Great Nicobar’s leatherbacks carry a unique genetic record that a megaport could bury

Last winter, female leatherbacks heaved themselves onto a beach bordering Galathea Bay on Great Nicobar Island to dig and lay eggs in 869 nests, according to data from the Andaman and Nicobar Forest Department obtained through the Right to Information Act. That is the highest number of leatherback nests recorded on any beach in the Indian Ocean in this year’s nesting season, ending in March.

In further good news, the numbers of leatherback nests not only on Galathea but also on the South Bay and West Bay beaches of Little Andaman Island have been going up in recent years, the data shows. Andaman and Nicobar is “the stronghold” of the Northeastern Indian Ocean leatherback population, said a researcher who did not wish to be named. “The population, being left alone, is thriving and likely overflowing” onto other beaches in the neighbourhood.

“It’s common sense that Galathea is the epicentre of that nesting population,” says Hiltrud Cordes of the Turtle Foundation, which works with communities in Indonesia to protect marine turtles. That is, although leatherbacks in the Northeastern Indian Ocean also nest on other beaches of Andaman and Nicobar and on Sumatra, Galathea Bay in Great Nicobar Island shelters the most nests of all sites that have been consistently monitored. The new nesting data indicate that Galathea alone hosts about 175 or more leatherback mothers per year, more than estimated for all of Sumatra or the Andamans.

Underscoring the significance of Galathea, the leatherbacks nesting there also have comparatively high genetic diversity, according to a study conducted in the winter of 2001-2002. Marine biologists Kartik Shanker and B.C. Choudhury, then at the Wildlife Institute of India, Dehradun, took tiny amounts of blood and tissue from more than 100 leatherback turtles nesting at Galathea. They then worked with the Centre for Cellular and Molecular Biology, Hyderabad, to examine the genetic material of the mitochondria, called mtDNA, in these samples.

Mitochondria are passed from mother to offspring; so capturing mtDNA enables scientists to trace the turtles’ maternal line of descent. “We wanted to create a baseline,” says Choudhury. They detected four distinct haplotypes — labelled A, D, E, and I — which are segments of genes that have passed intact through generations. The number of haplotypes provide a rough measure of genetic diversity, which tends to be higher for populations that are older in evolutionary terms.

Their study established that the genetic diversity of Galathea’s leatherbacks was among the highest measured anywhere in the world at that time. Back then, only the leatherbacks nesting on Terrengganu beach in Malaysia had been found to also have four haplotypes: A, D, E and H.

In 2015, Shanker, along with Peter H. Dutton, a marine biologist who has studied the genetics of leatherbacks worldwide, pulled together the available data to postulate that the world’s leatherbacks descended from a few that had survived the ice ages by nesting on beaches of the Sundaland biodiversity hotspot, which includes Great Nicobar Island. Modern-day leatherbacks, Dermochelys coriacea, have low genetic diversity overall, perhaps reflecting the population “bottleneck” about a million years ago, the authors speculated.

That limited diversity makes some conservationists worry about the capacity of these marine animals to adapt to intensifying challenges such as climate change. “Diversity is the raw material upon which adaptation works,” says Shanker, who is now at the Indian Institute of Science and a founder of Dakshin Foundation, which works on ecosystem health and human wellbeing in coastal and marine ecosystems.

“If you have two populations to conserve, and you could only pick one, you want to pick the one with greater diversity because that greater diversity has the raw material for the adaptation or the evolution of that lineage.”

However, the stability of Great Nicobar’s leatherback population is threatened by a massive development project along the Island’s southern and eastern coasts, including the construction of an international container transshipment terminal port on Galathea Bay.

Mariners from deep time

In 2026, Wesley D. Colombo of the Universidade Federal de Minas Gerais in Brazil and his colleagues integrated the world’s data on leatherback mtDNA to construct a family tree locating the primordial mother of all modern leatherbacks as having lived almost five million years ago in the Pacific Ocean. Her descendants spread to the Indian Ocean roughly three million years ago and to the Atlantic twice — about 1.7 million years ago and again, about a million years ago or less.

Leatherback turtles are “the closest thing we have to a living dinosaur,” writes marine biologist Carl Safina in his book Voyage of the Turtle. Their family, Dermochelydae, has been cruising the planet’s oceans for more than 100 million years — since when our own ancestors resembled shrews as puny as mice, and the Indian subcontinent was a triangular island that had just broken off the Gondwana supercontinent.

With the ability to traverse entire ocean basins, dive to depths that would crush humans in fractions of a second, and tolerate close-to-freezing temperatures, leatherbacks survived the great extinction event 66 million years ago that killed off other dinosaurs.

But only one species of leatherback, Dermochelys coriacea, made it through the ice ages, between 2.8 million and 700,000 years ago. By then, the Indian continental plate had hit the Eurasian, the Indian Ocean had expanded, and the cluster of small islands that had emerged between the Indian and the Pacific oceans, where several tectonic plates collided and folded, provided nesting beaches. Among these havens were the Andaman and Nicobar Islands.

New studies from other islands of the Indo-Pacific region are finding even higher diversity than was detected on Great Nicobar, however, thanks to advances in research and sustained interest in leatherback genetics. Researchers have found 11 haplotypes on Jeen Yessa (formerly Jamursba Medi) beach in West Papua, for instance. And on Sumatra, biologists have discovered eight haplotypes: D, E, I, and several variants of A, as well as an entirely new haplotype not seen anywhere else in the world.

