A clutch of a dozen turtle eggs lies buried in the bank of Australia’s Murray River. For the embryos inside, timing is everything. In a few days, they will all hatch together, finding safety in numbers in their vulnerable first moments. But such synchrony isn’t easy. To achieve it, the embryonic turtles need to coordinate the pace of their development, keeping in time with one another even before they experience the outside world.
Although all the eggs were laid at the same time, in the same nest, they experience radically different environments. Those at the top of the nest, buried in warmer sun-soaked soil, can be up to six degrees Celsius warmer than those at the bottom. That’s a problem because the embryos develop at different rates depending on how hot they are. Given the gradient of warmth in the nest, the topmost turtles should hatch well before their siblings at the bottom.
That’s not what happens. Ricky-John Spencer from the University of Western Sydney has found that the Murray River turtles can tell whether their clutch-mates are more or less advanced, and shift the pace of their own development accordingly. If their peers are racing ahead, they can play catch-up.
In 2003, Spencer collected clutches of wild eggs, split them into two groups, and incubated them at either 25 or 30°C. He reunited the eggs, and found that they still hatched together. Despite the developmental boost that the hotter half received, the colder ones still emerged in time with them. They either accelerated their development, or they hatched prematurely.
To work out which, Spencer’s student Jessica McGlashan captured pregnant Murray River turtles and allowed them to lay their eggs in a lab. Just as Spencer did previously, she split the clutches into two groups. In some cases, she incubated both groups at 26°C; in others, she incubated one group at 26°C and the other at 30°C. She reunited the eggs a week later and monitored the metabolism of each embryo by measuring how fast its heart was beating, and the amount of carbon dioxide it gave off.
McGlashan found that the embryos sped up their development if they were incubated with advanced peers, who had enjoyed a week at 30°C. In the weeks before hatching, their heart rates went up and they exhaled 67 per cent more carbon dioxide than turtles whose siblings had all stayed at 26°C.
When they finally emerged, McGlashan found that these embryos had exhausted more of their yolk supplies in their attempts to catch up. But the speedy developers hadn’t sacrificed their time in the egg by hatching prematurely. Once they were born, McGlashan found that they were just as physically capable as turtles that had grown at a more leisurely pace. If she flipped them over, they righted themselves in the same amount of time, if not quicker.
The embryos must have some way of communicating with one another but it’s not clear what that might be, or even whether it’s an active process. McGlashan notes that changes in the nest could trigger hormones that alter the turtles’ metabolism. For example, thyroid hormone helps to control their growth and development, as well as their metabolism. Embryos produce more of it when oxygen levels fall. It’s possible that as the fast-developing embryos use up oxygen around the nest and pump out more carbon dioxide, the stragglers automatically produce more thyroid hormone, which speeds up their own development.
Reference: McGlashan, Spencer & Old. 2011. Embryonic communication in the nest: metabolic responses of reptilian embryos to developmental rates of siblings. Proc Roy Soc B http://dx.doi.org/10.1098/rspb.2011.2074
Image by Peripitus
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