Oryzomys couesi

Oryzomys couesi, also known as Coues' rice rat, is a semiaquatic rodent in the family Cricetidae occurring from southernmost Texas through Mexico and Central America into northwestern Colombia. It is usually found in wet habitats, such as marshes, but also lives in drier forests and shrublands. Weighing about 43 to 82 g (1.5 to 2.9 oz), O. couesi is a medium-sized to large rat. The coarse fur is buff to reddish above and white to buff below. The hindfeet show some specializations for life in the water, such as reduced ungual tufts of hair around the digits. It has 56 chromosomes. There is much geographic variation in size, proportions, color, and skull features. Oryzomys couesi is active during the night and builds nests of vegetation that are suspended among reeds about 1 m (3.3 ft) above the ground. It is an excellent swimmer and dives well, but can also climb in vegetation. An omnivore, it eats both plant and animal food, including seeds and insects. It breeds throughout the year; females give birth to about four young after a pregnancy of 21 to 28 days. The species may be infected by several different parasites and by two hantaviruses. The species was first described in 1877, the first of many related species from the region described until the 1910s. In 1918, Edward Alphonso Goldman consolidated most into the single species Oryzomys couesi and in 1960 Raymond Hall united this taxon with its United States relative, the marsh rice rat (O. palustris), into a single widespread species; subsequently, many related, localized species retained by Goldman were also included in this taxon. After studies of the contact zone in Texas, where O. couesi and the marsh rice rat meet, were published in 1979 and underscored the distinctness of the two, they were again regarded as separate. Since then, some of the peripheral forms of the group, such as Oryzomys antillarum from Jamaica and Oryzomys peninsulae from the Baja California Peninsula, have been reinstated as species. Nevertheless, O. couesi as currently constituted is likely a composite of several species; a 2010 study, using DNA sequence data, found evidence to recognize separate species from the Pacific and eastern sides of the distribution of O. couesi and two additional species from Panama and Costa Rica. Generally, Oryzomys couesi is common and of no conservation concern, and it is even considered a plague species in places, but some populations are threatened. Oryzomys couesi and at least six more narrowly distributed species with peripheral distributions together form the O. couesi group within the genus Oryzomys. The eighth species of the genus, the marsh rice rat (O. palustris) is the only member of its own group (unless western populations are classified as a separate species, O. texensis). Oryzomys previously included many other species, which were reclassified in various studies culminating in contributions by Marcelo Weksler and coworkers in 2006 that removed more than forty species from the genus. All are placed in the tribe Oryzomyini ('rice rats'), a diverse assemblage of over a hundred species, and on higher taxonomic levels in the subfamily Sigmodontinae of the family Cricetidae, along with hundreds of other species of mainly small rodents. Edward Alston first described Oryzomys couesi in 1877, using three specimens from Mexico and Guatemala. He named the animal Hesperomys couesi, placing it in the now-defunct genus Hesperomys, and noted similarities to the marsh rice rat (then called Hesperomys palustris) and two species now placed in Tylomys. The specific name, couesi, honors American naturalist Elliott Coues, who had done much work on North American rodents. In 1893, Oldfield Thomas wrote that the species, by then placed in the genus Oryzomys as Oryzomys couesi, had caused much confusion about its identity, because the three specimens (one from Cobán, Guatemala, and two from Mexico) used by Alston in fact belonged to two or three different species. He restricted the name couesi to the animal from Guatemala, and introduced the new name Oryzomys fulgens for one of the Mexican animals. Several other related species were described from the early 1890s onwards and in 1901 Clinton Hart Merriam united many of those into a palustris-mexicanus group of species, which also included the marsh rice rat. Edward Alphonso Goldman revised North American Oryzomys in 1918 and consolidated many forms into a single species Oryzomys couesi, with ten subspecies distributed from southern Texas and western Mexico south to Costa Rica. He placed it in an Oryzomys palustris group with the marsh rice rat and several species