Article: Thomson, J.A., A. Gulick, and M. R. Heithaus. 2015. Intraspecific behavioral dynamics in a green turtle Chelonia mydas foraging aggregation. Marine Ecology Progress Series 532: 243-256. doi: 10.3354/meps11346.
Just imagine if a private investigator managed to slip a video camera onto you, waited for it to fall off, and then recovered it to analyze your every move and social encounter. A little creepy, sure, but chock full of information! Some researchers did just that to some unsuspecting sea turtles to study the mysteries of sea turtles’ interactions with each other.
Sea Turtle Interactions: A Scientific Knowledge Gap
Have you ever seen a sea turtle in the wild? If so, did you see one nesting on the beach? On the ocean’s surface breathing? While diving? Any of those experiences are amazing and you should count yourself lucky! Unfortunately, scientists tend to encounter sea turtles in the same sorts of ways. Now, why is that unfortunate? Because it means we don’t know what do they do in their natural offshore habitats when humans aren’t around. How much time do they spend alone versus with other turtles? When with other turtles, are they aggressive or do they sleep next to each other?
It turns out that behavioral interaction is a large gap in ecological knowledge for many large marine vertebrates, including sea turtles. Animal distribution is primarily driven by resource availability, but is also influenced by interactions such as competition and predation avoidance. Green turtles (Chelonia mydas) (Fig. 2) live in multiple habitat types: seagrass meadows for feeding grounds and deeper structurally complex areas—think reefs or rock ledges—for rest areas. Little is known about how turtles utilize these habitat types and if they must interact to do so.
A group of researchers from Florida International University set out to answer these questions in Shark Bay, Western Australia, where turtles aggregate throughout the year to forage on the seagrass (Fig. 2). They sought to detail the interactions of green turtles, determine what factors influence how often they encounter other turtles, categorize encounters by type and habitat, and test whether limited resting habitat leads to competitive interactions.
But sea turtles tend to be disturbed by human presence, preventing in-water observation. So how did they answer these questions?
Advances in animal-borne video have allowed researchers to observe behaviors of animals that previously eluded in-depth (or at-depth!) study (ex. diving seabirds). The researchers attached these cameras on adult green turtles using temporary glue. After 24-72 hours, the adhesive dissolved in the seawater, allowing the camera to float to the surface so that Thomson and colleagues could collect it. They collected 301 hours of footage from 93 turtles. That’s a lot of video to analyze! The group marked an interaction as an encounter anytime another turtle was present with the tagged turtle. Each encounter was analyzed for how long it lasted, how many turtles were involved, and what type of behaviors occurred (Table 1).
What Do Turtles Do?
Here are some stats (Fig. 3, Table 2):
- 52% of videos included an encounter with another individual.
- 0 to 29 turtles were seen per hour, with an average of 1/hour
- Tagged individuals encountered 1-7 turtles per encounter, with an average of 1.5.
- Encounters lasted 64.84 s (or just over 1 minute!) on average, 18 s median, or 20 minutes max.
- 55% of encounters occurred in deep structured habitat, which is noteworthy given that turtles only spend about 5% of their time in those areas.
- 26% of encounters occurred in shallow vegetated habitat.
- Most encounters in unstructured areas were with solitary individuals while those in structured ones were with 2 or more.
- Encounters lasted longer in deeper habitats.
Of all the specific behaviors examined, only a few show any patterns: following, biting, chasing, and group resting/cleaning. Following occurred more frequently in deep unstructured habitat. Biting and chasing were more likely to occur in structured habitat as was group resting/cleaning.
Based on results of statistical modeling, the authors also determined that encounters can best be classified into 3 types based on behaviors observed (Table 3). Regardless of class, these encounters occurred most in deep structured habitat. Strongly interactive/social encounters only occurred in that habitat.
- 37% were “passing encounters” with single individuals, of short duration (<18 s), and involving an approach but unlikely including any other behavior.
- 50% were “brief interactive encounters” likely including 1, or possibly 2, other turtles, which last longer than ~18 s, including approach and inspection behaviors along with a low to moderate chance of following, head bumping, body contact, mouth gaping, biting, chasing, and displacement but no group resting/cleaning.
- 13% were “strongly interactive/social encounters” involving 2-7 turtles, lasting 33s to 20 min, and including approach, inspection, and group resting/cleaning behavior and likely body contact, mouth gaping, biting, chasing, and displacement. Though infrequent, they accounted for 68% of total encounter time.
Forty one-on-one competitions occurred that included mouth gaping, biting, or chasing. 88% of those interactions happened in structured habitat.
Summary and Importance
It seems that green sea turtles do interact especially in deep structured habitat. Their more passive interactions in shallow areas makes sense in light of the great abundance of that habitat. But the deep and structured habitat in this area is more rare but immensely valuable as refuge (for example, from tiger sharks!) and resting. The hard surfaces allow for self- and symbiotic cleaning (Fig. 4) to remove the epibionts that collect on their carapaces (the scientific term for “shell”). Removal makes the turtles more streamlined for migration. Given the small amount and great importance of structurally complex habitat, it makes sense that more competitive and aggressive displays occurred here!
The research never ends, because there are always more questions!
Now that we know more about how and where sea turtles interact, what else is there? Well, does proximity of seagrass habitat to deep structured habitat affect the value of a given area? Could we create maps to predict where turtles are most likely to occur in order to better conserve them and protect resources? Could they overgraze areas close to shelter? What is the impact of competition on the population? Are poor competitors less likely to survive and reproduce? And in these competitions, who is the better competitor? These are just some of the many things we still don’t know! Isn’t that exciting?
Interested in seeing video footage of these behaviors? Check them out here!
Seriously, I think sea turtles “inspect” each other like dogs do.