Discovering the Life of a Hawksbill Turtle From a Single Bone

Jamie Clark is a Fisheries Biologist with CIMAS and the Southeast Fisheries Science Center, where she studies the life history and growth dynamics of fish and, previously, sea turtles using skeletal structures that record an individual’s biology in their bones. 

Sea turtles are both ecologically vital and complex marine reptiles. Hatchlings start out as eggs laid in nests on beaches all over the world. Once hatched, they float out to sea—otherwise known as the “the lost years” of a turtle’s life. They then move to near-shore feeding grounds, where they potentially stay for decades until reaching sexual maturity. Once fully mature, they migrate, maybe thousands of miles, back to their original nesting beaches for mating. After hatching, it may take 10 to 50 years (depending on the species), to reach sexual maturity. Such complex life cycles and migratory patterns create challenges to study and collect biological and ecological information. This gap in scientific knowledge creates challenges when evaluating management strategies used for populations at risk of extinction. 

The threat of extinction has been a consistent danger to Hawksbill Sea turtles. This is particularly due to prolonged harvest in some countries that these turtles frequent. Sea turtles are harvested at all life stages both for their meat and for their “tortoise shell.” Nonetheless, scientists can apply knowledge of species’ size-at-age relationships, growth patterns, and migration movements to better assess human impacts. This allows leaders in the field to deploy increasingly effective management strategies to avoid species extinction.

One way to help current populations is to utilize the trail of clues left over from deceased sea turtles. Using this data, we can recreate an individual’s growth, sexual maturation, and migration patterns. Scientists analyze the bones of sea turtles that wash ashore dead. Those bones are analyzed internally, similar to counting the rings of a tree. In collaboration with the National Sea Turtle Stranding and Salvage Network, we have been able to collect bones from a wide range of areas and time.

The humerus has been found to be the best bone to calculate age from, in most sea turtle species. This is due to the high ratio of compact bone (where the growth-ring is recorded) to spongy bone (eventually erasing initial growth-rings when the animal grows). To be able to count growth-rings like a tree, a sea turtle’s humerus goes through many steps before its clues are revealed. We cut cross-sections just below the neck of the humerus. We then decalcify that section and take even thinner sections to stain. This highlights the growth-rings, making them easier to count and measure.

 Hawksbill sea turtle

Figure 1. Example Hawksbill Sea Turtle humerus bone cross-section

Just like solving a mystery from a single clue, a scientist is trying to figure out the mystery of this sea turtle’s life from just its bone! We assign an age and calendar year to each growth-ring in a bone by back-calculating from the stranding date. Furthermore, scientists have found that there is a very close relationship between the measurements of bone growth-rings with sea turtle shell length. This means that we can back-calculate body size with each growth-ring that can be measured. Nonetheless, scientists can also calculate how much the turtle was growing each year by taking the difference of sequential body size estimates. Since growth rates slow as an individual reaches sexual maturity, we can estimate age and size at maturation. This can greatly differ between species and geographic regions.

Furthermore, the chemical composition (isotope ratios) of bone layers between the growth rings illustrate where turtles have previously been living and what they have been eating. By piecing together these clues from a sea turtle’s bone, scientists get a more detailed picture of the sea turtle’s life. Linking all of this information together is very helpful to analyze trends in age, growth, and habitat use over an extensive period of time and broad geographic range. By understanding the life-stage durations, growth patterns, and age at maturation, effective management strategies can be implemented to assist threatened species.

Using this ‘biological detective’ approach, scientists have found regional differences in growth patterns and have begun to determine the causation, whether this might be due to environmental variation and/or variability in preferred diet.