Oryzomys Couesi - Taxonomy - Texas To Nicaragua

Texas To Nicaragua

Taxonomic synonyms
  • Oryzomys jalapae apatelius
  • Oryzomys aquaticus
  • Hesperomys couesi
  • Oryzomys cozumelae
  • Oryzomys goldmani
  • Oryzomys jalapae
  • Oryzomys mexicanus peragrus
  • Oryzomys couesi pinicola
  • Oryzomys richardsoni
  • Oryzomys richmondi
  • Oryzomys jalapae rufinus
  • Oryzomys teapensis

Oryzomys populations from Texas to Nicaragua form a single Cytb clade, within which the average sequence divergence is 1.28%, and Hanson and colleagues proposed that the name Oryzomys couesi be restricted to this clade. These populations correspond to two subspecies recognized by Goldman (O. c. aquaticus and O. c. couesi) and an island form he retained as a species (O. cozumelae). Two other subspecies Goldman recognized, O. c. richmondi and O. c. peragrus, and a third, O. c. pinicola, that was described after Goldman's paper occur in the same region, but have not been studied genetically.

Skull of Oryzomys from Brownsville, Texas (aquaticus Allen, 1891)

The northernmost populations of Oryzomys couesi, those in southernmost Texas and nearby Tamaulipas, Mexico, are classified as the subspecies aquaticus, which was described as a separate species, Oryzomys aquaticus, in 1891. Here the range of O. couesi meets that of the marsh rice rat; in parts of Kenedy, Willacy and Cameron counties, Texas, and in far northeastern Tamaulipas, the two are sympatric (occur in the same places). In the contact zone, couesi occurs further inland, while the marsh rice rat lives along the coast. In experimental conditions, the two fail to interbreed and genetic analysis yields no evidence of gene flow or hybridization in the wild. Compared to populations further to the south, aquaticus is larger and paler and has a more robust skull. Specimens from Tamaulipas are slightly darker than those from Texas. The Cytb sequences of specimens of aquaticus form a separate group, but cluster among specimens of O. c. couesi from further south.

The form peragrus is known from further south in Mexico, in the Río Verde basin of San Luis Potosi, the state of Hidalgo, and far northern Veracruz. Late Pleistocene fossils of this form have been found in Cueva de Abre, Tamaulipas. According to Goldman, it is intermediate in color between O. c. aquaticus and O. c. couesi, but has a skull similar to that of aquaticus.

Goldman united populations ranging from northern Veracruz through eastern Mexico, Guatemala, Honduras, and Nicaragua south to far northwestern Costa Rica in the nominate subspecies, Oryzomys couesi couesi. He placed six other names as full synonyms of this form, which has its type locality in Guatemala—Oryzomys jalapae Allen and Chapman, 1897, from Veracruz; Oryzomys jalapae rufinus Merriam, 1901, from Veracruz; Oryzomys teapensis Merriam, 1901, from Tabasco; Oryzomys goldmani Merriam, 1901, from Veracruz; Oryzomys jalapae apatelius Eliot, 1904, from Veracruz; and Oryzomys richardsoni Allen, 1910, from Nicaragua. According to Goldman, individual variation within the subspecies is large, which has led to the large number of published synonyms, but populations from all parts of its range are essentially similar.

Skull of Oryzomys from Cozumel, Mexico (cozumelae Merriam, 1901)

The subspecies Oryzomys couesi pinicola was described in 1932 from a pine ridge in western British Honduras (now Belize); it is smaller and darker than nominate couesi, which also occurs in Belize, and has a more delicate skull. In 1901, Merriam described the Oryzomys of the island of Cozumel as a separate species, Oryzomys cozumelae, and Goldman kept it as such because of its large size, dark fur, and long tail. In 1965, however, Knox Jones and Timothy Lawlor judged the differences between cozumelae and mainland couesi trivial and found that cozumelae was inside the range of variation of mainland Oryzomys populations; accordingly, they demoted the island form to a subspecies. Mark Engstrom and colleagues, writing in 1989, reaffirmed this conclusion. For an island form, this population is highly genetically variable. In its Cytb sequence data, it falls among populations of nominate couesi. Oryzomys couesi is also found on Turneffe Atoll off the coast of Belize and Roatán off Honduras.

The Oryzomys of the eastern lowlands of Nicaragua was described as a separate species, Oryzomys richmondi, by Merriam in 1901, and Goldman retained it as a subspecies of O. couesi on the basis of its distinctly dark fur. In reviewing Nicaraguan Oryzomys in 1986, Jones and Engstrom did not keep richmondi as separate, because they thought the difference in color too small for the recognition of subspecies. Oryzomys dimidiatus, a small, dark Oryzomys with gray underparts, occurs with O. couesi in southeastern Nicaragua. According to Jones and Engstrom, rice rats from the island of Ometepe in Lake Nicaragua are distinctive in their large skull and small external measurements, with an especially short tail, soft fur that is orange-brown above and buffish below, and lack of sphenopalatine vacuities (openings in the roof of the mesopterygoid fossa, the gap behind the end of the bony palate). They considered that this population probably represented a separate subspecies, but declined to propose a new name because they had only one adult specimen. In Nicaragua, O. couesi occurs up to an altitude of 1250 m (4100 ft).

Read more about this topic:  Oryzomys Couesi, Taxonomy

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