Nocturnal emergence facilitated by thermally-induced hatching in the Chinese Softshell Turtle, Pelodiscus sinensis

The coincidence of hatching timing and emergence events with favourable conditions is crucial for survival. Nocturnal emergence has been widely documented across marine and freshwater turtles and has long been suggested as an adaptive behaviour that reduces risk of heat stress and predation. However, our knowledge of emergence timing is mainly based on the observation of emergence events, and whether the timing of hatching is influenced by diel environmental factors remains largely unknown. Herein, we visually monitored the activity of the Chinese softshell turtle (Pelodiscus sinensis)—a shallow-nesting freshwater turtle—from hatching to emergence. Our data demonstrated that (i) the embryos within a single nest exhibited synchronous hatching; in particular, (ii) their synchronous hatching coincided with the period of decreased nest temperature. Further analysis indicated that pipping was specifically more likely to occur during the period of rapidly decreasing nest temperatures. Together, these results provide strong evidence for the existence of synchronous hatching in this freshwater, shallow-nesting turtle species, and that the timing for nest emergence may have already been selected at the hatching stage.