Determining the age of sea turtles is notoriously difficult. The size of the creatures cannot be used as a measure, and the lack of teeth rules out another potential marker. Estimates vary on the age the marine reptiles can reach, with many experts suspecting that they can probably get to around 100 years old. But researchers have now demonstrated that the age of sea turtles, and even the time at which they reach sexual maturity, can be calculated by looking at the carbon signatures found in their shells, the result of the nuclear bomb tests carried out at the height of the Cold War. Their results are published in the Proceedings of the Royal Society B.
By looking at the scutes – or bony external plates – of dead Hawaiian hawksbill sea turtles, the researchers were able to count the growth lines within them (a little like counting tree rings). They were then able to calculate that the hawksbill turtles formed on average eight lines per year, allowing them to figure out the turtles' growth rates. In addition to this, they also worked out that the turtles reached sexual maturity at an astonishing 29 years of age, which goes some way to explain why the Hawaiian population of these reptiles has been so slow at recovery after being decimated for their desirable shell.
The nuclear bomb tests in the Pacific, such as this one on Bikini Atoll in July 1946, released masses of extra carbon-14 into the atmosphere, and subsequently the oceans. United States Department of Defense/Wikimedia Commons
Bomb radiocarbon is actually widely used to calculate the age of marine organisms, from corals and sponges to fish. The carbon-14 in the marine environment shifted dramatically after the nuclear testing that took place in the Pacific during the middle of the century. But until now, the use of this technique to get reliable age estimates for sea turtles had never been achieved, mainly because the reptiles get all their carbon-14 not from the environment, but from their diet, and this alters the amount of carbon-14 then locked away in their tissue.
By looking at the bomb-testing carbon-14 accumulated in the shells of the turtles and comparing it to the background rates found in Hawaii’s coral, as well as with captive turtles for which the growth rates are known, the researchers were able to calculate the ages of the wild turtles when they died, the rates at which they grew, and even at what point they reached sexual maturity. The scientists analyzed 36 different turtles that were collected from between 1962 and 2013 in order to give them a time frame spanning the period during which the nuclear bomb tests took place.
The researchers managed to acquire 36 Hawaiian hawksbill shells, many from museums due to their rarity. Kyle Van Houtan/Duke University
The researchers were also able to see how the turtles' diets have varied over the past 50 years, and it’s not great. Up until around the 1980s, the turtles were primarily omnivores, eating both animal and plant matter, but now they appear to be primarily herbivores, relying on things like algae. This could be an indication that there has been a shift in food supply that has forced them to become vegetarian, and could also be impeding their recovery.
This is the first reliable marker for aging sea turtles, and it is hoped that by using this method, researchers will be able to roll it out with other species worldwide, and to help expand our knowledge of these marine creatures.