In addition to whisker length and fur color, several other morphological differences distinguish the two, including the wider first upper molar in T. bolivaris.

Species of Hylaeamys and Euryoryzomys also differ from Transandinomys in some details of the skull and teeth and have shorter whiskers.

Species of Transandinomys live on the ground, are active during the night, eat both plant and animal matter, and construct nests of vegetation.

[8] In 2006, Marcelo Weksler published a broad phylogenetic analysis of Oryzomyini, the tribe to which Oryzomys and related genera belong, using morphological data and DNA sequences from the IRBP gene.

[9] Species of Oryzomys included in Weksler's study did not cluster together in his results, but instead appeared dispersed across Oryzomyini, indicating that the genus was polyphyletic and should be split up.

[11] Oryzomyini is one of several tribes within the subfamily Sigmodontinae of the family Cricetidae, which includes hundreds of other species of mainly small rodents, distributed chiefly in Eurasia and the Americas.

[15] Euryoryzomys species are in general slightly larger[16] and Hylaeamys are as large as Transandinomys, so that the only feature of external morphology that distinguishes the two genera is the length of the vibrissae.

[18] The fur is brownish (T. bolivaris) or reddish (T. talamancae) above and lighter below, appearing whitish, but the hairs on the underparts have gray bases.

The mesopterygoid fossa, the gap behind the back margin of the palate, does not usually reach forward between the maxillaries.

Unlike in many other rice rats, including Handleyomys alfaroi and Euryoryzomys species, the mesoflexus on the second upper molar, which separates the paracone (one of the main cusps) from the mesoloph (an accessory crest), is not divided in two by an enamel bridge.

[26] On the upper third molar, the hypoflexus (which in oryzomyines ranges from a slight indentation on the lingual, or inner, side of the tooth to a conspicuous valley between the main cusps)[27] is more developed in Euryoryzomys than in Transandinomys.

[28] The hypoflexid on the second lower molar, the main valley between the cusps, extends more than halfway across the crown; it is much shorter in H. alfaroi, Euryoryzomys, and Hylaeamys yunganus.

[29] Each of the upper molars has three roots (two at the labial, or outer, side and one at the lingual, or inner, side) and each of the lowers has two (one at the front and one at the back); both species of Transandinomys lack the additional small roots that are present in various other oryzomyines, including species of Euryoryzomys, Nephelomys, and Handleyomys.

[30] The distribution of Transandinomys extends from eastern Honduras through Nicaragua, Costa Rica, Panama, and Colombia south and east to southwestern Ecuador and northern Venezuela, generally west and north of the Andes.

T. bolivaris occurs from Honduras mainly on the Caribbean side of Central America south to western Colombia and northwestern Ecuador, up to 1800 m (5900 ft) above sea level.

[35] Both species live on the ground, are active during the night, are solitary, and feed mainly on fruits and seeds, but may also eat herbs and insects.