Transandinomys talamancae is a rodent in the family Cricetidae that occurs from Costa Rica to southwestern Ecuador and northern Venezuela. Its habitat consists of lowland forests up to 1,500 m (5,000 ft) above sea level. With a body mass of 38 to 74 g (1.3 to 2.6 oz), it is a medium-sized rice rat. The fur is soft and is reddish to brownish on the upperparts and white to buff on the underparts. The tail is dark brown above and lighter below and the ears and feet are long. The vibrissae (whiskers) are very long. In the skull, the rostrum (front part) is long and the braincase is low. The number of chromosomes varies from 34 to 54.
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 was placed in the genus Oryzomys until 2006, as Oryzomys talamancae, but is not closely related to the type species of that genus and was therefore moved to a separate genus Transandinomys in 2006. It shares this genus with Transandinomys bolivaris, which has even longer vibrissae; the two overlap broadly in distribution and are morphologically similar.
Active during the night, Transandinomys talamancae lives on the ground and eats plants and insects. Males move more and have larger home ranges than most females. It breeds throughout the year, although few individuals survive for more than a year. After a gestation period of about 28 days, two to five young are born, which reach sexual maturity within two months. A variety of parasites occur on this species. Widespread and common, it is of no conservation concern.
Transandinomys talamancae is a medium-sized, brightly colored rice rat. It is similar to T. bolivaris and the two are often confused. They are about as large, but in T. talamancae the tail is longer and the hindfeet shorter. Both species share uniquely long vibrissae, with both the mystacial (above the mouth) and superciliary vibrissae extending to or beyond the back margin of the ears when laid back against the head, but those in T. bolivaris are substantially longer. H. alfaroi, a widespread species ranging from Mexico to Ecuador, is also similar. It is smaller and darker, but young adult T. talamancae are similar in color to adult H. alfaroi and often misidentified. Hylaeamys megacephalus, with which T. talamancae was synonymized for some decades, is similar in body size, but is not known to overlap with T. talamancae in range.
The fur is short, dense and soft in Transandinomys talamancae ; in T. bolivaris, it is longer and even more soft and dense. The color of the upperparts varies from reddish to brownish, becoming lighter towards the sides and the cheeks. The underparts are white to buff, with the bases of the hairs plumbeous (lead-colored). The fur of T. bolivaris is darker: dark brown above and dark gray below. H. megacephalus also has darker fur. Juveniles have thin, gray fur, which is molted into the dark brown subadult fur when the animal is about 35 to 40 days old. This fur is replaced by the bright adult fur at age 49 to 56 days. Juveniles are never blackish as in T. bolivaris. The ears are dark brown, large, and densely covered with very small hairs.
The sparsely haired tail is approximately as long as the head and body. It is dark brown above and lighter below. In contrast, the tail of H. megacephalus has little to no difference in color between the upper and lower surface. In 2006, Weksler and colleagues noted tail coloration as a difference between the two species of Transandinomys (bicolored in T. talamancae and unicolored in T. bolivaris ), but in their 1998 study, Musser and colleagues could not find differences in tail coloration between their Panamanian samples of the two species.
The hindfeet are long and have the three central digits longer than the two outer ones. They are white to pale yellow above, where the foot is covered with hairs, which are longer than in T. bolivaris. The digits of the hindfeet are surrounded by ungual tufts of silvery hair that are longer than the claws themselves. The claws are short and sharp. Parts of the sole are covered by indistinct scales (squamae), which are usually entirely absent in T. bolivaris. The pads are moderately large.
The length of the head and body is 105 to 151 mm (4.1 to 5.9 in), tail length 105 to 152 mm (4.1 to 6.0 in), hindfoot length 26 to 32 mm (1.0 to 1.3 in), ear length 17 to 24 mm (0.67 to 0.94 in), and body mass 38 to 74 g (1.3 to 2.6 oz). As in most oryzomyines, females have eight mammae. There are 12 thoracic vertebra with associated ribs, 7 lumbars, and 29 caudals; a pair of supernumerary (additional) ribs is occasionally present.
