Compared with endotherms, homeotherms have it easy: environmental temperature has little effect on our biological processes as we generate our own warmth internally. In contrast, ectotherms – which don't intrinsically maintain a constant body temperature – must constantly cope with varying thermal conditions, seeking warm or cool areas to maintain their body temperature within the optimal physiological range. But how do embryonic reptiles cope with fluctuating temperatures? It was generally accepted that they could not behaviourally thermoregulate. First, the embryos are bound within an eggshell, preventing any major relocation. Further, the machinery required to both detect thermal heterogeneity and relocate to a more favourable environment was thought to be lacking in their early life stages. However, Wei-Guo Du, Bo Zhao and Ye Chen from Hangzhou Normal University, China, and Richard Shine from the University of Sydney, Australia, challenged this point of view in their recent Proceedings of the National Academy of Sciences study. The authors specifically assessed the capacity of turtle embryos to change their position in the eggshell depending on the positioning of the heat source outside of the egg.
The team initially decided to assess whether significant temperature gradients could occur on the surface of the eggs. They observed that the part of the eggshell closest to a heat source could be a much as 1°C warmer than the opposite part of the egg, thereby definitely establishing that thermal heterogeneity occurs within these eggs.
Next, Du and colleagues exposed turtle eggs in their first month of development to a heat source coming from the side or from above. The authors assessed the initial position of the embryo in the egg by candling (transmission of bright light through the egg) and then periodically sampled eggs to dissect and record the embryos' displacement within the egg according to their initial position. In both laboratory and simulated nest settings, the team discovered that the `immobile' turtle embryos were indeed moving around in their eggs, specifically repositioning themselves closer to the heat source. In addition, when they shifted the heat source midway through their experiment from a lateral to a dorsal position, the embryos closely followed the heat source thereby providing further support to their initial results.
This elegant study certainly shows that even early in their development, turtles can behaviourally thermoregulate by moving around in their eggshell depending on the thermal gradient in their environment. This behavioural adjustment could provide them with a strong selective advantage when it comes to their development. Indeed, minute differences in temperature can have a drastic effect on developmental rates during early life stages. Further, this study opens up a suite of questions regarding this remarkable feat in turtle embryos as these results suggest that the structures necessary to elicit this behaviour (sensing a temperature gradient) and to accomplish it (locomotory capacity) are in place very early in turtle development, certainly warranting further research.