Transandinomys bolivaris


Transandinomys bolivaris, also known as the long-whiskered rice rat, is a rodent in the genus Transandinomys. It is found in humid forest from northeastern Honduras to western Ecuador, up to above sea level. Since it was first described in 1901 from Ecuador, six scientific names have been introduced for it, but their common identity was not documented until 1998 and the species has long been known under the name Oryzomys bombycinus, described from Panama in 1912. The name Oryzomys bolivaris was used before it was moved to the new genus Transandinomys with Transandinomys talamancae in 2006.
It is a medium-sized rice rat distinguished by its very long vibrissae —those above the eyes are up to long. The fur, which is soft and dense, is usually dark brown above and light gray below; it is darker in juveniles. The feet are long and the tail is about as long as the head and body. The skull is narrow and has a broad interorbital region. The species generally lives on the ground. Although it is rare, its conservation status is thought to be secure.

Taxonomy

In 1901, Joel Asaph Allen described four new species of rice rat in the genus Oryzomys: three from Ecuador and the Peruvian Oryzomys perenensis. The three Ecuadorian species–Oryzomys bolivaris from Porvenir, Bolívar Province; Oryzomys castaneus from San Javier, Esmeraldas Province; and Oryzomys rivularis from Río Verde, Pichincha Province—were each based on a single specimen collected in 1899 or 1900. He distinguished the three on the basis of coloration, size, and relative tail length. Philip Hershkovitz listed all three among the many synonyms of "Oryzomys laticeps" in a 1960 paper.
Edward Alphonso Goldman described Oryzomys bombycinus in 1912 from four specimens from Panama. He compared it to Oryzomys talamancae and placed it with the "Oryzomys laticeps group". Three years later, he described Oryzomys nitidus alleni from Costa Rica as a subspecies of Oryzomys nitidus, without mentioning bombycinus. He revised Oryzomys of North America in 1918 and recognized Oryzomys bombycinus as the only member of its own group, with alleni as a subspecies distinguished by the proportions of the skull. He also mentioned that the group occurred in Ecuador and indicated that O. bombycinus probably reached Colombia. Goldman considered the group to be similar to O. talamancae, but suggested that bombycinus and alleni might only be subspecies of O. nitidus. In 1939, Oliver Pearson added a third subspecies, O. b. orinus, from eastern Panama, and in 1966 the species was first recorded from Colombia. Ronald Pine reviewed Oryzomys bombycinus in 1971, when 59 specimens of it were known, and first recorded the species from Nicaragua and Ecuador. He kept the three described subspecies—alleni from Nicaragua to western Panama, bombycinus from central Panama, and orinus from eastern Panama to Ecuador.
Alfred Gardner and James Patton suggested in 1976 that Allen's O. rivularis may be the same species as O. bombycinus. They considered O. bolivaris as probably the same as O. nitidus and listed castaneus as a synonym of O. capito. In 1984, Benshoof and colleagues reported the first record of Oryzomys bombycinus from Honduras. Guy Musser and Marina Williams reviewed O. talamancae in 1985 and included O. castaneus as one of its synonyms, though without having examined the holotype. In the 1993 second edition of Mammal Species of the World, Musser and Michael Carleton used the name Oryzomys bolivaris for the species previously known as O. bombycinus, and in 1998, Musser and colleagues fully documented the allocation of the names bolivaris, castaneus, rivularis, bombycinus, alleni, and orinus to the same species, Oryzomys bolivaris. They noted its similarity to O. talamancae, but did not attempt to determine phylogenetic relationships among the species they discussed. In their limited material, they found geographic variation within the species inconsequential and they recognized no subspecies.
In 2006, Marcelo Weksler published a phylogenetic analysis of Oryzomyini, the tribe to which Oryzomys is allocated, using morphological and DNA sequence data. His results showed species of Oryzomys dispersed across Oryzomyini and suggested that most species in the genus should be allocated to new genera. Later in the same year, he, together with Alexandre Percequillo and Robert Voss, named ten new genera for these species, including Transandinomys, which has Oryzomys talamancae as its type species. They also included Oryzomys bolivaris in Transandinomys, so that it is now named Transandinomys bolivaris, although it had not been included in Weksler's phylogenetic study. The two species are morphologically similar, but they could identify only one synapomorphy for them: very long superciliary vibrissae. Transandinomys is one of about 30 genera in Oryzomyini, a diverse assemblage of American rodents of over a hundred species, and on higher taxonomic levels in the subfamily Sigmodontinae of family Cricetidae, along with hundreds of other species of mainly small rodents.
Various authors have used the common name "long-whiskered rice rat" for this species, but several other names have been proposed. In 1918, Goldman named O. bombycinus bombycinus the "Long-Haired Rice Rat" and O. b. alleni the "Allen Rice Rat". Musser and Carleton, writing in the 2005 third edition of Mammal Species of the World, used "Long-whiskered Oryzomys", the 2009 IUCN Red List gave "Bolivar Rice Rat", and Thomas Lee and colleagues used "Long-whiskered Trans-Andean Rice Rat" in 2010.

