Unlocking the Mysteries of Sea Turtles Using Their Bones
Every year, hundreds of sea turtles wash up on our beaches. These strandings, while so often tragic, provide a unique opportunity to study these mysterious creatures that spend nearly their entire lives roaming the ocean. This nomadic lifestyle poses a significant challenge for scientists, particularly those who study their growth, like me. However, by borrowing techniques from anatomy and forensic science, we can use the bones of dead stranded turtles, which contain growth rings similar to tree rings, to learn more about their growth, diet, and habitat use. Sea turtle bones provide a window into their past lives and are key to unlocking ecological mysteries that have perplexed sea turtle biologists for decades.
Bone Growth Rings
Studying how fast turtles grow is notoriously difficult. Traditionally, one must first locate, capture, tag, and measure a turtle and then try to capture that same turtle again in the future. As you might imagine, this is no easy task. Despite harnessing substantial (wo)manpower over multiple years, mark-recapture studies often generate datasets plagued by small sample sizes and irregular time intervals. Analyzing sea turtle bones provides a means of circumventing some these issues.
As it turns out, sea turtle humerus bones contain records of growth in the form of annual growth rings that can be revealed by staining sections of bone (Fig. 1). Most animal tissues are replaced regularly with new cells (e.g., skin), but sea turtle humerus bones are unique in that they seem to retain their structure for multiple years, although they do eventually break down from the inside out (the dark brown center of Fig. 1b).
Previous research shows that there is a relationship between the diameter of a growth ring and the length of a turtle’s shell—take the difference of body size estimates from adjacent growth rings, and voilà, you have a growth rate. In fact, many growth rates. From a single turtle we can get up to 12 years of growth information! What’s more, turtle bones also provide one of the only practical methods for aging sea turtles. We can determine roughly how old turtles were when they died by counting the number of visible growth rings (and adding the number of rings likely broken down in the center of the bone).