Maclear’s rat

On the vulnerability of rats

The extinction of countless birds, mammals, and reptiles in modern times can be traced back to rats that arrived on European ships and expanded their range. In fact, however, there are also many species from the family of true mice and rats (Muridae) that have been lost in recent centuries. That rats are not only particularly adaptable, ecologically unspecialized food pests and disease carriers, but can also be vulnerable, was shown above all by the history of Australia’s Christmas Island in the Indian Ocean.

Just under 13 years after miners founded a settlement near the capital, Flying Fish Cove, to mine phosphate, two Old World rats and mice (Murinae) disappeared while the black rat (Rattus rattus) spread across Christmas Island. Of the island’s two formerly endemic rats—Maclear’s rat and the bulldog rat—there is still no trace today.

Maclear’s rat was named after the captain of the research ship HMS Flying Fish, John Fiot Lee Pearse Maclear, who brought the first settlers to Christmas Island in 1886. Maclear took several specimens of Maclear’s rat back to England, where the British zoologist Oldfield Thomas described it scientifically in 1887. Ten years later, in 1896, the settlement for the miners was founded. In A monograph of Christmas Island (1900), the zoologist and botanist Charles William Andrews still described rats as the most common mammals on the island between 1897 and 1898:

“In every part of the island that I visited, swarms of rats appeared. During the day one sees nothing of them, but immediately after sunset one sees them running in all directions, and throughout the whole forest there echoes a peculiar distressed squeaking as well as the sounds of constant fighting. These animals (…) are almost completely free of fear, and if one shines a lantern into the undergrowth, they come closer to inspect the novel phenomenon.”

A monograph of Christmas Island, 1990, C. W. Andrews.

When Andrews returned to Christmas Island in 1908, he found the island almost as he remembered it. Only a few new roads had been added—and the native rats had disappeared.

Maclear’s rat – fact sheet

alternative names	Captain Maclear’s rat, Christmas Island rat
scientific names	Rattus macleari, Mus macleari
original range	Christmas Island (Australia)
time of extinction	1904
causes of extinction	black rats introduced to the island together with parasites
IUCN status	extinct

Rats, fleas, and trypanosomes

In On the fauna of Christmas Island (1909), Andrews was convinced that both species must have become extinct. Even then, he speculated that the cause of their disappearance lay in the presence of black rats, which had arrived on Christmas Island as ship rats in a load of hay in December 1899, when the supply ship S.S. Hindustan (or Hindoustan) docked, together with epidemic pathogens.

About 100 years later, researchers were finally able to solve the mystery of the historical mammal extinction on Christmas Island by examining DNA samples from rats in museum collections that had been collected on the island during the extinction window between 1888 and 1908. The team led by Kelly B. Wyatt of the American Museum of National History was thus able to show in a study in 2008 that endemic rats collected before the black rat reached Christmas Island showed no pathogenic trypanosomes, whereas animals collected later did.

Trypanosomes are single-celled flagellates that occur as endoparasites in various vertebrates. Many species are harmless; others cause animal epidemics or disease in humans. Trypanosomes are usually transmitted by insects. Ultimately, the parasite Trypanosoma lewisi, found in rat blood, was responsible for the extinction of the endemic immunologically naive rats on Christmas Island. It was transmitted to the island rats by fleas that had infested the introduced black rats.

Maclear’s rat: the time of its disappearance can be narrowed down

Exactly when Maclear’s rat became extinct can be narrowed down using historical documents and eyewitness accounts. Jane Pickering and Christopher A. Norris analyzed documents discovered at the University of Oxford in 1996, as well as specimens of Maclear’s rat from various zoological collections. Among other things, the documents contain notes on a lecture by Karl Richard Hanitsch, then director of the Raffles Museum in Singapore, who was on Christmas Island in September and October 1904 and was unable to find any of the native rats.