with more limited distributions, which he regarded as related to O. couesi but distinctive enough to be classified as separate species. In the 1930s, a few more forms related to O. couesi were described. As then recognized, the ranges of the marsh rice rat, a United States species, and Oryzomys couesi meet in southern Texas. In 1960, Raymond Hall reviewed specimens from this contact zone and found no grounds on which to separate the two species; thus, he reduced O. couesi to a subspecies of the marsh rice rat. Other workers continued this lumping and by 1971 all other species Goldman had placed in the O. palustris group were classified under the marsh rice rat, together with Oryzomys azuerensis from Panama, described as a species in 1937. Additional studies of the palustris–couesi contact zone in Texas published in 1979, using more specimens and characters, indicated that the two species are in fact easily distinguishable there; therefore, O. couesi has since been regarded as a species distinct from the marsh rice rat. Afterward, some of the other forms synonymized under O. couesi or O. palustris were resurrected as separate species—Oryzomys nelsoni from the Marías Islands, western Mexico, and Oryzomys antillarum from Jamaica. In 2009, Michael Carleton and Joaquin Arroyo-Cabrales reviewed western Mexican Oryzomys, reaffirmed the distinctness of O. nelsoni, and reinstated O. peninsulae from the tip of the Baja California Peninsula and O. albiventer from interior Mexico as species. Still, O. couesi included 22 synonyms, and Carleton and Arroyo-Cabrales wrote that further research on O. couesi and related species would certainly result in the recognition of additional species. A 2010 study by Delton Hanson and colleagues used DNA sequence data from the mitochondrial gene cytochrome b (Cytb) and two nuclear markers, exon 1 of the interphotoreceptor retinoid-binding protein gene (Rbp3) and intron 2 of alcohol dehydrogenase gene 1 (Adh1-I2) to study relationships among populations of the marsh rice rat and O. couesi. The Cytb data placed all studied specimens of O. couesi in a clade sister to the marsh rice rat; the mean genetic distance between the two groups was 11.30%, much larger than the distance between sister species in the related genera Melanomys and Nectomys (7.48% and 7.52%, respectively). Within the O. couesi clade, two populations from Panama and Costa Rica were successively basal to the other specimens, which fell into two large subclades—one containing animals from the Pacific seaboard from western Mexico to El Salvador and the other containing rats from the eastern seaboard from Texas to Nicaragua. The Panamanian and Costa Rican populations differed by 6.53% to 11.93% from the others and the western and eastern subclades differed by 4.41% on average. Data from both of the slower-evolving nuclear markers Rbp3 and Adh1-I2 also placed examples of Oryzomys in two main clades, but did not recover the western and eastern groups of O. couesi as separate clades. In addition, Adh1-I2 placed the Costa Rican population within the marsh rice rat clade and placed some western O. couesi specimens closer to the marsh rice rat than to the O. couesi group. The combined dataset supported the western and eastern clades within O. couesi and placed the Costa Rican population marginally closer to the marsh rice rat than to O. couesi. Using the genetic species concept, the authors suggested that the four groups they found within O. couesi should be recognized as distinct species. If this suggestion is followed, the eastern subclade would retain the name Oryzomys couesi, the western group would be named Oryzomys mexicanus, and the appropriate names for the Panamanian and Costa Rican species remain unclear. Populations of Oryzomys couesi from Jalisco, western Mexico, east to El Salvador form a single Cytb clade, which Hanson and colleagues proposed to recognize as the species Oryzomys mexicanus. These animals differ by 4.4% from Oryzomys couesi in the strict sense, which occurs to the north and east, are separated by mountain ranges from the latter, harbor different species of hantavirus, and according to Merriam (1901) have more robust skulls, with larger molars, stronger zygomatic arches (cheekbones), and better developed ridges along the margins of the interorbital region of the skull (between the eyes). Within the 'Oryzomys mexicanus' clade, Cytb sequence differences average 2.06% and western (Jalisco to Oaxaca) and eastern (Chiapas and El Salvador) groups form distinct subclades; Hanson and colleagues recognized these as different subspecies, mexicanus in the west and zygomaticus in the east.