No evidence of fine scale thermal adaptation in green turtles

Dominic Tilley,Samantha Ball,Jacqui Ellick,Brendan J Godley,Nicola Weber,Sam B Weber,Annette C Broderick

doi:10.1016/j.jembe.2019.04.001

Abstract

Adaptation to increasing temperatures may enable species to mitigate the long-term impacts of climate change. Sea turtles have temperature dependent sex determination (TSD) and variation in the thermal reaction norm, which influences offspring sex ratio, has been suggested as a potential adaptive mechanism to rising global temperatures. Here, we investigate the sex ratio of green turtle Chelonia mydas offspring from nests on beaches with notable differences in their thermal properties, to look for evidence of localised adaptation. We compared pivotal temperatures and hatch success in both the laboratory and in situ using eggs laid on two nesting beaches (dark vs. pale sand) at Ascension Island that represent the extremes of the range of incubation temperatures experienced by this population. We found no effect of beach of origin on pivotal temperatures, hatch success, or hatchling size in the laboratory or the wild. This suggests that turtles from the same rookery are not locally adapted to different thermal conditions experienced during incubation. Under predicted climate change scenarios, this will result in reduced hatch success and an increased proportion of female offspring unless temporal or spatial range shifts occur.

Highlights

Clear patterns of spatiotemporal shifts in biotic and abiotic trends have unequivocally been associated with a response to climate change (Laloë et al, 2014; Parmesan and Yohe, 2003; Rosenzweig et al, 2008), as species are forced to adapt, disperse or disappear (Parmesan, 2006)
The different albedos of these beaches means that sand temperature on North East Bay (NEB) is consistently ~2°C warmer than Long Beach (LB) (Hays et al, 1995; Weber et al, 2012) with conditions approaching the limit of known thermal tolerance
For clutches incubated in situ from which we sampled hatchlings for sexing (n = 26 clutches; LB = 12, NEB = 14), we obtained a best fit model suggesting a field pivotal temperature of 28.9 °C with a mixed proportion of sexes occurring between 27.1 °C and 30.6 °C for both LB and NEB combined (Fig. 1 - A)

Summary

Introduction

Clear patterns of spatiotemporal shifts in biotic and abiotic trends have unequivocally been associated with a response to climate change (Laloë et al, 2014; Parmesan and Yohe, 2003; Rosenzweig et al, 2008), as species are forced to adapt, disperse or disappear (Parmesan, 2006). Species that exhibit temperature dependent sex determination (TSD) are sensitive to climatic variation (Refsnider and Janzen, 2016) and there are concerns that increasingly imbalanced sex ratios may affect the long term viability of some populations (Laloë et al, 2016; Nelson et al, 2004). Plasticity in the thermal reaction norm, which influences offspring development and sex, has been suggested as one mechanism that may allow adaptation to changing climatic conditions (Refsnider and Janzen, 2016). Sea turtles have been shown to respond to climate change by changing the phenology of nesting (Weishampel et al, 2010), which may ensure clutches incubate under conditions within their thermal norm. For instance loggerhead turtles (Caretta caretta) at the northern extreme of their range produce near balanced offspring sex ratios (Hawkes et al, 2007), but green turtles (Chelonia mydas) show a marked contrast in offspring sex ratio from the northern to southern end of the Australian Great Barrier Reef, likely as a result of differing incubation temperatures (Jensen et al, 2018)

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Discussion

Conclusion