Leatherback sea turtle

• 1. Introduction

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  The leatherback sea turtle, Dermochelys coriacea, holds a record for endurance that few animals can match. Scientists once tracked one individual from Jen Womom beach in Tambrauw Regency, in West Papua, Indonesia, all the way to the United States. The foraging journey covered 20000 kilometres and lasted 647 days.

  That traveller belongs to a very odd kind of reptile. It has no hard shell. It hunts in water close to freezing, though it breathes air and is cold-blooded by ancestry. It is also the largest turtle alive today.

  So how is such an animal built, and how does it stay warm in the deep ocean? Where does it go, and why does a species with the widest range of any sea turtle now count its nesting females in the tens of thousands? Some of its beaches have gone almost silent.

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• 2. Skin Instead of Shell

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  In 1761, Domenico Agostino Vandelli described an animal captured at Ostia and donated to the University of Padua by Pope Clement XIII. He called it Testudo coriacea. That was only the first of many names. Henri Blainville coined the genus Dermochelys in 1816, and Leopold Fitzinger placed it in a family of its own in 1843. In 1884 the American naturalist Samuel Garman described the species yet again, as Sphargis coriacea schlegelii. The two subspecies that resulted were later judged invalid.

  The scientific name translates as Leathery Skin-turtle, and it fits. Other sea turtles wear a bony shell covered in plates called scutes. The leatherback has thick, oily skin with tiny bone fragments, known as osteoderms, embedded inside it. Seven ridges run along its back from head to tail. Those ridges inspired the names lute and luth, a comparison to the seven strings of the instrument. The source suggests the more likely origin is the lute's ribbed, shell-shaped back.

  This flexible covering is an engineered marvel. The osteoderms are made of a bone-like mix of mineral and collagen, and their jagged edges interlock through zig-zag joints called sutures. The lungs and the carapace expand with a breath of air and shrink during deep dives, so flexibility matters. The zig-zag pattern leaves no easy path for a tooth to puncture the armour, and the joints stop cracks from spreading. Researchers also suspect the skin lets the turtle grow or shrink its fat deposits as food supplies change.

  The colouring is dark grey to black, with white-pink speckles. The underside is pale, a pattern called countershading. There are no true teeth. Instead the upper lip carries sharp points, and backward-pointing spines line the throat to keep slippery prey from escaping.

  The scale is enormous. Adults average between 250 and 700 kg, and the front flippers of large animals can span 2.7 metres from tip to tip. They carry no claws. The largest verified specimen washed up at Sandspit, a beach in Pakistan, measuring 213 cm along the curve of its shell and weighing 650 kg. An older contender, the Harlech turtle, was said to reach 256.5 cm and 916 kg. Its remains at the National Museum Cardiff suggest a length closer to 1.5 metres, which casts doubt on the weight as well.

  Fossil-calibrated genetic studies suggest the leatherback family split from the one holding the other six sea turtles roughly 49 to 70 million years ago. Its sister group on the family tree is the extinct Protostegidae, which also included species without a hard carapace.

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• 3. Cold and Deep

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  Imagine a hunter chasing prey in water at just 0.4 degrees Celsius. One leatherback was seen doing exactly that. After each foraging dive it returned to surface water at 17.5 degrees to recover its body heat, then plunged back into the near-freezing depths.

  The reason for such dives is jellyfish. Adults live almost entirely on them, and they follow the jellyfish through the day. The turtles prefer deeper water in daylight and shallower water at night, when the jellyfish rise. Soft-bodied relatives such as siphonophores, salps and pyrosomas also make the menu, along with squid. Some reports add small crustaceans, fish, sea urchins, snails, seagrasses and algae. An adult eats the equivalent of 73 percent of its own body weight each day.

  Leatherbacks have been recorded diving as deep as 1280 metres. Most dives last between 3 and 8 minutes, though a few stretch to 30 or even 70 minutes. Pursuit of prey below 1000 metres takes them beyond the limits of every other diving four-limbed animal except beaked whales and sperm whales.

  Staying warm in that cold is a puzzle with a twist. Early studies found resting metabolisms about three times higher than expected for a reptile this size. Later work that included turtles of every size class found the resting rate of a large adult is in line with predictions. The real secret is constant motion. Wild individuals may spend as little as 0.1 percent of the day resting, and their swimming muscles generate heat. Add a thick fat layer, brown adipose tissue, large body size and countercurrent heat exchangers in the flippers and around the windpipe, and the result is striking. Adults have been found with core temperatures 18 degrees above the surrounding sea. The swimming muscles even work independently of temperature.

  Speed comes with the package. The 1992 Guinness Book of World Records lists the leatherback at 35.28 km/h in water, making it the fastest-moving reptile. Typical cruising speeds run from 1.80 to 10.08 km/h.

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• 4. Ocean Wanderers

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  From Alaska and Norway in the north to Cape Agulhas in Africa and the southernmost tip of New Zealand, leatherbacks turn up across a huge stretch of sea. No other sea turtle spreads so widely. They live in all tropical and subtropical oceans, and their range reaches well inside the Arctic Circle.

