Why Baby Leatherback Turtles Have Trouble Finding the Sea

When sea turtles hatch, they use the moon and natural light as their guide to the water. Even when they can’t see the sea, they can tell the difference between the dimmer shore and the brighter skies over the water.

Hatchlings usually emerge in one large group and crawl toward the water, which is the lowest and brightest horizon.

But not all sea turtles are able to easily make a beeline to the shore. Researchers recently studied baby leatherback sea turtles to find why they get disoriented when they are trying to find the sea.

“The motivation for our study was to better understand the visual capabilities of leatherback sea turtles in context (i.e. behaviorally) as they compare to the other species. Leatherbacks are the most distantly related of all extant sea turtle species, and so, it stands to reason that those capabilities could differ significantly from others,” lead author Samantha Trail, a Ph.D. student at Florida Atlantic University, tells Treehugger.

“Seafinding is a very important and conserved behavior with conservation implications, so it is important to understand this process and how it might be affected by different conditions or sensory thresholds.”

Straight or Circles

Most sea turtle hatchlings take a straight path from their nests right to the water’s edge. But leatherback (Dermochelys coriacea) hatchlings often wander around in circles.

“Historically, leatherbacks have been noted to exhibit orientation circles (tight crawls often in a complete circle) more often than other species although why was not clear,” Trail says. 

Circling wastes energy, takes longer for them to enter the ocean, and then puts them at an increased risk of danger from predators like birds and crabs.

To study why some hatchlings circle and particularly why this behavior is common in leatherbacks, researchers first had to learn how sensitive the leatherback babies were to light. They compared their reactions to loggerhead hatchlings and found that leatherbacks were 10 to 100 times less sensitive to light wavelengths.

They also found that there were no obvious structural differences—such as a larger cornea or lens to help gather light—that might offer improved vision in dim lighting conditions.

These discoveries prompted the hypothesis that circling behavior might be related to the amount of light on the beach when hatchlings made their trek from nest to sea. So then they compared how the two species behaved in the bright light of a full moon versus only the light from stars during a new moon.

“Our research revealed that increased circling correlates with new moon conditions in which light levels are low. These light levels are much closer to the visual perception thresholds of leatherbacks than other species, although still below,” Trail says.

“Discrimination thresholds on the other hand suggest that at least some wavelengths, leatherbacks would not be able to significantly discriminate the duneward from the landward direction.”

The results were published in the journal Animal Behaviour.

A Longer Trip

Although leatherbacks often have trouble determining the horizon over the sea, they eventually crawl there, even during the low light of a new moon.

“It just takes them longer because they stop occasionally to circle, which we think enables them to re-evaluate, and eventually confirm, the correct crawl direction,” Trail says.

Researchers were curious why leatherbacks would be less sensitive to light than loggerheads, particularly because that inability can be perilous. They hypothesize that other differences in their visual abilities helps them find prey, mates, or habitats in the open ocean where they live. That habitat is quite different from the shallow, coastal waters where most loggerheads live.

“These are important findings, because they have implications for conservation efforts. Leatherbacks might be more susceptible to disorientation due to artificial lighting (i.e. skyglow),” Trail says. “It is also interesting to note how different the visual capacities are of sea turtles that share the exact same nesting beaches.”