Transandinomys talamancae is a rodent in the family Cricetidae that occurs from Costa Rica to southwestern Ecuador and northern Venezuela.

The species was first described in 1891 by Joel Asaph Allen and thereafter a variety of names, now considered synonyms, were applied to local populations.

It was lumped into a widespread species "Oryzomys capito" (now Hylaeamys megacephalus) from the 1960s until the 1980s and the current allocation of synonyms dates from 1998.

It shares this genus with Transandinomys bolivaris, which has even longer vibrissae; the two overlap broadly in distribution and are morphologically similar.

[17] In the same year, Wirt Robinson and Markus Lyon named Oryzomys medius from near La Guaira, Venezuela.

He placed both panamensis and carrikeri as synonyms of Oryzomys talamancae and mentioned O. mollipilosus and O. medius as closely related species.

O. talamancae was the only member of its own species group, which Goldman regarded as closest to Oryzomys bombycinus (=Transandinomys bolivaris).

[23] They also examined the holotypes of panamensis, carrikeri, mollipilosus, medius, and magdalenae and identified them as examples of Oryzomys talamancae.

[25] In 2006, Marcelo Weksler published a phylogenetic analysis of Oryzomyini ("rice rats"), the tribe to which Oryzomys is allocated, using morphological data and DNA sequences from the IRBP gene.

[54] The skull has a long rostrum (front part), a broad interorbital region (between the eyes), and a low braincase.

[31] The incisive foramina (openings in the front part of the palate) are short and do not reach between the first molars;[40] they are longer in H. alfaroi.

[37] The bony palate is long and extends beyond the end of the molar row and the back margin of the maxillary bones.

[61] The sphenopalatine vacuities (openings in the roof of the mesopterygoid fossa, behind the palate) are also small,[31] as are the auditory bullae.

[31] The mental foramen, located in the diastema between the lower incisor and the first molar, opens towards the side, as usual in oryzomyines.

[64] The upper and lower masseteric ridges, which anchor some of the chewing muscles, do not join into a single crest and extend forward to below the first molar.

In this species, but unlike in many other rice rats, including H. alfaroi and E. nitidus, 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.

[63] The hypoflexid on the second lower molar, the main valley between the cusps, is very long, extending more than halfway across the tooth; in this trait, the species is again similar to T. bolivaris but unlike H. alfaroi.

[68] 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 lower molars has two (one at the front and one at the back); T. talamancae lacks the additional small roots that are present in various other oryzomyines, including species of Euryoryzomys, Nephelomys, and Handleyomys.

[80] The karyotype of an Ecuadorean sample from north of the Gulf of Guayaquil is similar to that of Venezuelan animals at 2n = 36, FN = 60; it includes four acrocentric and two subtelocentric pairs and no submetacentrics.

Musser and colleagues termed the difference between the two Ecuadorian forms "impressive"[80] and noted that further research was needed to understand the karyotypic differentiation within the species more fully.

[83] The distribution of Transandinomys talamancae extends from northwestern Costa Rica south and east to northern Venezuela and southwestern Ecuador, up to 1,500 m (5,000 ft) above sea level.

[85] Transandinomys talamancae reaches the northern limit of its range in Costa Rica, but except for one record from the far northwest (in Guanacaste Province near the southern margin of Lake Nicaragua), it is known only from the southeastern third of the country.

Along the Pacific coast in Colombia and Ecuador, it is found on the coastal plain and the adjacent foothills of the Andes.

[43] There is a record from the Orinoco Delta of northeastern Venezuela, well within the range of Hylaeamys megacephalus, but Musser and colleagues suggest that this is based on mislabeled specimens.

Hylaeamys perenensis[Note 6] does, however, occur further south along the eastern foothills of the Cordillera Oriental in Colombia and it is possible that the two overlap in this area.

[90] The animal nests above ground level and occasionally enters burrows also used by the pocket mouse Liomys adspersus.

[91] Its diet is omnivorous: including both plant material such as seeds and fruits; and adult and larval insects.

[93] Fleming estimated that population densities reached peaks of up to 4.3 per ha (1.7 per acre) late in the rainy season (October–November), but dropped to near zero around June; however, these figures may well be underestimates.

[99] Animals become sexually mature when less than two months old; in Fleming's study, some females in juvenile fur, probably less than 50 days old, were already pregnant.

[103] Ten species of mites (Gigantolaelaps aitkeni,[104][Note 7] Gigantolaelaps gilmorei, Gigantolaelaps oudemansi, Gigantolaelaps wolffsohni, Haemolaelaps glasgowi, Laelaps dearmasi, Laelaps pilifer, Laelaps thori, Mysolaelaps parvispinosus,[110] and Paraspeleognathopsis cricetidarum),[111] thirteen chiggers (Aitkenius cunctatus,[112] Ascoschoengastia dyscrita, Eutrombicula alfreddugesi, Eutrombicula goeldii, Intercutestrix tryssa, Leptotrombidium panamensis, Myxacarus oscillatus, Pseudoschoengastia abditiva, Pseudoschoengastia bulbifera, Trombicula dunni, and Trombicula keenani),[113] and four fleas (Jellisonia sp., Polygenis roberti, Polygenis klagesi, and Polygenis dunni) have been found on T. talamancae in Panama.