  Genetically, three major populations exist, in the Atlantic, the eastern Pacific and the western Pacific. Recent estimates put annual nesting at 26,000 to 43,000 females. In 1980 the figure was 115,000.

  The Atlantic group roams from the North Sea to the Cape of Good Hope. Off Canada, leatherbacks feed in the Gulf of Saint Lawrence and as far north as Newfoundland and Labrador. Nesting beaches are few. The key ones lie in Suriname, Guyana and French Guiana, in Antigua and Barbuda, in Trinidad and Tobago, and in Gabon. The beaches of Mayumba National Park in Gabon host the largest nesting population in Africa, and possibly in the world. Nearly 30,000 turtles visit each year between October and April. A few hundred nest annually on Florida's eastern coast, and Costa Rica's Gandoca and Parismina beaches also serve as nesting grounds. Females tagged in French Guiana have turned up on the far side of the ocean, in Morocco and Spain.

  The Pacific splits in two. One group nests in Papua, Indonesia, and the Solomon Islands, then forages in the Northern Hemisphere along the coasts of California, Oregon and Washington. The eastern Pacific group feeds along South America's west coast and nests in Mexico, Panama, El Salvador, Nicaragua and Costa Rica, as well as eastern Australia. One well-studied feeding area lies near the mouth of the Columbia River. The WWF estimates only 2,300 adult females remain in the Pacific, making it the most endangered marine turtle subpopulation. In the Indian Ocean, little research exists, but nesting is known from Sri Lanka and the Nicobar Islands.

  The Pacific crossing from Indonesia to California jellyfish grounds covers about 6000 miles. Along the way, common remoras latch onto the turtles and ride with them. The arrangement helps the remoras and neither helps nor harms the turtle.

  Adults have few serious enemies at sea. Killer whales, great white sharks and tiger sharks can overwhelm one occasionally. Yet one medium-sized adult chased off a shark that had tried to bite it, then turned on the boat carrying the people watching.

  By eating so many jellyfish, leatherbacks keep those populations in check. That matters to people, because jellyfish feed heavily on larval fish, and the adults of those fish are commercially caught.

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• 5. Nesting in the Dark

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  Courtship happens at sea, and males never come ashore once they enter the water. A male approaches a female, which may signal her readiness with a pheromone, and tests her willingness with head movements, nuzzling, biting or flipper strokes. Males can mate every year, while females mate every two to three years. Several males usually mate with one female, though this brings her offspring no particular advantage.

  Each female comes ashore at night, when predators and heat are least of a threat. Her eyes are poorly suited to seeing in the dark on land. She uses the contrast between a dark forest behind the beach and the brighter, moonlit sea to find her way, heading toward the dark to nest and back toward the light to leave. Beaches with soft sand are preferred, because her softer shell and underside are easily bruised by hard rock. The typical nesting event takes 108.1 minutes from landing to departure.

  She digs the nest above the high-tide line with her flippers. A clutch averages around 110 eggs. Afterwards she fills the hole and scatters sand to hide it. A single female may nest up to ten times in a season, with about nine days between visits, the shortest gap of any sea turtle. Unlike other sea turtles, which nearly always go back to their own hatching beach, leatherbacks may pick another beach in the same region.

  The eggs meet danger immediately. Ghost crabs, monitor lizards, raccoons, coatis, dogs, coyotes, genets, mongooses and shorebirds all raid nests. Hatchlings face frigatebirds and raptors on the sand, then cephalopods, requiem sharks and large fish in the water. Nesting females have been attacked by jaguars in the American tropics and by saltwater crocodiles in Papua New Guinea.

  Inside the egg, cell division starts within hours of fertilisation, then development pauses while the eggs are laid. Embryos remain extremely vulnerable to movement until their membranes form fully, after 20 to 25 days of incubation. Hatching comes after roughly 60 to 70 days. The temperature of the nest decides the sex of the hatchlings, and it also shapes their bodies. Warmer nests produce lighter turtles with smaller flippers, while cooler nests give heavier ones with longer flippers.

  Nesting seasons differ by place. In Parismina, Costa Rica, they run from February to July, and in French Guiana from March to August. That temperature rule, unknown to scientists until the 1980s, would prove fatal to a famous beach in Malaysia.

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• 6. The Lost Beach

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  The beach of Rantau Abang in Terengganu, Malaysia, once held the largest nesting population in the world, with 10,000 nests a year. In the late 1960s, more than 3,000 females nested at Terengganu. The last official count, in 1993, found a mere two.

  The main cause was people eating the eggs. Conservation efforts began in the 1960s, but they backfired. Workers dug up the eggs and incubated them at artificial sites, which accidentally exposed them to high temperatures. Scientists only learned in the 1980s that sea turtles have temperature-dependent sex determination. It is suspected that nearly all the hatchlings raised this way were female.