The recovered records also show that the bacteriologist Herbert E. Durham was on the island between November 1901 and March 1902. He collected 19 rat specimens, some of which Pickering and Norris later identified as hybrid forms between the black rat and Maclear’s rat. They concluded that Maclear’s rat had been genetically swamped by the black rat, which they considered a factor that had contributed to the extinction of Maclear’s rat. This hypothesis was later disproved by various scientists. On the one hand, hybridization between the two species is unlikely because of the phylogenetic distance, and on the other hand, more recent studies show that all rats collected by Durham can be clearly assigned to either one species or the other. This is also confirmed by Wyatt’s molecular genetic study from 2008.

Since Maclear’s rats were among the rats collected by Durham, they must still have existed in early 1902. The fact that none of the specimens were Christmas Island rats points to the rarity of that species. Most scientists agree that Maclear’s rat disappeared in 1904. The rarer Christmas Island rat probably died out somewhat earlier. The IUCN also states that Maclear’s rat was last seen in 1904 and that extensive searches for the rodent remained unsuccessful.

In a 2014 article for the Australian Zoologist, the biologist Peter T. Green redefined the temporal sequence once again. According to Green, the black rats and thus the trypanosomes did not reach Christmas Island until September 1900, and the extinction of Maclear’s rat occurred between April and October 1904.

Two cases of coextinction: with Maclear’s rat, by the way, the flea species Xenopsylla nesiotes, which parasitized exclusively in the fur of these rats, and the tick species Ixodes nitens, which as an ectoparasite is known only from specimens of Maclear’s rat, also became extinct.

Eradicating animals is easier than bringing them back

The extinct Maclear’s rat was recently in the media, for example in Stern, because Danish researchers launched an attempt to bring the species back to life. The evolutionary biologist Tom Gilbert and his colleagues found two preserved specimens of Maclear’s rat in a museum in Oxford, from which they were able to isolate DNA fragments. They published the results of their research in the scientific journal Current Biology in April 2022.

DNA fragments alone cannot be used to reconstruct a complete genome, which is why Gilbert and his team used the genome of the brown rat (Rattus norvegicus), a living relative of the extinct Maclear’s rat, as a guide in order to put the DNA pieces into the correct order. But even in this way, only 95% of the genome of Maclear’s rat could be deciphered. More than 1,600 genes could be reconstructed to less than 90%, and 26 genes not at all. The gene regions for functions that are actually vital—the immune system and the sense of smell—were missing completely.

By means of genome editing, it would be possible, the scientists say, to recreate Maclear’s rat by mapping it with the brown rat. However, even then a large number of genes would either resemble the extinct species only partially or, in the worst case, come one hundred percent from the brown rat. The result would be a hybrid of a living brown rat and the extinct Maclear’s rat. And whether this hybrid form would be viable is also questionable. A reintroduction of this hybrid Maclear’s rat into its original habitat would probably be out of the question. The reconstructed species would likely lack attributes that are crucial for its survival in its natural environment.

Why de-extinction does not work

It was not necessarily the goal to bring Maclear’s rat back to life; rather, the researchers wanted to show the possibilities and limits of de-extinction. It is especially about the limits, because Gilbert points out to SingularityHub that it would make more sense to invest the money used for attempts to resurrect extinct species in protecting species that are still alive.

Current resurrection projects focus on animals such as woolly mammoths (Mammuthus primigenius) or passenger pigeons. Gilbert and his team give a successful reconstruction of extinct species little chance of success. The researchers do believe that genomes of species can be reconstructed all the better the smaller the evolutionary divergence between the extinct and the still-living species is, or the more closely related the species are, but even this only within certain limits. With the help of the black rat, the genome of Maclear’s rat could thus be restored to 96.56%—compared with 95.15% through mapping with the brown rat. The divergence between the passenger pigeon and its closest living relative, the band-tailed pigeon (Patagioenas fasciata), however, is far greater than that between the rats, Gilbert says, which makes genome reconstruction impossible.