Christmas Island forest skink

Another loss for Australia’s fauna

The Christmas Island forest skink is, alongside the Christmas Island pipistrelle (2009) and the Bramble Cay melomys (between 2009 and 2011), among the most recent losses suffered by Australia’s wildlife. The last known Christmas Island forest skink was named Gump—a female that died in human care on March 31, 2014. It is probably the first endemic reptile of Australia to have gone extinct since European colonization, according to ecologist John Woinarski in a 2014 article for The Conversation.

In the late 2000s, when it was already clear that hardly any forest skinks remained, researchers on Christmas Island in the Indian Ocean tried to track down animals for a captive breeding program, but were able to find only three females—among them Gump. Until the very end, the scientists hoped Gump would survive long enough for a male to be found so the species could be saved. That did not happen; despite every effort, no other forest skink was found. Gump eventually died less than five months after the species had been placed on the list of Australia’s threatened species. Conservation measures could therefore no longer take effect.

Besides the Christmas Island forest skink, there are or were four other reptile species endemic to the 135-square-kilometer Christmas Island: Lister’s gecko (Lepidodactylus listeri), Sadleir’s bow-fingered gecko (Cyrtodactylus sadleiri), the Christmas Island blue-tailed skink (Cryptoblepharus egeriae), and the Christmas Island blind snake (Ramphotyphlops exocoeti). With the exception of Sadleir’s bow-fingered gecko, all of these species have declined over the last 40 years almost to total eradication.

Christmas Island forest skink – fact sheet

alternative names	Christmas Island whiptail-skink, forest skink
scientific names	Emoia nativitatis, Lygosoma nativitatis, Emoia nativittatis
original range	Christmas Island (Indian Ocean, Australia)
time of extinction	2014
causes of extinction	animals introduced to the island, habitat loss
IUCN status	extinct

Forest skinks: still widespread into the 1990s

Into the 1990s, forest skinks were widespread on Australia’s Christmas Island and then—without warning—they became fewer and fewer until they finally disappeared altogether. This pattern of a rapid and sudden disappearance of a species without a precise cause is already known from the Christmas Island pipistrelle.

Australian herpetologist Hal Cogger recalls still seeing more than 80 forest skinks in 1998 basking in the sun around a single fallen tree and searching for food. In Australian endangered Species: Christmas Island Forest Skink (2013) for The Conversation, Woinarski and Cogger write that forest skinks were widespread on Christmas Island well into recent decades.

Then the decline began. Up to 2003, forest skinks could still be found scattered across remote regions of the island. By 2008, a survey showed that they were present in only a single area on the island. Subsequent searches failed completely, and no animals could be located, leading researchers to assume that the reptile no longer occurred in its former preferred habitat. The last sighting in the wild is said to have occurred in 2010, according to the conservation organization IUCN, which has listed the Christmas Island forest skink as extinct on its Red List since 2017.

Why did the Christmas Island forest skink go extinct?

Why the number of forest skinks, and other reptiles, on Christmas Island declined so rapidly is still not fully understood; scientists assume that a combination of factors led to the decline. Herpetologists Michael J. Smith and Hal Cogger examined the possible causes behind the falling population numbers of several native reptile species on Christmas Island in a 2012 paper. They concluded that predation by introduced species was probably the decisive factor in the decline of the island’s native reptiles. But other processes such as interspecific competition may also have played a role, especially since five additional non-endemic reptile species now live on Christmas Island.

Invasive species as a threat to the endemic fauna

The invasive species on Christmas Island include black rats (Rattus rattus) together with their parasites. They probably arrived there accidentally on a ship around 1889, which also quickly led to the extinction of the endemic Maclear’s rat and the bulldog rat. The introduced rodents eat skink eggs and juveniles.

Feral cats and domestic cats also arrived on Christmas Island by ship and took a heavy toll on the native fauna. An analysis of 19 cat stomachs showed that 30 to 40 percent of their contents consisted of native geckos and skinks such as the forest skink; this is reported in the Final Report of the Christmas Island Expert Working Group (2010). Also spread across Christmas Island are scolopendras, or giant centipedes (Scolopendromorpha)—nocturnal, venomous predators that hunt lizards, skinks, birds, and mice.

