Coues’ Rice Rat, Oryzomys couesi

Coues’ rice rat (Oryzomys couesi) is a semiaquatic rat that can be found in southern Texas, throughout Mexico and Central America, and into the northwestern portion of Colombia. It prefers a habitat with abundant water, but it is able to live in arid habitats and shrub lands. Coues’ rice rat forms the group O. couesi, along with six other species of rat. All of these rats are in the Oryzomys genus, along with the marsh rice rat who is the sole member of its group. There are more than forty species that were removed from the Orysomys genus in 2006, when Marcelo Weksler and his associates studied the species and found them to be wrongly classified. However, all of these species are classified within the Oryzomyini group, commonly known as rice rats.

Edward Alston, who examined three individuals from Guatemala and Mexico, first described Coues’ rice rat in 1877. Alston called it Hesperomys couesi, placing it in the now non-existent genus Hesperomys. He compared the rat to the marsh rice rat, which was called Hesperomys palustris at the time, as well as two species in the Tylomys genus. Once the rat was placed in the Orysoys genus, it received the specific name couesi, derived from Elliott Coues, and American naturalist who studied many North American rodents.

There has been much confusion on the classification the Coues’ rice rat. Oldfield Thomas write of this confusion in 1893, asserting that two or three of the individuals that Alston studied were actually separate species from Oryzomys couesi. Thomas called the individuals from Mexico Oryzomys fulgens and classified the specimen from Guatemala as Oryzomys couesi. Beginning in the 1890s, many species were classified, and in 1901, Clinton Hart Merriam classified several of these into the palustris-mexicanus group, which contains the marsh rice rat.

In 1918, Edward Alphonso Goldman reviewed the classifications of may rice rats, and placed ten subspecies under Coues’ rice rat. These can be found in a range extending from southern Texas to western areas of Mexico, and into Costa Rica. He placed Coues’ rice rat in the Oryzomys palustris group, noting that although many of the species were related to it, they were distinct enough to be considered separate species. A few more species relate to Coues’ rice rat were classified in 1930.

After examining the ranges of the marsh rice rat and Coues’ rice rat, which overlap in southern Texas, Raymond Hall concluded that the species held no grounds for a separate classification, so he placed Coue’s rice rat under the marsh rice rat as a subspecies. By 1971, the grouping of these rat species was complete, and all species that had been previously placed in the O. palustris group by Goldman, were classified under the marsh rice rat.

In 1979, more research was conducted on the similarities and differences between the marsh rice rat and Coues’ rice rat where their range overlaps in Texas. These studies revealed that the two species were actually distinct from each other, and as a result, Coues’ rice rat is now considered a distinct species from the marsh rice rat. After this, many subspecies under O. couesi and O. palustris were reclassified as well, including Oryzomys nelsoni, which can be found in western Mexico and the Marías Islands and the Jamaican Oryzomys antillarum.

In 2010, Delton Hanson and his associates conducted extensive DNA research on the marsh rice rat and Coues’ rice rat. He found that the two species were not as related as had previously been thought, and using this genetic data, Hanson was able to conclude that four species within O. couesi should be classified as distinct species. Although the specimens from Panama and Costa Rica did not show a clear distinction, the western group should be called Oryzomys mexicanus and the eastern subclade should hold the name Oryzomys couesi.

The western subclade, called Oryzomys mexicanus by Hanson, makes up a single clade or original group, and their range includes east Salvador, Jalisco, and western Mexico. There are differences between these rats and Coues’s rice rat, including their range and skull structure. There were also differences within the O. mexicanus clade, so Hanson and his associates called the differing specimens subspecies, classifying the eastern subspecies as O. zygomaticus and the western subspecies as O. mexicanus.

First described as a distinct species in 1897 by Joel Asaph Allen, specimens from Jalisco were classified as Oryzomys mexicanus. Allen described another species, Oryzomys bulleri from near Nayarit, but he did not compare it with O.mexicanus. In 1901, Merriam described another species from Nayarit, called Oryzomys rufus, mentioning its color variances from O. mexicanus. In 1918, Goldman classified these three species as synonyms of O. couesi mexicanus. Carleton and Arroyo-Cabrales supported this in 2009, because the differences between the species were age related and only variances within the species. Another subspecies known as Oryzomys couesi lambi, described by Burt in 1934, bears color and structure differences from O. mexicanus, although more information is needed to know whether it is actually a subspecies.

In 1901, Merriam described three subspecies of Coues’ rice rat all occurring in central Mexico, including crinitus, aztecus, and albiventer. In 1915, Goldman described a fourth subspecies, known as regillus, from central Mexico, differing in color and size from Coues’ rice rat. In 2009, Carleton and Arroyo-Cabrales evaluated the classifications of these four subspecies, finding that albiventer was actually a distinct species, and that the other three subspecies needed more research to correctly classify them. The specimens originally described were found at the outermost portions of its range, and it was thought that they only represented larger individuals of O. mexicanus, instead of a separate species.

