Tongan Ground Skink

From the South Pacific to the museum – The last Tongan ground skinks

The Tongan ground skink, a giant lizard, is known only from two specimens that are now housed in the Natural History Museum of Paris. The French doctors and naturalists Jean-René-Constant Quoy and Joseph Paul Gaimard collected the two animals between 1826 and 1829 in the South Pacific during their circumnavigation aboard the Astrolabe, an expedition ship of the French Navy.

The French explorer Jules Dumont d’Urville recorded anecdotes and reports about the journey with the Astrolabe in a series of volumes titled Voyage de la corvette l’Astrolabe exécuté par commande du roi, pendant les années 1826 – 1827 – 1828 – 1829 sous le commandement de M. J. Dumont d’Urville, Capitaine de vaisseau, which he published between 1830 and 1835.

Based on the two collected specimens, French herpetologists André Marie Constant Duméril and Gabriel Bibron published the scientific first description of the Tongan ground skink about twelve years later (1839) under the name Eumeces microlepis. They listed Tongatapu, the main island of the Kingdom of Tonga, as the type locality. The species epithet “microlepis” means “small body scales” and refers to the many small scales in the middle of the body.

Since the early 19th century, the lizard species has not been documented, and search expeditions have been unsuccessful. As a result, scientists generally believe that the Tongan ground skink is extinct. The animals Quoy and Gaimard discovered on Tongatapu between 1826 and 1829 were likely among the last survivors of the species. The International Union for Conservation of Nature (IUCN) also lists the species as “extinct.”

Tongan ground skink – Fact sheet

Alternative name	Tonga ground skink
Scientific name	Tachygyia microlepis, Tachygia microlepis, Eumeces microlepis, Otosaurus microlepis, Liosoma microlepis, Lygosoma microlepis, Riopa (Eugongylus) microlepis
Original Range	Tongatapu, Tonga (South Pacific)
Date of extinction	early 19th century
Causes of extinction	habitat loss, introduced mammals on the island
IUCN status	extinct

In search of the Grey Ghost

Although many believed that the Tongan ground skink had long been extinct, the British naturalist John R. H. Gibbons was the first to actively search for this large skink species. In January 1985, he visited the island of Tongatapu, but without concrete results. He returned in October of the same year, combing the island’s last remaining forest patch with students, again to no avail.

Gibbons’ third and final visit to Tongatapu took place in February 1986. This time, he took a different approach: he placed an advertisement in a local newspaper, offering a reward of 100 Tongan dollars for a photo or specimen of the Tongan ground skink. In the newspaper, he referred to the skink as the “Grey Ghost,” speculating that it lived underground and was only active at night or after heavy rains.

Following the publication, several individuals reported possible sightings to the Tonga Chronicle. However, herpetologists Ivan Ineich and George R. Zug, in their 1996 publication Tachygyia, the Giant Tongan Skink: Extinct or Extant?, concluded that these sightings were likely of other skinks of the genus Emoia. Only three of the seven Emoia species on Tonga reach a snout-to-vent length of over ten centimeters, making it easier to rule out smaller lizards. The two museum specimens of the Tongan ground skink have snout-to-vent lengths of 17.7 and 14.3 centimeters, respectively.

Before Gibbons could continue his search, he and his family tragically died in a boating accident in Fiji in November 1986. In October 1993, Ineich and Zug searched the last remaining forest patch on Tongatapu for the Tonga ground skink. Despite the area’s rich biodiversity, they could not locate the lizard. Since then, no further expeditions have been conducted to find the elusive Tongan ground skink.

Sightings of the Tongan ground skink?

Although Gibbons could not find a living Tongan ground skink, he collected reports of possible sightings of the large lizard. Before his death, he sent an unpublished manuscript with his notes to Ivan Ineich, who, along with George R. Zug, mentioned these in a 1996 article in the Journal Cryptozoology.

Gibbons recorded three eyewitness accounts that are likely the most detailed and credible. An American gardener at Tupou College on Tongatapu reported seeing a huge lizard with a snout-to-vent length of 25 to 28 centimeters on his plantation. The animal was dull green with black spots on its back and had a tail about one-third the length of its body. It did not have a crest on its back.

A second eyewitness reported seeing a large lizard with similar coloring while mowing and burning high grass and brush near a small isolated forest. The body diameter was said to be 3.5 centimeters. The lizard ran from the fire toward the forest, a place avoided and feared by locals because it contains a large ancient burial mound of 300 Tongan nobles. The third eyewitness, a Tongan student, claimed to have occasionally seen a very large lizard on Tongatapu, which he called moko, not fokai (iguana).

