Synchronous hatching in freshwater turtles : metabolic and endocrine physiological mechanisms

Synchronous hatching occurs in many aquatic turtles and appears to have evolved to maximise hatchling survival despite predation and stochastic environmental conditions. In most cases, synchronous hatching occurs when cooler, underdeveloped embryos in a nest either accelerate their development through metabolic compensation or hatch early. Synchronous hatching is more broadly a form of environmentally cued hatching (ECH), which generally allows embryos to alter the time of hatching in relation to the environment through phenotypic plasticity and hatch early, delayed, and/or synchronously. The mechanisms of ECH and synchronous hatching in reptiles are still poorly understood but both cause variation in developmental stage at hatching and reduce variation in incubation time to increase an individual’s chance of survival through a tradeoff between the risks and benefits of hatching. Reptilian embryonic development and incubation period are sensitive to temperature changes. Thermal gradients in shallow nests influence rates of embryogenesis, but how these are influenced is diverse given daily temperature fluctuations and seasonal trends. Differences in incubation conditions cause eggs at the top of a nest to experience warmer temperatures than eggs at the bottom, which increases metabolic and developmental rates of embryos and causes asynchronous development within clutches. Despite asynchrony in development, most clutches still hatch synchronously, which is thought to be an important survival strategy for hatchlings. Cold-incubated embryos are triggered to either hatch early and at a less developed stage or metabolically compensate to accelerate developmental rate and hatch fully developed. Metabolic compensation and matching of circadian rhythms in heart rates of embryonic turtles indicate the potential for communication between embryos in a nest. I aimed to identify the mechanisms of hatching synchrony in the eastern long-necked turtle (Chelodina longicollis) and the painted turtle (Chrysemys picta) to determine if they synchronously hatch via hatching early or metabolic compensation. The Murray River shortnecked turtle (Emydura macquarii) is known to synchronously hatch by metabolically compensating during embryogenesis. In E. macquarii, metabolic compensation occurs because embryos initially incubated at different temperatures are still able to hatch at the same size and developmental stages after eggs are re-introduced to one another. Cold incubated embryos appear to “catch up” by increasing their developmental rate, which is correlated with higher metabolic rates during late development and reduced residual yolk sacs at hatching. In this project, asynchronous conditions were induced in clutches of C. longicollis and C. picta, and incubation periods and hatchling morphology were compared to determine whether they still hatched synchronously and whether cold-incubated embryos “caught up” developmentally with their warmer siblings. I also assessed metabolic rates via oxygen consumption and heart rates to determine if synchronous hatching occurred as a result of metabolic compensation. The effects of a group environment during incubation on egg development and incubation period were also investigated in C. longicollis during the final 3 weeks of development