Oryzomys dimidiatus

Oryzomys dimidiatus: A Mysterious Rice Rat

Imagine stumbling upon a creature so enigmatic that it has eluded scientific classification for over a century. That’s the story of Oryzomys dimidiatus, also known as Thomas’s rice rat or the Nicaraguan rice rat. First discovered in 1904, this rodent has been a puzzle to biologists ever since.

Discovery and Classification

Was it really that hard to classify? In 1905, Oldfield Thomas described the first known specimen as Nectomys dimidiatus. However, in 1948, Philip Hershkovitz took a closer look and placed it in its own subgenus of Oryzomys. This decision was based on the distinct characteristics that set this species apart from others.

Physical Characteristics

What does this rice rat look like? Oryzomys dimidiatus is a medium-sized rodent, with gray-brown upperparts and grayish underparts. Its tail is only slightly darker above than below, giving it a unique appearance. The species is semiaquatic, spending time in water—much like its cousin, the Oryzomys palustris.

Measurements

The head and body length of this rice rat ranges from 110 to 128 mm, making it a medium-sized rodent. The tail is about as long as the head and body combined, with about 15 rings per centimeter. Its hindfoot measures between 27 and 28 mm, while its ear length varies between 13 and 15 mm.

Weight

The weight of Oryzomys dimidiatus is around 46 grams, with testes measuring about 11 mm in length. The species has a short snout, well-haired ears hidden by fur, and off-white or brownish hands and feet.

Conservation Status

Is this rice rat safe from extinction? Currently, Oryzomys dimidiatus is assessed as ‘Data Deficient’ due to the limited availability of information about its conservation status. The species occurs in southeastern Nicaragua and has been found near streams and dense cane.

Specimens

The first specimen was collected on November 5, 1904, by W. G. Palmer. A second specimen was obtained in 1966 and published in 1971 by Hugh Genoways and Knox Jones. Fiona Reid reported a third specimen found in 1997.

Systematics

How does this rice rat fit into the family tree? Marcelo Weksler removed most species from Oryzomys in 2006, keeping O. dimidiatus as part of the genus. The species is recognized as one of eight species in the genus and is further classified within the O. couesi section.

Skull Characteristics

The skull of Oryzomys dimidiatus is lightly built with narrow nasals and a broad braincase. Its zygomatic plate is broad, and the incisive foramina are broadest in their back halves. The mesopterygoid fossa is perforated by sphenopalatine vacuities.

Molar Crowns

The molar crowns of this rice rat have smaller additional roots, adding to its unique characteristics.

Conclusion

Oryzomys dimidiatus is a fascinating creature that continues to intrigue scientists with its elusive nature. Despite being discovered over a century ago, there’s still much we don’t know about this semiaquatic rodent. As conservation efforts continue, perhaps more will be revealed about the life and habits of this mysterious rice rat.