Modelling the pipping to emergence interval and its influence on the thermosensitive period for sex determination in the green sea turtle Chelonia mydas

Sea turtle survival rate and hatchling sex ratios are closely linked to incubation temperature and duration, both of which are critical for understanding species resilience to climate change. This is particularly important for sea turtles currently nesting in thermally stressed environments, such as the North West Indian Ocean green sea turtle population. However, there is a methodological discrepancy between field studies (which usually measure incubation duration from oviposition to emergence) and constant temperature incubation studies (which measure from oviposition to either pipping or hatching time). The period between hatching and emergence can span several days, potentially affecting estimates of the thermosensitive period (TSP) when sex is determined. This study investigates the often-overlooked interval between hatching and emergence, a developmental phase essential in hatchling survival. We evaluate two non-invasive methods to estimate this interval: (1) deploying tri-axial accelerometers and temperature loggers to detect movement associated with hatching, and (2) comparing laboratory and field incubation durations using fitted models. Results reveal that field-based durations consistently exceed laboratory-based predictions, primarily due to the inclusion of the emergence to pipping time. Across seven nests, using emergence instead of pipping to mark the end of incubation raised mean TSP temperature by 0.1-0.5 °C and lengthened the inferred TSP by about four days. Our findings highlight the implications of this discrepancy for modelling TSP timing; by excluding the pipping to emergence time. While this bias may seem minor, the narrow thermal range of sex determination means it could significantly skew sex ratios and lead to artificial feminisation estimations. By refining incubation duration metrics, this work enhances the accuracy of sex ratio predictions and provides new insights into the hidden pipping dynamics informing conservation strategies such as beach nourishment and hatchery practices.