Maslim As-singkily, a conservation scientist at the Wageningen University and Research in the Netherlands, says the discovery of higher genetic diversity on Sumatra isn’t surprising. “There aren’t a lot of studies for the Indian Ocean population. So, there will be new information when you study it.”

As-singkily and his co-authors argue that the leatherbacks nesting on Sumatra are genetically distinct enough to be managed separately from the Northeast Indian Ocean population. “High haplotype diversity is often associated with regions that have experienced historical connectivity and may act as critical genetic reservoirs for migratory species, making Sumatra a key site for maintaining the species’ genetic health and long-term resilience,” they write.

According to Shanker, the Sumatran population only looks more genetically diverse than the Andaman and Nicobar ones because As-singkily and others sequenced longer segments of mtDNA. “It’s the same one as in Great Nicobar,” he emphasises. The 2001 Galathea study had examined snippets of mitochondrial DNA of less than 400 base pairs (molecular units that are strung together to make up DNA), whereas recent studies use longer lengths of mitochondrial DNA, of close to 800 base pairs.

There are other indications that leatherbacks from Sumatra and Great Nicobar are related. In January 2023, a leatherback tagged on Galathea also nested in Along beach in Sumatra 10 days later, according to Cordes. And several of the leatherbacks tagged with satellite transmitters on Little Andaman followed the Sumatran coastline on their way toward Australian waters, notes Shanker.

As-singkily, on the other hand, believes that the Andaman and Nicobar leatherbacks are more closely related to those that had once nested on Terrenganu, Malaysia, based on discussions with fisherfolk who suggest that the turtles from the Indian Ocean also use Malacca Strait to travel eastward to the South China Sea. But this matter cannot be resolved by further genetic studies of Terengganu’s leatherbacks since that population is now all but extinct.

A single female delighted conservationists by returning to nest three times in Malaysia this past September, after a complete absence of turtle nesting for nine years. According to marine biologist Nicolas Pilcher of Save Our Seas Foundation, a combination of excessive egg collection, deaths by drowning in offshore fishing nets, and unscientific hatchery management virtually extirpated Terengganu’s population that had once nested in the thousands — a reminder of how vulnerable leatherbacks are to human activity on and near a key nesting beach.

Information black hole

No new genetic data from the Andaman and Nicobar Islands has emerged since that pioneering work, done a quarter century ago. The tsunami of 2004 destroyed the beach, and although the beaches of Great Nicobar have since reformed and the leatherbacks have returned, the research focus has shifted to Little Andaman for logistical and safety reasons, says Shanker.

Also deterring genetics research on leatherbacks is the arduous process of getting permission from the Indian Government to take tissue samples, says Chowdhury. Leatherback turtles are a Schedule I species with the highest level of protection under the Indian Wild Life (Protection) Act, 1972. “Most often, by the time your grant period is over, all this bureaucratic procedure is still not over, and your time is gone,” he says. Even if permission is granted, “they will add about 20 different clauses. You cannot collect blood samples from more than five individuals and so on. Sometimes the sample size would be so low that it’s not even worth trying to do any work,” he adds.

A team led by Shanker at Dakshin Foundation does have tissue samples from leatherbacks nesting on Little Andaman, collected over the last 15 years, however. Thus far, Dakshin researchers have sequenced shorter lengths of mtDNA from nine leatherbacks on West Bay beach, finding three haplotypes — A, E and I. Given the limited number of leatherbacks studied, that represents relatively high diversity, but the sample size is too small to be statistically meaningful. The team is currently studying longer sequences of base pairs, which are required to compare the genetic diversity of Little Andaman leatherbacks with those nesting on Sumatra and elsewhere in the world.

The genetic profile of Great Nicobar’s leatherbacks is likely to be very similar to that of Little Andaman leatherbacks, given that a leatherback tagged on Little Andaman in 2014 reportedly turned up on Galathea in 2021. But the mtDNA haplotypes on these two islands may not be identical. As-singkily reports that different beaches on Sumatra turned up different haplotypes, with the brand-new haplotype being found only in a single leatherback.

A state-of-the-art genetic study at Galathea, examining longer haplotype sequences over several nesting seasons, could well yield far greater diversity than measured in the 2001 study. Such a study being undertaken at this time appears unlikely, however, given that the container terminal planned at Galathea Bay could render the nesting beach inaccessible to marine turtles.

All research on Great Nicobar’s multitude of species is currently being conducted by government-affiliated departments or agencies. That recent nesting data for Great Nicobar had to be obtained through a Right to Information request, whereas earlier surveys were published in the scientific literature, offers a glimpse of the secrecy shrouding information about wildlife on the island.

As survivors from deep time, leatherbacks are often described as resilient. But unlike whales and dolphins, which also descend from an ancestor that originally lived on land, marine turtles never evolved a way to have their offspring in the water. “Their last trace of land-oriented existence — the females’ need to return to shore to lay eggs — is their Achilles’ heel,” writes Safina in Voyage.

Leatherbacks may have survived threats that killed off other dinosaurs, but their need to return, not only to land but to the beach of their birth to perpetuate their life cycle “is a hallmark of vulnerability, not resilience,” Safina added in an interview.

That vulnerability is about to be tested in an experiment that experts say has never been tried before — the loss of a major nesting beach, alongside massive ship traffic, and likely increased fishing as a projected 325,000 newcomers to Great Nicobar seek sustenance. “What happens when that stronghold breaks?” asked the researcher who wished to remain anonymous. “That’s the million-dollar question that all of us would not like to test.”

Banner image: Leatherback hatchlings at Galathea Bay on Great Nicobar Island. Image by Dhritiman Mukherjee.