditions. 3. Here, we propose a new thermal model of TSD using a mechanistic approach. We built this model from a collection of published data of physiological processes (i.e. the growth of the embryo, the growth of gonads and the activity of the enzyme aromatase) underlying the TSD mechanism, for the European pond turtle ( Emys orbicularis ). This new approach provides integration of incubation temperature fluctuations, as well as the cumulative and differential effect of high and low temperatures on sexual differentiation to embryo sex determination. 4. The significant consistency obtained between observed and predicted sex-ratios both at diverse constant and fluctuating incubation temperatures provides hope to develop an efficient method to predict sex-ratio under natural conditions. The reliable validity of this new model could have wideranging implications for the understanding of the TSD mechanism, as well as its evolutionary and ecological consequences in natural populations.