But the greatest danger to the native reptile fauna comes from two other species: the yellow crazy ant (Anoplolepis gracilipes) and the common wolf snake (Lycodon capucinus). The yellow crazy ant probably reached Christmas Island accidentally from Africa as an introduced species sometime between 1919 and 1934, where it threw the entire ecosystem into disarray. These supercolony-forming ants do not just eat fruit and rob birds of their food base; they also attack animals such as chicks and reptiles. Since yellow crazy ants are found mainly along the coasts or in coastal areas of the island, however, Smith and Cogger assume they alone are not responsible for the decline of Christmas Island’s reptiles.

This is where the common wolf snake, introduced in the early 1980s, enters the picture. It probably played the most important role in the disappearance of the forest skink, because it feeds mainly on lizards and skinks. In the case of the Christmas Island blue-tailed skink, its presence has already led to extinction in the wild. According to the IUCN, the common wolf snake could be directly responsible for the extinction of the forest skink; Smith and Cogger share that view.

Habitat loss through phosphate mining and urbanization

Besides invasive species, the loss of natural habitat may also have contributed in small part to the decline of the native fauna, the IUCN notes. Phosphate has been mined and exported on Christmas Island since 1890, destroying 25 percent of the island’s forest habitat, much of it tropical rainforest. The discovery of phosphate also ultimately led to the settlement of Christmas Island.

Smith and Cogger acknowledge that mining and urbanization certainly had negative effects on habitat, but say the native reptiles retreated into less disturbed areas. They therefore do not see forest clearing, for example, as the main cause of reptile decline. The scientists also point out that reptiles disappeared even from parts of the island where no habitat destruction occurred.

Christmas Island forest skinks are poorly studied

Skinks (Scincidae) of the species-rich genus Emoia are distributed on islands from the northwestern to the South Pacific. According to the Reptile Database, 78 species are currently distinguished, which can be divided into eight groups based on morphological criteria, each of which also occurs on other islands. This species diversity probably goes back to the evolutionary phenomenon of adaptive radiation.

A genetic analysis from 2018 showed that within the genus Emoia, the Christmas Island forest skink is most closely related to Boettger’s emo skink (Emoia boettgeri). This species lives about 4,000 kilometers away in Micronesia in the Pacific Ocean. The evolutionary divergence of the traits of the two species must have taken place around 13 million years ago during the late Miocene.

It was not that long ago that the Christmas Island forest skink went extinct, yet surprisingly little is known about how it lived or what it looked like. From the scientific first description of the species, published by the Belgian-British zoologist George A. Boulenger in 1887, the following, among other things, emerges regarding the external features of the species:

“(…) lacertiform; (…) snout long, blunt. Lower eyelid with an undivided transparent disc. (…) Ear opening oval, (…) toes somewhat elongate, slightly flattened at the base, compressed at the end (…). Brown above, highly iridescent, with small golden and black spots, most numerous on the sides and limbs; underside white.”

Report on a Zoological Collection made by the Officers of H.M.S. Flying Fish at Christmas Island, Indian Ocean. Proceedings of the Zoological Society of London 1887, pp. 516-517, G. A. Boulenger.

For his description of the new species, Boulenger had only “a single female specimen, without tail” as the holotype. The head length was 1.5 centimeters and the body measured 5.6 centimeters. Woinarski gives a total body length of around 20 centimeters for the Christmas Island forest skink, with the tail making up about two thirds of that length.

Cogger summarized his observations on the preferred habitat of the diurnal, ground-dwelling Christmas Island forest skink in 1983 in Reptiles and Amphibians of Australia as follows: The species was found in forest clearings, usually in leaf litter, but occasionally also in low-growing vegetation or on tree stumps. The forest skink was said to be abundant wherever sunlight penetrated the canopy, especially along tracks or roads bathed in sunlight.