Effects of semi-constant temperature on embryonic and hatchling phenotypes of six-tubercled Amazon River turtles, Podocnemis sextuberculata

Purpose: We evaluated how constant incubation temperatures affect life-history traits pre-hatching and post-hatching of the six-tubercled Amazon River turtle, Podocnemis sextuberculata. Methods: We incubated eggs from natural nests at ten semi-constant temperatures between 22.26±1.01°C and 37.37±0.38°C (2013) and at six temperatures between 25.75±0.22°C and 36.17±0.15°C (2016). In 2013, we raised hatchling for 90 days to evaluate effects of temperature on early hatchling growth. We evaluated maternal effects in 2016. Results: P. sextuberculata displays temperature-dependent sex determination and produces males at colder and females at warmer temperatures (TSD Ia). The estimated pivotal temperature was 33.73 ± 0.15°C and the transitional range of temperatures (TRT) 1.16 ± 0.59°C. Semi-constant temperatures below 26°C and above 38°C were lethal. Intermediate temperatures (32.25°C and 31.5°C, respectively) were optimal for hatching success and produced larger hatchlings that grew slower early in life compared to colder or warmer conditions, which produced smaller hatchlings. Warmer incubation temperatures within the optimal range (28°C-37°C) accelerated embryonic development. In contrast, comparisons of 30, 60 and 90 days-old suggests that warmer incubation temperatures reduced growth and mass gain rates post-hatching, such that incubation temperature effects on body size at emergence disappeared by 3 months of age. Conclusions: Six-tubercled Amazon River turtles showed the highest pivotal temperature reported for any turtle. The relatively narrow TRT may limit the evolutionary potential of this vulnerable turtle in the face of global warming. Future incubation experiments at a finer scale (33°C-36°C) are warranted to refine the sex-ratio reaction norm. Field studies that monitor natural nests are imperative to evaluate conservation measures and the effect of female-biased illegal hunting and climate change. By providing data about the thermal biology of an understudied lineage of non-model species, our study helps fill gaps in our understanding of the evolution of vertebrate sex determination and its potential adaptive value.Data files can be opened using any spreadsheet reader, that opens .xls file format. All analyses were performed using R v. 3.5.2Funding provided by: PetrobrasCrossref Funder Registry ID: http://dx.doi.org/10.13039/501100004225Award Number: Funding provided by: Ministerio de Ciência, Tecnologia, Inovação e Comunicação, Governo do Brasil*Crossref Funder Registry ID: Award Number: Funding provided by: iGUi Ecologia*Crossref Funder Registry ID: Award Number: Funding provided by: Institute of Food and Agricultural Sciences, University of FloridaCrossref Funder Registry ID: http://dx.doi.org/10.13039/100007861Award Number:Two semi-constant-incubation-temperature experiments were conducted in 2013 and 2016. Freshly-laid eggs were collected at the Middle Solimoes River within the Mamiraua Sustainable Development Reserve, Central Amazon, Brazil and transported to the laboratory at the Mamiraua Sustainable Development Institute (Tefe, Amazonas State, Brazil). Egg incubation started