Oryzomys rats from Costa Rica, Colombia, and Panama have previously been denoted to as O. c. couesi. After conducting studies on specimens from these areas, Hanson and his associates found that two of the Costa Rican individuals differed so much from Coues’ rice rat that they should be a separate species altogether. However, because they could study enough specimens, the status of these rats could not be changed or confirmed.

In Panama, these rats are rare, but were first described in 1912 by Goldman, who named the species he studied Oryzomys gatunensis. He noted that although the rats had similarities to the richmondi subspecies of Coues’ rice rat, it was not similar enough to be classified as a subspecies, and so it remained a distinct species. Bole described another species from Panama in 1937, called Oryzomys azuerensis, but Goldman suggested that both azuerensis and gatunensis should be classified as subspecies of Coues’ rice rat. Bole noted that azuerensis was not located close enough to O. couesi within its range. This was resolved in 1966, when Charles Handley reviewed mammals from Panama, and classified both species as subspecies under the marsh rice rat, where Coues’ rice rat was classified at the time. After Coues’ rice rat was reclassified as a distinct species, both subspecies were changed as well, although Hanson and his associates did not consider azuerensis a distinct species.

The populations of Oryzomys rats ranging from Texas to Nicaragua retained the name Oryzomys couesi. The rats that inhabit the far southern areas of Texas and Tamaulipas, Mexico are under Coues’ rice rat as the subspecies aquaticus. In this range, Coues’ rice rat merges with the marsh rice rat, and in northeastern Tamaulipas, as well as Willacy, Cameron, and Kenedy counties, the two species are sympatric because they occur together. Although they are sympatric, the marsh rice rat appears in coastal areas, while Coues’ rice rat prefers an inland habitat. Studies conducted on interbreeding between the two species show that hybridization is not successful in captivity or in the wild.

Goldman classified many groups of Coues’ rice rat occurring from northern Venezuela, throughout Mexico, and into Honduras, Guatemala, and Nicaragua, and as far south as northwestern Costa Rica as the subspecies Oryzomys couesi couesi . He also classified six other species as a synonym of the O.c.c. subspecies, including Oryzomys teapensis, classified by Merriam in 1901, and Oryzomys richardsoni, classified by Allen in 1910. This occurred because although there are major variances in size and color between the many species, they are essentially the same rats.

In 1901, Merriam described the Oryzomys rats found on the island of Cozumel, classifying them as Oryzomys cozumelae, a distinct species from Coues’ rice rat. In 1965, Knox Jones and Timothy Lawlor found that the differences between the two species were not enough to classify them as separate and placed Oryzomys cozumelae under couesi as a subspecies. Another subspecies of Coues’ rice rat that was once considered distinct is Oryzomys richmondi, described by Merriam in 1901, and it occurs in Nicaragua. Jones and Engstrom did not support this and re-classified it as a subspecies in 1986.

Coues’ rice rat can reach an average body length of 3.9 to 5.6 inches, and a weight of 1.5 to 2.9 ounces. Studies have shown that the size of these rats can vary depending upon their location, and in Texas and El Salvador, males are slightly larger than females. The short fur is typically red or buff in color on the upper parts, while the lower parts are pale and the backend is dark. There is fur located on the digits, and the paw pads are reduced, and these are traits that aid in its semiaquatic lifestyle.

Coues’ rice rat lives on the ground, but it does spend much of its time in the water, and is also a skilled climber. Studies have shown that this rat may forage under the water, giving it a separate food source from other rats in its range. It is a nocturnal creature that takes to the water when frightened. It will make spherical nests that are suspended in the air by reeds, made from intertwined vegetation. In Texas, where Coues’ rice rat is bigger, the nests are larger as well.

The population density of this species can vary depending on its location, but the there is an average of two to twelve per acre. In Texas, males can move up to 502 feet every 24 hours while females tend to move an average of 413 feet. It is thought that these rats can mate year round. After a pregnancy period of up to 28 days, two to seven young are born, although three young is average. At seven weeks of age, the young are sexually mature, and the lifespan of each rat is short.

The diet of Coues’ rice rat consists of plant materials like seeds and green vegetation, but they are also known to eat animals like crustaceans and small fish, as well as insects.  In Cozumel, the boa constrictor, an introduced species, is a common predator of Coues’ rice rat. These rats can contract many parasites including, ticks, mites, fleas, and worms. Although the Hantavirus did occur in some specimens studies, there are no recorded infections in humans caused by these rats.

Because Coues’ rice rat has a large range and it is able to live in many habitats, it is in no danger of extinction. However, some local populations are threatened by habitat loss. In Texas, it is highly threatened by habitat loss, which has caused it to have a fragmented range in that area. Benson and Gehlbach estimated that it numbered 15,000 at most in Texas in 1979. One study conducted in 2001 suggested that because of habitat loss due to climate change, the populations in Texas are likely to become extinct. It is considered a pest in many areas of its range. Populations in Cozumel have also declined due to habitat loss, but it also thought that predation by introduced species may be a cause for is decline as well. Coues’ rice rat as a species appears on the IUCN Red List with a conservation status of “Least Concern”.

Image Caption: Two Central American rodents: Oryzomys couesi (top) and Tylomys panamensis (bottom). Credit: E.R. Alston/Wikipedia