Jamaican Rice Rat, Oryzomys antillarum

The Jamaican rice rat (Oryzomys antillarum) is an extinct species of rodent that was once found in Jamaica. This rat is thought to have been a divergence of O. couesi, which can be found in mainland Central America. It is thought that this rat went extinct in the late 19th century due to habitat destruction and by predation and competition from introduced species like the mongoose and the brown rat.

Elliott Coues noted two specimens of Oryzomys in his 1877 monograph about North American rodents, which appears in the United States National Museum (USNM). He indicated that although they had similarities to marsh rice rat, they differed in color. Instead of classifying these specimens as a distinct species, Coues only described them in order to avoid any conflict with possible names that he did not know about.

It was not until 1898 that the Jamaican rice rat was classified as its own species, when Oldfield Thomas studied a specimen that had been in the British Museum of Natural History since 1845. Thomas specified that it was a distinct species that he named Oryzomys antillarum, but it was related to O. couesi. He also stated that there was a good chance that if there were any living relatives of this rice rat, they would be found in unexplored regions of Hispaniola or Cuba.

In 1918, Edward Alphonso Goldman revised North American Oryzomys, leaving the Jamaican rice rat as a distinct species. However, he did note that because it was so similar to O. couesi that it may have been brought to Jamaica by humans. In 1920, Harold Anthony found remains of the Jamaican rice rat in coastal caves, suggesting that they may have been a vital component to the diet of the barn owl. Further studies conducted on these remains by Clayton Ray for his 1962 thesis suggested that it is not a distinct species at all, but simply varying species of Oryzomys palustris, which had previously been classified with numerous subspecies including O. couesi. Later, O. couesi of Central American and Mexico was again classified as a separate species, and the Jamaican rice rat was reclassified as a subspecies known as Oryzomys couesi antillarum.

Gary Morgan reclassified the Jamaican rice rat as a distinct species in a 1993 review where he cited an unpublished paper by himself, Humphrey, and Setzer. He stated that it was closely related to O. couesi, but in 2005 when Guy Musser and Michael Carleton wrote the third edition of Mammal Species of the World, they did not cite Morgan and continued to classify the Jamaican rice rat as a subspecies. Two papers, published in 2006 and 2009, cited Morgan and classified the Jamaican rice rat as a distinct species.

According to Carleton and Arroyo-Cabrales, who published the 2009 paper, which classified the Jamaican rice rat as a distinct species, there are eight species in the genus Oryzomys, of which the Jamaican rice rat is one. These rats occur throughout many regions of the Americas, and it is thought that there is not enough information to understand their diversity and accurately classify them.

The Jamaican rice rat was medium sized, and the largest specimen studied had a body length of ten inches, with five inches making up the tail. According to Thomas’s 1898 description, the Jamaican rice rat had many colors in its fur. Its dorsal body would have been red in color, while its underbelly would have faded from yellowish to red on the sides. The tail held little hair, and was lighter brown on the underside.  Goldman’s description of different specimens showed that they were redder in color, but he thought this might have been a reaction to the alcohol in which they were stored. Coues had described the same specimens as rusty red in color.

Because of the confusion on the color of these specimens, Andrew Arata compared them with specimens of Oryzomys palustris natator, which were also red in color, and found that the Jamaican rice rat was actually redder than Oryzomys palustris natator.

The facial structure and teeth of the Jamaican rice rat is very similar to that of Oryzomys couesi. It is strong and supported well-developed brow ridges, which can be found above the eyes. The roof of the mouth stretched over the third molars. One difference between the two rats is that the Jamaican rat had nasal bones that extended past the premaxillaries, whereas these bones were adjacent in Oryzomys couesi. The cheek bones in the Jamaican rat appear to be more developed that those of O. cousei.

According to a study conducted in 2202, the oldest and most reliably dated subfossil was found in a series rock and radiocarbon dated to between 10,250 and 11,260 years before present. It was found in many other undated sites that were not colonized by humans. One site, called Wallingford Roadside Cave, did not contain subfossils of Oryzomys. Data that supports the arrival of the Jamaican rice rat before humans arrived disproves the “species introduction” theory, and it is thought that instead of being brought to the island, the rats must have migrating over the water during a rafting event that most likely occurred 125,000 years ago.

The occurrence of the Jamaican rice rat’s predecessor would have been aided by the last glacial period, when much of the land between Jamaica and Central America became visible, and by sea currents that decreased the distance between the two locations. Rafts made from vegetation most likely carried the rats to Jamaica. The Jamaican rice rat, and any rat in the Oryzomys genus, is semiaquatic, and this helps to support their appearance on Jamaica.

Ray suggested that because of its wide range and the various sites it has been found in, the Jamaican rice rat must have had a large distribution in different habitats before humans appeared on the islands.  The Jamaican rice rat was the only Sigmodontine rodent found to occur on the Greater Antilles.

Because the Jamaican rice rat decreased rapidly after European colonization, there are not many historical records pointing to the species. It is thought that this also occurred due to the fact that the settlers did not distinguish between the Jamaican rice rat and introduced species such as the black rat and the house mouse. In 1756, Patrick Browne described a “Water-Rat”, and other rodent species, in the Civil and Natural History of Jamaica.

Edward Long described four Jamaican rats in his 1774 History of Jamaica. These included the “black house-rat, which was thought to have been brought from Europe and two native species. Philip Henry Gosse, in his 1851 A Naturalist’s Sojourn in Jamaica, described both a brown and black rat, a house mouse, and the same “Water-Rat” that Browne had described in 1756, although he described it as the Cane-piece Rat or Mus saccharivorus.

Both Thomas and Ray suggested that the “Cane-piece Rat” was most likely a brown rat by studying its measurements. Gosse mentioned an explorer Anthony Robinson, who had described the “brown rat” species and its body length, which measured twenty inches, of which half the length comprised the tail. Although Ray did not examine Robinson’s unpublished paper, he concluded that the rat Robinson described could possibly be the Jamaican rice rat on the basis that the brown rat had not come to the Americas until approximately the 1800’s.

It is thought that the Jamaican rice rat went extinct in the 1870’s, most likely due to the introduction of the small Asian mongoose.  However, Ray considered humans to be the main cause of the Jamaican rice rat’s extinction, causing major habitat destruction after arriving in 1655. Because the majority of the land was used for cultivating crops, the rat was reduced to competing with introduced rodent species for food and shelter in an unfamiliar environment filled with humans. The Jamaican rice rat appears on the IUCN Red List as an “Extinct” species.

Image Caption: Skull of the holotype of Oryzomys antillarum. Credit: Clayton E. Ray/Wikipedia