Oryzomys Couesi - Ecology and Behavior

Ecology and Behavior

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 2300 m (7500 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 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.