  In 2008, two turtles nested at Rantau Abang, but their eggs were infertile. The proposed South China Sea subpopulation, those that nest in Malaysia, has effectively been wiped out. Small nesting sites survive in southern Thailand, where 18 turtles nested in 2021.

  In mid-2007, Malaysia's Fisheries Department revealed a plan to clone leatherbacks to rebuild the population. Some conservation biologists were sceptical. Cloning had only worked on mammals such as dogs, sheep, cats and cattle, and questions remained about the health and lifespan of cloned animals.

  The turtle still appears on Terengganu's tourism advertisements, as the state's main animal. In Malaysia, where it is practically extinct locally, the eggs are treated as a delicacy. Across the Caribbean, some cultures regard them as aphrodisiacs. Egg harvesting in Asia is cited as the most significant cause of the global decline.

  But eggs are only one of the dangers the species faces, and many of the others come from the sea itself.

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• 7. Plastic and Lights

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  Almost a quarter of leatherback strandings in Florida, 23.5 percent, are caused by vessel strikes. That is the leading cause among 603 strandings recorded between 1980 and 2014. Boats are one hazard among many.

  Bycatch is another. As a species of open water, the leatherback is sometimes caught accidentally. Ropes from lobster pots can entangle it. The turtle's great size makes excluder devices on nets less effective for mature adults. In the eastern Pacific alone, an average of 1,500 mature females were accidentally caught each year in the 1990s.

  Plastic may be the cruellest trap. Leatherbacks mistake balloons and floating bags for jellyfish. An estimated one-third of adult Pacific leatherbacks have swallowed plastic. In California, people used upward of 19 billion plastic bags a year, and the state banned them in 2016. Even small amounts of debris can block a turtle's gut. Plastic also displaces food, slowing growth and delaying maturity. When researchers autopsied leatherbacks from 1885 to 2007-34 percent of the cases showed plastic blockage. The highest risk of swallowing it lies off San Francisco Bay, the Columbia River mouth and Puget Sound.

  Chemical pollution matters too. High levels of phthalates have been measured in egg yolks. In western Atlantic turtles, tests found arsenic, cadmium, lead, mercury and selenium in the salt glands and red blood cells, with higher concentrations in the blood. Larger turtles carried more cadmium and mercury.

  On land, the problem is light. Hatchlings are drawn toward brightness because, on a natural beach, the sky over the sea is the lightest place and the dunes or forest the darkest. Streetlights and buildings send them crawling the wrong way. On Florida's Atlantic coast, some densely used beaches have lost thousands of hatchlings to artificial light.

  The turtle itself is not much hunted for food, since its flesh is too oily to be popular. Even so, a few are caught for meat by subsistence fisheries. Protection has therefore had to come from many directions at once.

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• 8. Guardians and Traditions

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  On the 2nd of June 1970, the United States listed the leatherback as endangered, and the Endangered Species Act of 1973 ratified that status. In 2012, the National Oceanic and Atmospheric Administration designated 41,914 square miles of Pacific Ocean off California, Oregon and Washington as critical habitat. Canada made exploiting the species in its waters illegal under the Species at Risk Act. Australia lists it as vulnerable under national law, and Queensland lists it as endangered.

  Internationally, the species is on CITES Appendix I, which makes trade in the animal or its parts illegal. The Zoological Society of London lists it as an EDGE species. The IUCN Red List classes it as Vulnerable overall. Four subpopulations are far worse off. The east Pacific, southwest Atlantic, southwest Indian Ocean and west Pacific groups are all Critically Endangered. The northwest Atlantic group is Endangered, and the northeast Indian Ocean and southeast Atlantic groups are Data Deficient. The Leatherback Trust was founded to protect the species and set up a sanctuary in Costa Rica, Parque Marino Las Baulas.

  Local efforts fill in the picture. At Matura Beach in Trinidad, more than 2,000 females come ashore each year, and Earthwatch volunteers work with scientists and a local group, Nature Seekers. In Costa Rica, the village of Parismina has run a hatchery since 1998, while the Parismina Social Club collects donations for beach patrols. Dominica has patrollers from DomSeTCo. Brazil's TAMAR project protects nests and works to prevent accidental kills by fishing boats. Only seven nesting females were counted there at the last official tally. In January 2010, a female at Pontal do Paraná laid hundreds of eggs in a state where no leatherback nesting had been reported before. Ireland and Wales run a joint research effort that uses tagging and satellite tracking.

  For some peoples, the turtle is far more than a conservation target. The Seri people of Sonora, Mexico, believe it is one of their five main creators. They hold ceremonies when one is caught and released. Alarmed by the decline, young people and elders formed Grupo Tortuguero Comaac, which blends traditional knowledge with Western technology. On New Zealand's Banks Peninsula, the Koukourārata hapū hold the turtle in spiritual regard. In 2021, the Department of Conservation laid a leatherback to rest in a hilltop cave on Horomaka Island, dug by the hapū.

  Research from 2020 gives one reason for hope. A method using genetic markers from five marine turtle species calculated the natural lifespan of the leatherback at 90.4 years. A turtle protected today could still be swimming decades from now.

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