Human light disorients and jeopardizes turtle hatchlings

Imagine baby turtles flapping their flippers on miniature treadmills before going for a swim in bright pink bathing suits.

That was the scene in a Florida lab as biologists tried to mimic the experience of sea turtles hatching into a disorienting world of urban lights. The researchers found that lights cast on the beach can disorient the hatchlings and expose them to environmental hazards as the animals rest between racing to the water from their nests in the sand.

The International Union for the Conservation of Nature classifies green sea turtles (Chelonia mydas) as endangered and loggerheads (Caretta caretta) as threatened. Both species are rebounding in Florida, but residential light pollution on beaches disorients hatchlings as they come out from the sand and dart toward the nearby waves in a frenzy to swim straight to the Gulf Stream. In Boca Raton, disorientation can affect up to half of all hatchlings, so thousands of newborn turtles may wander more than 1.5 kilometers before winding up stuck in vegetation or a pool or on a road.

“When they’re crawling for these extended periods, they stop often and rest, so it makes them more vulnerable to predators or being overheated,” said Sarah Milton, lead author on the paper published in the Journal of Experimental Biology.

Tipped off by a local nature center about nest sites and hatching periods in the summer of 2013, Milton — an associate professor of biological sciences at Florida Atlantic University — and her team monitored miles of Boca Raton’s beach. They captured 150 freshly-hatched loggerhead and green sea turtles emerging from their nests.

In the lab, the Milton and her colleagues placed the hatchlings one by one before a disorienting light on improvised tiny turtle treadmills — belt sanders wrapped with hairbands for traction and hooked to power sources to adjust the contraptions’ speed. As the hatchlings walked 500 meters on the treadmill, the researchers noted the time the animals spent moving and pausing and switched the belt on and off to match their condition throughout the night. Before dive boats released the turtles into the Gulf Stream, the biologists tethered each for two hours to the roof of a small water tank using pink bathing suits. They wanted to see whether such a long period of energy-intensive disoriented dashing could exhaust hatchlings’ swimming abilities.

The following summer, the team took to the beach again to track disoriented hatchlings and collect corroborating field data on the distances they traveled and the frequency and length of their breaks.

Contrary to expectations, the results showed that disorientation didn’t significantly weaken the hatchlings’ capacity to swim, Milton said.

“They did not build up high levels of lactate that would interfere with the muscle’s ability to swim — like you get after you’ve sprinted for a long time — so they could swim when they got into the water,” she said. “When the lactate built up to a point where they were tired, they stopped for a little bit, and that let it go down.”

But, Milton said, “since they were resting so much, it was adding a lot of time to the time they were on the beach.”

These extra minutes or hours on the beach increase disoriented hatchlings’ risk of being eaten by ghost crabs (Ocypode ceratophthalmus), raccoons (Procyon lotor), foxes and other predators and of overheating after sunrise, Milton said.

The study’s findings are consistent with previous ones indicating that disoriented hatchlings “stop and crawl more slowly when they’re on the beach for a long time,” she said.

Although regulations require beach-facing residents to control their light usage or use lamps that don’t attract hatchlings during turtle nesting season, not everyone follows the rules.

“This report adds more scientific weight to the argument for local lighting ordinances,” Milton said. “Where somebody had a light on, the turtles would head right for it, completely the wrong way. It is critical people keep their lights out.”

Header Image: A newly-hatched sea turtle proves its swimming is unaffected by disorienting light in the lab. Jay Paredes