Rusli, M.U., D.T. Booth, and J. Joseph (2016). Synchronous activity lowers the energetic cost of nest escape for sea turtle hatchlings. J. Exp. Biol. 219, 1505-1513. doi:10.1242/jeb.134742
Sea turtles have a rough start in life. Their first mission is to dig out of over a foot of wet sand (with a pair of tiny flippers, no less) and dash for the ocean. Once in the water, the hatchings swim as fast as they can out to sea, where they remain until they are large enough to breed. To make matters worse, they have to do all this with only the energy contained in their egg yolk, since hatchlings do not begin feeding until they are well out to sea.
It’s no surprise that this “post-hatching dispersal phase” is a critical time in the life of a sea turtle.
But hatchlings have one possible advantage – they don’t need to dig themselves out alone.
Synchronous digging activity in sea turtle hatchings has been proposed as an example of social facilitation. Social facilitation is the concept that one animal’s performance of a certain behaviour (e.g. a deer bolts when it hears a noise) increases the probability of other animals to either engage in or increase the intensity of the same behaviour (e.g. the other deer in the herd also bolt, even if they did not hear the dog).
Because all eggs within a nest (called a “clutch”) experience similar temperatures, they tend to hatch at about the same time. Since turtle hatchlings do not dig take long rest periods between bouts of intense digging activity, social facilitation may help co-ordinate clutch digging (e.g. one turtle’s digging causes the ones nearby to also start digging).
Researchers out of the University of Queensland and Universiti Malaysia Terengganu investigated how the size of the clutch influenced the time and energy needed for green sea turtle hatchlings to escape the nest. They hypothesized that due to the combined effort of more individuals, larger clutches would allow hatchlings to reach the surface more quickly, and using less energy, than smaller clutches.
To estimate energy use during nest escape, the researchers used a technique called “open-flow respirometry” to measure the oxygen consumption rate of a digging clutch of eggs. By comparing the difference in oxygen content of the air going into a sealed chamber, and the air coming out of the chamber, the team was able to calculate the amount of oxygen consumed by the nest. Since oxygen is involved in converting energy in useable form, oxygen consumption rate is a good indicator of its metabolic rate.
The oxygen consumption rate was measured in buried egg clutches as the turtles hatched and dug their way to the surface. By varying the number of eggs in each clutch, the researchers were able to test the hypothesis that larger clutches would reduce the energetic demands placed on each individual hatchling as they made their way to the surface.
The group also estimated the overall energy content of some freshly laid eggs and of a small subset of hatchlings using ballistic bomb calorimetry (sounds awesome, right?), which essentially measures how much heat is given off when the substance (an egg’s yolk, a hatchling’s carcass) is burned. The researchers then calculated how much energy was used during development and hatching, and how much was available for nest escape and dispersal into the sea.
With a little help from my friends
As hypothesized, it seems that many flippers make light work. The more eggs in a clutch, the faster the hatchlings were able to dig their way through 40 cm of sand. Since the total energetic cost of nest escape increased with each extra day the turtles spent digging, hatchlings that reached the surface faster also tended to have lower average metabolic rates during the digging period.
Hatchling energy budgets & future survival
The researchers calculated that the average energetic cost of escaping through the sand barrier ranged from 4.4 kJ to 28.3 kJ per individual turtle, and the cost deceased as the number of individuals in the clutch increased. This means hatchlings spent 11 to 68% of their remaining energy stores on digging out from the nest!
Being born into a large family has its benefits! It can result in as much as a 4-fold reduction in an individual’s energetic cost of nest escape. This can make a huge difference, as energy saved during the initial dig out can be re-allocated to swimming away from shore or fending off starvation, giving the hatchings a better chance of surviving until they are big enough to defend themselves.
Aside from making it easier to reach the surface, hatching with a bunch of brothers and sisters has other advantages. Predators can easily pick off one or two hatchlings on their own, but can be overwhelmed by large numbers of turtles emerging simultaneously, ensuring at least some hatchlings make it into the surf. It has been suggested that social facilitation also applies in this circumstance as well, and that hatchlings tend to make their dash towards the waterline in large groups.
Confirmation that hatchlings from large clutches have better energy reserves when entering the sea also challenges the wisdom of some active conservation efforts, and underscores the importance of basic research when designing a species-specific conservation program. In places like Malaysia, where portions of this study were conducted, a common strategy is to split natural clutches when relocating them to hatcheries. Hatchlings from these smaller split clutches may emerge with reduced energy reserves, and be at a disadvantage when they are returned to the wild.