Most people in Hawai‘i are familiar with the Hawaiian green sea turtle, frequently spotted on island beaches. Far less common is its cousin, the hawksbill turtle. With only 100 breeding adults left in Hawaii, much is still unknown about the endangered species. But recent research suggests the answers may lie in its shell. HPR’s Molly Solomon reports.
Kyle Van Houtan shows me a taxidermy model of a hawksbill turtle from the Philippines. The speckled pattern on its shell has been polished. The sheer brilliance of it is a reminder of why so many were harvested in the past. “The shell is ornate, beautiful, and has these wonderful hues from a dark ebony to a light amber or honey color,” said Houtan, a biologist with the National Oceanic and Atmospheric Administration and an adjunct professor at Duke University. “It’s easily fashioned into bracelets, barrettes, and jewelry.”
Van Houtan says these shells do more than just showcase the beauty of these animals. They also hold a lot of important data. “We knew they have an abundant keratin archive that they’re carrying around on their back,” he explained. “What we wanted to see was a way to access that and maybe map it onto time.”
Van Houtan and his team set out to do just that. They studied the keratin, or hard tissue, from 36 hawksbill turtle shells from around the world. They shaved layers from the longer pieces towards the back of the shell. “And when we cross sectioned that and polished it, we found these growth lines.”
Finding those growth lines was a huge victory, said Van Houtan. Largely because little is known about some of the most basic biological questions regarding sea turtles. “There’s a lot of mystery around their basic life history, the kind of things we’d want to know to manage and conserve their populations, particularly for hawksbills because they’re rare in Hawai‘i,” said Van Houtan. “We don’t know quite a bit about them.”
Van Houtan and his team used radiocarbon dating from atomic bomb fallout. They compared the carbon-14 found in coral reef systems with the levels in the hawksbill shells to estimate the turtle’s age, growth, and sexual maturity. He says the turtle’s growth lines are similar to the idea of tree rings. Except instead of one growth mark a year, hawksbills have about eight. The markings can be analyzed to find everything from how old the turtle is to breeding habits. “What we found was the hawksbills here in Hawai‘i probably first start breeding at about 29 years of age,” said Van Houtan. “That was kind of a surprise, because globally for hawksbills in other populations, it’s about 15 years. So this is about twice that.”
Van Houtan says that could explain why the hawksbill turtles in Hawai‘i haven’t recovered as quickly as expected. The turtles are still extremely rare, with only about 100 breeding adults left. Van Houtan also noticed a shift in eating habits. Up until recent decades, hawksbills ate a variety of organisms, both plants and animals. He says now, they’re vegetarians. “That raises a red flag,” he said. “It indicates to us that the population may be suffering from a lack of resources or what it needs from its habitat to thrive. Maybe they’re not here.”
Future studies will likely focus on these recent diet changes and whether the declining health of our coral is a factor. Van Houtan’s findings were reported in the journal Proceedings of Royal Society B.