These reports suggest the presence of a large, ground-dwelling lizard on the island of Tongatapu. It is clear that the lizard is very rare and only occurs in certain places, so many locals have never seen it. While Gibbons interpreted the eyewitness reports as evidence of the survival of the Tongan ground skink, Ineich and Zug believe it was a different skink: Emoia trossula. Although this species rarely grows longer than 16 centimeters, the described coloration matches. The Tongan ground skink was likely dark brown and did not have black markings on its back. Additionally, the description of the lizards running away fits Emoia trossula better, as the Tongan ground skink, due to its body structure, was likely more of a crawler.

Does the Tongan ground skink still exist?

A study published in the journal Salamandra in 2024 offers hope: Herpetologists Ivan Ineich and Wolfgang Böhme believe it may have been premature to declare the Tongan ground skink extinct. They consider that this ground-dwelling lizard might have survived in remote areas in the southern part of the island nation of Tonga. Satellite islands or other peripheral islands were less exposed to human interference than Tongatapu, where the only known specimens of the skink were likely collected. The main island of Tonga is ecologically heavily impacted: since human settlement, it has been greatly influenced by agriculture. Only three percent of the island’s area still has forest fragments.

Due to a strong morphological similarity – possibly through convergent evolution – between the Tongan ground skink and the Caribbean endemic galliwasps (Diploglossidae), and given that the Caribbean lizards share a similar habitus, it is assumed that they share similar ecological parameters. Ineich and Böhme propose using knowledge about the galliwasps to define biotopes where the Tongan ground skink could survive. Galliwasps are mostly found in forests, live on the ground, or burrow. They are nocturnal or crepuscular.

Ineich and Böhme also suspect that the Tongan ground skink is a terrestrial forest inhabitant and might also be found in densely vegetated coastal areas where small rocks and possibly rotting tree stumps provide shelter.

They consider it unlikely that the Tongan ground skink has survived on the ecologically heavily impacted island of Tongatapu. Therefore, they suggest potential habitats in the national park on the larger and less ecologically altered island of ‘Eua, where the last remaining rainforest of the archipelago is located, as well as the small island of Kalau, situated south of ‘Eua. This secluded island is seen as a potential habitat because it may have been less affected by human activities.

Encouragement and hope are drawn from the example of the terror skink (Phoboscincus bocourti), described in 1876 based on only one known specimen and then thought to be extinct – until its rediscovery in 1993. This skink, endemic to the islands of New Caledonia, was believed to be extinct for the same reasons as the Tongan ground skink. However, scientists discovered specimens on two small islands off the coast of the Isle of Pines, which belongs to the French overseas territory of New Caledonia.

Why did the Tongan ground skink go extinct?

In the 1980s, German ornithologist Dieter Rinke spent 18 months researching the Tongan islands in search of rare and endangered birds and reptiles, including the Tongan ground skink. In an article for the journal Oryx (1986), he summarized his findings:

Despite extensive questioning with the aid of a photograph, I found no evidence to indicate that this skink still survives. Native people did not know of any ground dwelling reptile species this large (up to 35 cm). Although the species would probably very cryptic, as its congeners are, it should certainly be known by planters. If the type specimens really were collected on Tongatapu, the species must now be considered extinct.

The Status of Wildlife in Tonga, Oryx Vol. 20 (3), S. 150, D. Rinke.

Plantations Instead of Forests

The presumed extinction of the Tongan ground skink is primarily attributed to habitat loss. As a forest dweller, the species depended on the presence of forests, which are now almost nonexistent on Tongatapu. Over the 3,000 years or more that the most populous island of the archipelago has been inhabited by humans, 90 percent of Tongatapu has been deforested. Today, the island’s vegetation mainly consists of cultivated crops, primarily pumpkins and zucchini, or lies fallow. Almost the entire island is divided into small family plots, government parcels, and towns or villages.

The few remaining forest areas on Tongatapu are merely secondary forests, further disturbed by numerous pigs living on the island that forage for food.

Introduced species

The Dutch navigator Abel Tasman, who discovered the Tonga islands in 1643, the British explorer James Cook in the 1770s, and later European explorers brought domestic cats and rats (Rattus rattus and R. norvegicus) to the island of Tongatapu. Dumont d’Urville also noted the presence of rats in his travel report (1830-1835).

Some experts speculate that introduced domestic dogs may have posed a threat to the Tongan ground skink. However, Zug and Ineich believe that the greater danger came from introduced pigs, which could eat the skink’s eggs or young.

Predatory mammals introduced to islands, such as rats or cats, often have devastating effects on native reptiles. Feral cat populations now exist on virtually every human-inhabited island in the Kingdom of Tonga. For a ground-dwelling lizard like the Tongan ground skink, these predators pose a significant and ongoing threat, especially to juveniles.

