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).

Fig. 1. (A) stained and (B) isotopically sampled humerus bone sections from a loggerhead sea turtle that stranded dead in North Carolina. Yellow dashed lines indicate location of annual growth rings with back-assigned year, turtle shell length, and age estimates for the newest and oldest growth rings are also provided. New bone tissue is added on the outer edge of the bone, whereas older bone tissue is slowly broken down in the inner part of the bone (i.e., central dark brown portion of B). Reproduced from Ramirez et al. 2015.

Sea Turtle Forensics

Sea turtle bones are not just recorders of age and growth. They also archive past diet and movement in the form of stable isotopes and trace elements (Box 1 below). Different prey items and areas of the ocean have different chemical signatures, so as turtles move through the environment and forage, they pick up these unique chemical ‘fingerprints’ in their tissues.

For example, nitrogen isotope ratios increase in animal tissues as you move up the food web. As you are what you eat, if we have isotopic information on all parts of the food web we can identify the types of prey turtles eat (e.g., jellyfish vs. crab vs. fish), and in some cases the specific species (if the food web is simple). Similarly, with knowledge of regional differences in chemical signatures, we can determine what regions of the ocean turtles frequented in the past.

The true power of sea turtle bones is thus the ability to combine many sources of information (e.g., size, age, growth, diet, habitat use) through time for each turtle. By doing so we have been able to identify the specific sizes and ages that turtles migrate between open ocean and nearshore habitats (1,2,3) and study how their growth rates change in response to this shift in habitat use (4,5). We can also study how turtle diets vary regionally or with age and how this variability influences their growth rates, the subject of my PhD research.

Box 1: Elemental Chemistry 101

As you might (or might not) recall from high school chemistry class, elements are alternate forms of atoms that differ in the number of protons in their nucleus—e.g., carbon (6 protons), nitrogen (7 protons), oxygen (8 protons). Stable isotopes, in contrast, are elements that differ in the number of neutrons in their nucleus, and thus are different forms of the same element—e.g., naturally occurring carbon isotopes can have 6–8 neutrons. Trace elements are simply those elements present in a sample or the environment in very small amounts (e.g., <0.1% of a tissues’ composition).

Using the Past to Inform the Future

Studying sea turtle growth and the factors that contribute to growth variability is important to understanding sea turtle population dynamics and developing effective conservation strategies. Anything that causes turtles to grow slower can increase the time it takes them to mature and reduce their survival, both of which can ultimately delay the recovery of these threatened and endangered species. And, if our collection of bone samples is robust and consistent, we can examine how growth rates change in response to catastrophic events, such as oil spills.

Understanding when and where sea turtles occur in the ocean is similarly important. Sea turtles are global travelers, regularly crossing ocean regions and the jurisdictions of many countries. As such, they are exposed to a wide suite of threats and variable levels of protection. The information gleaned from turtle bones thus provides critical new information about their ecology pertinent to their conservation and management.

Matt participated in Oregon State's GRADx talks in February where he went into detail on his research and discussed his journey and the decisions he's made through his graduate student experience. From his personal opinions on the ocean, to his passion for sea turtle conservation, it's an informative and engaging talk. 

Here is a link to the recording.

You can also read more about Matt's journey on his website.

References

(1) Ramirez MD, Avens L, Seminoff JA, Goshe LR, Heppell SS (2015) Patterns of loggerhead turtle ontogenetic shifts revealed through isotopic analysis of annual skeletal growth increments. Ecosphere 6:1–17. doi:10.1890/ES15-00255.1

(2) Turner Tomaszewicz C, Seminoff JA, Peckham SH, Avens L, Kurle CM (2017) Intrapopulation variability in the timing of ontogenetic habitat shifts in sea turtles revealed using δ15N values from bone growth rings. Journal of Animal Ecology 86:694–704. doi: 10.1111/1365-2656.12618

(3) Ramirez MD, Miller JA, Parks E, Avens L, Goshe LR, Seminoff JA, Snover ML, Heppell SS (2019) Reconstructing sea turtle ontogenetic habitat shifts through trace element analysis of bone. Marine Ecology Progress Series 608:247-262. doi: 10.3354/meps12796

(4) Snover ML, Hohn AA, Crowder LB, Macko SA (2010). Combining stable isotopes and skeletal growth marks to detect habitat shifts in juvenile loggerhead sea turtles Caretta caretta. Endangered Species Research 13:25-31. doi: 10.3354/esr00311

(5) Ramirez MD, Avens L, Seminoff JA, Goshe LR, Heppell SS (2017) Growth dynamics of juvenile loggerhead sea turtles undergoing an ontogenetic habitat shift. Oecologia 183:1087–1099. doi: 10.1007/s00442-017-3832-5