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. It is a forest species and occurs in both evergreen and deciduous forest. Although its distribution broadly overlaps that of T. bolivaris, it is more widely distributed in South America because of its greater tolerance of dry forest habitats.
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. In contrast, T. bolivaris and H. alfaroi occur further north, into Honduras and Mexico respectively. It occurs throughout Panama at low elevations. Along the Pacific coast in Colombia and Ecuador, it is found on the coastal plain and the adjacent foothills of the Andes. The southernmost known record is in far southwestern Ecuador, but the species may range into nearby Peru.
It also occurs throughout northern Colombia at low elevations and into western Venezuela west of Lake Maracaibo and at the foot of the western part of the Venezuelan Coastal Range east to Guatopo National Park. Hylaeamys megacephalus occurs further to the east in the eastern portion of the coastal range, separated by the coastal Eastern Caribbean Dry Zone. 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. The species has also been found on the narrow strip between the Llanos and the Andes (Cordillera Oriental and Cordillera de Mérida) in eastern Colombia and northwestern Venezuela. The unforested Llanos separate these areas from Hylaeamys populations. Hylaeamys perenensis 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.
Transandinomys talamancae is a common, even abundant, species. Its ecology was studied by Theodore Fleming in the Panama Canal Zone. It lives on the ground and is active during the night. The animal nests above ground level and occasionally enters burrows also used by the pocket mouse Liomys adspersus. Its diet is omnivorous: including both plant material such as seeds and fruits; and adult and larval insects.
Males tend to travel longer distances than females. The average home range size in Fleming's study was 1.33 hectares (3.3 acres); males had larger home ranges on average. Specimens that were once captured tended to be captured more frequently than those that had never been captured. 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. In central Venezuela, population densities vary from 5.5 to 9.6 per ha (2.2 to 3.8 per acre).
In Panama, this species breeds year-round without apparent seasonal variability. According to Omar Linares's Mamíferos de Venezuela (Mammals of Venezuela ), reproductive activity is highest in June–July and December. In the laboratory, the gestation period is 28 days; Linares reports 20 to 30 days in the wild. Females produce an average of six litters per year and there are two to five (average 3.92) young per litter, so that a single female may produce about 24 young per year; this is likely an overestimate because most females would not live for a full year. Larger females may have larger litters. 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. The oldest specimen Fleming observed was nine months old; he estimated that animals were unlikely to live for more than a year in the wild and that the mean age at death was 2.9 months.
Ten species of mites (Gigantolaelaps aitkeni, Gigantolaelaps gilmorei, Gigantolaelaps oudemansi, Gigantolaelaps wolffsohni, Haemolaelaps glasgowi, Laelaps dearmasi, Laelaps pilifer, Laelaps thori, Mysolaelaps parvispinosus, and Paraspeleognathopsis cricetidarum ), thirteen chiggers (Aitkenius cunctatus, Ascoschoengastia dyscrita, Eutrombicula alfreddugesi, Eutrombicula goeldii, Intercutestrix tryssa, Leptotrombidium panamensis, Myxacarus oscillatus, Pseudoschoengastia abditiva, Pseudoschoengastia bulbifera, Trombicula dunni, and Trombicula keenani ), and four fleas (Jellisonia sp., Polygenis roberti, Polygenis klagesi, and Polygenis dunni ) have been found on T. talamancae in Panama. G. aitkeni has also been found on this species in Colombia. In addition, the sucking lice Hoplopleura nesoryzomydis and Hoplopleura oryzomydis occur on T. talamancae.
A widespread and common species, Transandinomys talamancae is listed as "Least Concern" by the IUCN Red List. It occurs in numerous protected areas, tolerates disturbed habitats well, and no important threats are known.