Zug and Ineich, who had the opportunity to search for the missing Tongan ground skink on Tongatapu and ‘Eua in October 1993, came to the following conclusion:

It seems most unlikely that a semi-fossorial lizard could survive under these circumstances. Therefore, we now agree with Rinke (…) that the giant Tongan skink is extinct on Tongatapu. However, giant scincoid anguid lizards have managed to survive in the West Indies despite cats, rats, pigs, and mongooses (…). If Tachygyia occured on ‘Eua, sufficient undisturbed forest remains for its continual survival there.

Tachygyia, the Giant Tongan Skink: Extinct or Extant?, Cryptozoology 12, 1996, S. 34, I. Ineich & G. R. Zug.

John R. H. Gibbons, who actively searched for the Tongan ground skink on Tongatapu in 1985 and 1986, also believed that the species was once common there but disappeared as more forested habitats were converted to plantations and cats and rats reached the island.

Lizards on the menu

It is not unlikely that the Tongan ground skink was once part of the diet for Tongans. The French biologist Jacques Labillardière wrote in Voyage in Search of La Pérouse (1800) about his visit to Tongatapu in 1793, noting that the inhabitants offered him captured lizards, which were “very good to eat.” Whether these were Tongan ground skinks or another lizard species is unknown. However, archaeological research has shown that the Fiji banded iguanas (Brachylophus), which can grow up to one meter in length, were a common food source for Tongans on the Tonga islands.

Another extinct species from the Tonga islands is the Tongatapu rail, known only from a description and a contemporary painting. The factors leading to its extinction in the late 18th century are believed to be the same as those that affected the Tongan ground skink.

Mysterious lifestyle of the Tongan ground skink

Little is known about the Tongan ground skink, except that it is terrestrial. Only contemporary reports, anecdotes, and legends, as well as comparisons with closely related species, provide clues about its lifestyle.

The Tonga ground skink is part of Tongan legends, in which it is attributed with special powers. An encounter with the rare animal is considered an omen for a significant family event, such as a wedding or a funeral. This legend suggests that seeing the Tongan ground skink is considered a great stroke of luck, hinting at its hidden lifestyle.

Herpetologists Ivan Ineich and Wolfgang Böhme, in their 2024 study, suggest that Duméril and Bibron discovered the Tongan ground skinks during the rainy season. Heavy rainfall could have forced the animals out of their underground hiding places. This is supported by a travel description from Jules Dumont d’Urville in 1832, in which he reports heavy and frequent rains.

The Tongan ground skink was very likely a semifossorial forest dweller, similar to its relatives in New Caledonia and New Guinea. This means it was capable of digging and lived partially underground, unlike fully fossorial creatures (like naked mole-rats or moles).

The IUCN notes that similar species are crepuscular predators that stay in the underbrush. This could also be true for the Tongan ground skink. Given the strong morphological similarity between the Caribbean endemic double-tongued skinks and the Tongan ground skink, a similar lifestyle is likely. Double-tongued skinks live on the ground or burrow and are nocturnal or crepuscular.

Tongan ground skink – Truly a distinct genus?

After its initial description in 1839 as Eumeces microlepis, the genus classification of the Tongan ground skink has been changed multiple times. It was not until 1952 that the American herpetologist Myron Budd Mittleman proposed the monotypic genus Tachygyia in A Generic Synopsis of the Lizards of the Subfamily Lygosominae. This genus is characterized by a movable, scaly lower eyelid without a spectacle, a visible ear opening that is half the size of the eye or smaller, and very robust and long limbs that overlap significantly when held against the body.

Since Tachygyia is a monotypic genus found only on a remote Pacific island, its classification has been questioned by some authors, including Ineich and Böhme. There are several similar cases where monotypic island genera have turned out to be species-rich local variants of larger genera through radiation. One example is the extinct Cape Verde giant skink, long considered the sole species of the genus Macroscincus. A revision of Cape Verde skinks in 2010 showed that the Cape Verde giant skink was actually a giant form within the species-rich genus Chioninia. Similarly, the Mauritius giant skink, which went extinct between 1600 and 1650, was later identified as a member of the genus Leiolopisma.

Ineich and Böhme propose that the Tongan ground skink might be an aberrant member of the genus Emoia, whose skinks are distributed from the northwest to the southwest Pacific. These reptiles are characterized by the presence of nasal scales (supranasalia), a transparent scale in the movable lower eyelid, and well-developed limbs. The Tongan giant skink shares many similar traits with the species in this group but differs in some specific features such as the scaly lower eyelid. It could therefore represent a particularly divergent line within the Emoia radiation in the central Pacific, explaining its unique evolutionary history.