The bomb pulse

Answers concerning the life-history traits of sea turtles sometimes come from the most unlikely sources. Most recently, the nuclear tests during the mid-twentieth century proved key to determining the age and growth rates of sea turtles in Hawaii. To understand the improbable link between Hawaiian sea turtles and nuclear weapons, we have to go back in time by over half a century.

During the 1940s and early 1960s nuclear tests were being carried out by various nations across the globe. As a direct result of this, the concentration of carbon-14 (14C) in the atmosphere nearly doubled within a decade. In 1963 over 100 nations signed a treaty agreeing to ban nuclear weapon tests in the atmosphere, in outer space and under water. Since then the atmospheric concentration of 14C has been decreasing at a steady rate due to natural exchanges with the biosphere. As atmospheric 14C is assimilated in the biosphere it can be found in all plants and in the animals that eat them. Interestingly, the 14C concentrations inside an organism mirror those present in the atmosphere and because the temporal change in the concentrations of 14C is well-documented, scientists can accurately determine the age of an organism based on its 14C content. This technique, known as bomb-radiocarbon dating (or bomb-pulse dating), is similar to the more widely-known radiocarbon dating used to date fossils.

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The levels of carbon-14 in the atmosphere have been relatively stable over long time periods, with the exception of a large addition of carbon-14 in 1955–1963 as a result of nuclear bomb tests. The boxed region in a is shown in more detail in b. (Source: Nature)

A team of researchers from NOAA and Duke University recently applied bomb-radiocarbon dating to the hard tissue of 36 hawksbill turtle shells collected since the 1950s. This allowed them to approximate growth rate and reproductive maturity of these turtles and gave them new insights into this hawksbill population. Their results, published in Proceedings of the Royal Society B, show that this sea turtle population starts breeding at an average age of 29 years (range from 23 to 36 years). This is much later than other populations of this species, and may be a reason why this population, one of the smallest in the world, is not rebounding. In addition, the research reveals that these turtles’ diet has changed over time: they were omnivores until the 1980s but are now mostly herbivores. This indicates a dramatic change in the turtles’ food supply, which could be a sign of long-term ecosystem changes occurring in Hawaii.

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Researchers have generated new growth curves for the Hawaiian hawksbill sea turtles by studying the interior structure of the turtles’ posterior marginal scutes and using bomb-radiocarbon dating.  (Source: Proceedings of the Royal Society B)

Bomb-radiocarbon dating appears to be more accurate in assessing the development of sea turtles than previously-used methods. It also has many applications in other research fields, including investigating Nazi war crimes and accurately determining wine vintages. However, as the atmospheric 14C issued from the bomb pulse disappears, so does our ability to use it to accurately age organisms. It is estimated that the bomb pulse will die out within the next two decades. Until then, there is no doubt that the bomb pulse will continue to be of unexpected scientific significance.

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Terrestrial basking and climate change

Terrestrial basking is a rare behavior observed in populations of green sea turtles in Hawai’i, Western Australia, and the Galápagos Archipelago. Being cold-blooded, the main reason why turtles bask on land is probably to regulate their body temperature but scientists speculate that terrestrial basking may also aid immune function, predator avoidance, and may even prevent unwanted courtship.

New research published in Biology Letters examined the relationship between terrestrial basking and climate. Having counted the number of turtles that bask on one Hawaiian beach every day for six years, the researchers found that terrestrial basking peaks in the year when the sea surface temperatures are lowest. Terrestrial basking generally happens when sea surface temperatures fall below 23°C. This suggests that terrestrial basking is a response to seasonally cool ocean temperatures.

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Basking varies seasonally in concert with cool SST. Green circles are standardized anomalies of the number of turtles observed basking weekly at Laniakea, Oahu. Blue circles are weekly AVHRR SST data for this location. Thick dark lines are the Fourier series for each timeseries. (Source: Biology Letters)

However, since the sea surface temperatures at the sites where turtles bask on land is warming on average 0.04°C per year, the researchers predict that in the future the waters will be warm enough that the turtles will no longer come on land for warmth. The researchers estimate terrestrial basking may cease in Hawaii by 2039, in Australia by 2086, and in the Galápagos by 2102. Since other populations of marine turtles are successful without having to resort to terrestrial basking, this will probably not have drastic negative impacts on these green turtle population, but this does mean that beach goers of the future will not have the privilege of sharing the beach with napping turtles.

Terrestrial basking

Turtles usually only come ashore to nest. It is rare to see a marine turtle on land otherwise. But it does happen: at certain beaches turtles come ashore to bask in the sun during the day. The only species known to exhibit this unique behaviour is the green turtle. It regularly comes to bask on the beaches of Hawai’i, and there have also been reports of turtles basking in the Galápagos Archipelago and in Western Australia. While basking the turtles will seemingly close their eyes and fall asleep. This creates the unique situation for beach goers to have to share the beach with napping turtles!

The obvious benefit of basking for reptiles is thermoregulation: raising body temperature accelerates metabolic processes like digestion and growth. Being cold-blooded, this is probably the main reason why these green turtle bask and nap on land. A possible added benefit of being on land is the reduction in exposure to marine predators such as tiger sharks. Finally, it could be that staying on land is also more energy-efficient as the turtle does not have to periodically swim to the surface for air. There are turtles that seem to prefer to bask at the same spot every day. On some beaches, tape is placed around the turtles’ favourite sleeping spots to ensure the turtles can rest undisturbed. It is also reported that certain groups of turtles always bask together, like friends. Occasionally Hawaiian monk seals also join the turtles for their nap, like in the pictures below!

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A big thank you to Mark Sullivan for all the great pictures of the turtles with the seals. Check out his great work with Hawaiian monk seal! I also thank Rebecca Scott for the original post idea.