After hatching, young sea turtles embark on a harrowing dash to the sea across a predator-filled beach. But new research suggests that baby leatherback sea turtles have a difficult time determining the direction of the sea, leading them to stall more often than other turtle species and crawl in circles on the sand.
The circling was already reported in leatherbacks (Dermochelys coriacea), but it wasn’t clear why the turtles have this problem.
Since sea-finding largely relies on night-time visual cues, and — both at Florida Atlantic University — investigated how the soft-shelled leatherbacks differ in their vision and beach behaviours from hard-shelled sea turtles like loggerheads (Caretta caretta).
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The researchers brought leatherback hatchlings into the lab and put them in a Y-shaped maze, with one arm of the maze illuminated with a specific wavelength and intensity of light and the other dark. Based on how frequently the turtles crawled into the dark or illuminated arms, the team determined the leatherbacks’ sensitivity to a broad spectrum of light, finding that their vision is 10 to 100 times less sensitive than that of loggerhead or green sea turtles (Chelonia mydas), according to results of previous studies on these comparator species.
When the team then compared how loggerheads and leatherbacks judged light intensity, they found that leatherbacks sometimes couldn’t discriminate between light levels in dim conditions.
The leatherbacks’ difficulty carried over into field experiments on the beach, where the sea tends to be brighter than the land at night as it is more reflective. With the horizon and habitat illuminated by a full moon, both loggerheads and leatherbacks went straight to the waves. But under darker new moon conditions, leatherbacks wheeled on the sand far more than loggerheads.
“In trying to distinguish between a dim direction and a slightly less dim direction, [leatherbacks] seem to have trouble,” says Trail.
Circling may be about gathering more information to verify the correct direction, says Trail. But delaying has drawbacks. “There are high predation rates on the beach, and the longer you stay [there], then the more likely you are to get eaten or disoriented,” she says.
Hatchlings reabsorb calorie-rich yolk into their bodies soon after hatching, says at Pendoley Environmental in Western Australia, but “if you’re crawling around in circles trying to find the ocean, you’re chewing up that energy reserve”.
Leatherbacks’ visual difficulties on land may provide benefits when they are hunting jellyfish — their favoured food — in deep, dark water. Their eyes probably provide a relatively bright but coarse image useful for spotting slow-moving prey, but at the expense of sensitivity.
Pendoley — who wasn’t involved with this research — notes that scientists have known about light cues in turtles since 1911.
“So to think 110 years later that we still haven’t really figured it out is interesting,” she says.
Trail thinks it is possible that leatherbacks may be particularly susceptible to artificial light pollution, which can disorient hatchlings by making it harder to distinguish the direction of the sea.
Animal Behaviour
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