Coues's rice rat
Oryzomys couesi, also known as Coues's 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.
Several common names have been proposed for Oryzomys couesi and the synonyms currently associated with it. Eliot in 1905 and Goldman in 1918 gave separate common names for each of the species and subspecies they recognized. Many authors have used "Coues' Rice Rat" or some variation thereof for O. couesi, but "Coues' Oryzomys" has also been used.
Oryzomys couesi is a medium-sized to large rat with coarse fur that is buff to reddish above, becoming paler towards the sides and cheeks and darker on the rump and face. The underparts are white to buff. The fur is shorter, brighter, and more intense in color than in the marsh rice rat. The snout ends bluntly and the moderately large eyes show reddish eyeshine. The small ears are black on the outside and the inside is covered with short, gray to buff or red hairs. The long tail is dark brown above and white to light brown below. The feet are long and stout. On the forefeet, the ungual tufts (tufts of hair on the digits) are present. Many of the pads on the hindfeet are reduced, as are the ungual tufts, and small interdigital webs may be present in at least some specimens. Some of these traits are common adaptations to life in the water in oryzomyines. As in most other oryzomyines, females have eight mammae. Head and body length is 98 to 142 mm (3.9 to 5.6 in), tail length is 107 to 152 mm (4.2 to 6.0 in), hindfoot length is 27 to 33 mm (1.1 to 1.3 in), ear length is 13 to 18 mm (0.51 to 0.71 in), and body mass is 43 to 82 g (1.5 to 2.9 oz). Studies in Texas and El Salvador found that males are slightly larger than females.
The stomach has the characteristic pattern of sigmodontines (unilocular-hemiglandular): it is not split in two chambers by an incisura angularis and the front part (antrum) is covered by a glandular epithelium. The gall bladder is absent, a synapomorphy (shared-derived character) of Oryzomyini. The karyotype includes 56 chromosomes and a fundamental number of 56 autosomal arms (2n = 56, FNa = 56). The autosomes include 26 pairs of acrocentric chromosomes, with a long and a very short arm, and one medium-sized submetacentric pair, with one arm shorter than the other. The X chromosome is either acrocentric, with a long and a short arm, or subtelocentric, with a long and a vestigial arm. The form of the sex chromosomes has been used to distinguish the marsh rice rat from Oryzomys couesi, but there are no consistent differences between the two.
As is characteristic of Sigmodontinae, Oryzomys couesi has a complex penis, with the baculum (penis bone) ending in three cartilaginous digits at its tip. The outer surface of the penis is mostly covered by small spines, but there is a broad band of nonspinous tissue. The papilla (nipple-like projection) on the dorsal (upper) side of the penis is covered with small spines, a character Oryzomys couesi shares only with Oligoryzomys and the marsh rice rat among oryzomyines examined. On the urethral process, located in the crater at the end of the penis, a fleshy process (the subapical lobule) is present; it is absent in all other oryzomyines with studied penes except the marsh rice rat and Holochilus brasiliensis.
The distribution of Oryzomys couesi extends from southern Texas and central Sonora, but not the central plateau of Mexico, through Central America south and east to northwestern Colombia; see under "Taxonomy" for details. The species has also been found in late Pleistocene cave deposits in Mexico and Honduras. It is common in watery habitats, such as marshes and small streams, but also occurs in forests and shrublands with sufficient cover. In addition, it is found in sugarcane and rice fields. In Texas, it occurs in marsh vegetation along resacas (oxbow lakes) and in Veracruz, it has even been found on the dry coastal plain among shrubs. It occurs from 2,300 m (7,500 ft) altitude down to sea level. On Cozumel, the proportion of juveniles and females is higher near roads that function as habitat edges. Cozumel rice rats rarely cross roads, which may isolate subpopulations on the island.
Oryzomys couesi lives on the ground and is semiaquatic, spending much time in the water, as Alston in his original description already recognized, but is also a good climber. A study in Costa Rica found that O. couesi is an excellent swimmer, diving well and using its tail to propel itself. It is probably able to forage underwater, which may help differentiate its niche from that of the ecologically similar cotton rat Sigmodon hirsutus, which also swims well, but does not dive. When disturbed, O. couesi will enter the water and swim away. It is primarily active during the night. Oryzomys couesi builds globular nests of woven vegetation suspended among reeds, about 1 m (3.3 ft) above the water or the ground; in Texas, larger individuals make larger nests. It does not usually make its own runways in vegetation, but may use those of other rodents, such as cotton rats.
Population densities range from 5 to 30 per ha (2 to 12 per acre). On Cozumel, density is around 14.5 to 16.5 per ha (5.9 to 6.7 per acre), but shows large seasonal variation. In western Mexico, one study found densities of 3 per ha (1.2 per acre) in cloud forest and 1 per ha (0.4 per acre) in a disturbed area. In 24 hours, male Texas O. couesi move up to 153 m (502 ft) and females up to 126 m (413 ft). The diet includes both plant material, including seeds and green parts, and animals, including small fish, crustaceans, snails, insects like ants and beetles, and other invertebrates. It probably breeds around the year and after a pregnancy of 21 to 28 days, the female produces litters of two to seven young, with an average of 3.8, according to Reid's Mammals of Central America & Southeast Mexico. In 28 pregnant females from Nicaragua, litter size varied from one to eight, averaging 4.4. The young become reproductively active when seven weeks old and the life cycle is short.
The introduced snake Boa constrictor preys on O. couesi on Cozumel. Parasites recorded on O. couesi in Veracruz include unidentified ticks, mites, fleas, and fly larvae. The flea Polygenis odiosus was found on an Oryzomys couesi from Cozumel. Out of ten O. couesi in San Luis Potosí, five each were infected by the nematode worms Hassalstrongylus musculi and H. bocqueti, with about 25 worms per rat, and two were infected by one or two cestodes of the genus Raillietina. The mites Eubrachylaelaps circularis and Gigantolaelaps boneti have been found on Oryzomys couesi in Oaxaca, the sucking louse Hoplopleura oryzomydis in Nicaragua, the mites Laelaps oryzomydis, Echinonyssus microchelae, Ornithonyssus bacoti, Prolistrophorus frontalis, and Prolistrophorus bakeri in Colima, and the apicomplexan Eimeria couesii in Mexico. The species is infected by two hantaviruses—Catacamas virus in Honduras and Playa de Oro virus in western Mexico—which are related to the Bayou virus infecting the marsh rice rat, a common cause of hantavirus infections in the United States. No hantavirus infections in humans have been linked to O. couesi hantaviruses, however. Chiapas O. couesi easily survive experimental infection with several arboviruses, including the Venezuelan equine encephalitis virus, suggesting that the species may serve as a reservoir for that virus.
The IUCN lists Oryzomys couesi as "Least Concern", because it is a widely distributed, common species with broad habitat tolerance that occurs in many protected areas. Habitat destruction, such as drainage of wetlands, may threaten some populations. In many areas, it is so common that it is considered a plague species. Populations even persist in the Valley of Mexico, as evidenced by a photograph published in 2006. However, the species is listed as threatened in Texas, where its distribution is very limited, because of habitat loss. In 1979, Benson and Gehlbach estimated the size of the Texas population to be about 15,000. A 2001 study predicted that climate change would drive the Texas population to extinction, because no suitable habitats would continue to exist. The Cozumel population has declined substantially since the mid-1980s, perhaps due to habitat disturbance and predation by introduced species.
