The birth process of sea turtles provides some of the most endearing images in nature. Mother turtles trudge up on to the beach at night, dig a nest and lay their eggs, then haul themselves back into the ocean. Weeks later, dozens of adorable little hatchlings, tiny flippers churning through sand, scamper into the sea for the first time.
But we rarely see what happens immediately after the mother lays her eggs and before she slides back into the sea.
“People find that stage boring, and they just don’t understand it,” said Malcolm Kennedy, a zoologist and professor of natural history at the University of Glasgow. “The turtles flap around, and that’s it. Everyone goes home.”
But that flapping and scattering of sand fascinated Dr. Kennedy, who released a study last week in Royal Society Open Science with Tom Burns, also a zoologist at the University of Glasgow. They found that the turtles are actually creating decoy nests designed to fool predators like mongooses, dogs and wild pigs, and prevent them from sniffing out the real nests and devouring their eggs. Their findings challenge earlier theories that the turtles were disguising their nests and, in some cases, easing hatchlings’ trips to the water.
Dr. Kennedy and Dr. Burns observed two species of sea turtles — the massive leatherbacks and the smaller hawksbill — in Trinidad and Tobago from 2013 through 2019. They found that the turtles did not linger near the nests they had just dug up. They moved as far as several feet, in multiple random directions, stopping to scatter sand at various “stations” along the way, ranging from two to more than a dozen.
The turtles also spent a good deal of energy and time — roughly 30 minutes — on this endeavor, exposing themselves to dangers from predators and the hot, rising sun.
“It just emphasizes how important this activity is for them,” Dr. Kennedy said. “Why would they spend that time disguising an area that is not near their nests?”
They think that a predator might dig into one or two enticing spots left by a sea turtle, and give up.
“If a species of turtle always did the same thing, then a predator could learn it, and track back to find where the nest is,” Dr. Kennedy said. “But if they move randomly, then the predator can’t learn any patterns.”
Dr. Kennedy said the leatherbacks and hawksbills are related by a common ancestor over 100 million years ago, and added that it is possible they learned the decoy behavior in order to thwart small predators from digging after their eggs during the age of the dinosaurs.
There are more than a half dozen species of sea turtles, and others may not engage in such behaviors. Roldán A. Valverde, a biologist at Southeastern Louisiana University and the scientific director of the Sea Turtle Conservancy, said that the olive ridley turtle tends to lay her eggs and head straight back to the ocean. Dr. Valverde said he is “skeptical” of the study and noted that the leatherback’s enormous size — they can grow to over 1,000 pounds — and exaggerated movements could account for the seemingly random sand scattering.
Dr. Kennedy and Dr. Burns presented a similar theory of decoy nests in a paper to the Canadian Journal of Zoology in 2016, but it did not pass peer review.
Alexander Gaos, a research ecologist for the federal Marine Turtle Biology and Assessment Program, praised the new study’s methodology, but wondered if its finding was mostly “a matter of semantics.”
“Whether you call it decoy or a disguise, it’s still the same activity,” he said.
Dr. Kennedy said he welcomes these challenges, but believes most turtle scientists need to study this behavior more rigorously.
“That’s the fun thing about science,” he said. “Who wants to do just what everyone else believes.”