Description

Transandinomys bolivaris is a medium-sized rice rat with very long superciliary vibrissae. According to Fiona Reid's Mammals of Southeastern Mexico & Central America, it is distinguishable from any similarly sized rice rats by the length of these whiskers; T. talamancae also has long superciliary vibrissae, but not as long as in T. bolivaris. In both species, the mystacial vibrissae are also long and extend beyond the ears when laid back, but they are again much longer in T. bolivaris. The vibrissae are mostly dark, but translucent at the tips. Handleyomys alfaroi, a rice rat with which young T. bolivaris are often confused, is much smaller. In T. bolivaris, the cheeks may be light gray,, or reddish. The ears are dark brown to black and are sparsely haired.
The fur, which is soft, dense, and thick, is dark brown to gray on the upperparts, grading to black on the midback and yellowish brown on the sides. The underparts are sharply different in color. There, the hairs are dark gray at their bases and white at the tips, so that the fur appears grayish white. The fur is shorter and darker than in T. talamancae and softer and thicker than in H. alfaroi. Young animals have darker, finer, and softer fur. Pine separated the subspecies alleni and orinus on the basis of their darker fur, but Musser and colleagues could not confirm this pattern and found paler and darker specimens within the same geographical regions.
The tail appears naked and is shorter than or about as long as the head and body; it is longer in T. talamancae. Its coloration is variable; it is dark brown above and at the sides and light brown and often white to a greater or smaller extent below, and in some specimens the tail has the same color above and below. The scales on the tail are smaller than in T. talamancae.
The forearms are dark gray. The forefeet are unpigmented and ungual tufts of white hairs surround the equally unpigmented claws. The animal has long, narrow hindfeet, longer than in T. talamancae, with usually smooth soles. The three middle digits are much longer than the outer two. Six pads are present on the sole. The upper surface and the sides of the hindfeet are white and appear naked, although short, white hairs are present; these hairs are longer in T. talamancae. Ungual tufts of long, white or gray hairs are present around the claws, which are short and lack pigment.
Head and body length is, tail length, hindfoot length, ear length, and body mass. Females have four pairs of mammae, as usual in oryzomyines. Like most rice rats, T. bolivaris has twelve thoracic and seven lumbar vertebrae. A study in Costa Rica found that there are 58 chromosomes, including many that are large and have two arms, and the fundamental number of arms is 80, a highly differentiated karyotype. The karyotype of T. talamancae is variable, but has fewer chromosomes and major arms. H. alfaroi has more chromosomes and major arms.

Skull and teeth

The skull is relatively long and has a long, narrow rostrum, broad interorbital region and narrow braincase with almost vertically oriented walls at the sides and behind. It differs from that of T. talamancae in various proportions. Although the subspecies previously recognized in "Oryzomys bombycinus" have been separated by small differences in skull features, Pine rejected these on the basis of his much larger samples. Musser and colleagues agreed, but noted that Colombian animals appeared to have larger skulls. The zygomatic plates are broad and the zygomatic arches behind them are nearly parallel to each other. The margins of the interorbital region contain prominent beads, which extend to the braincase as temporal ridges; these are usually less well-developed in T. talamancae. Unlike in T. talamancae and H. alfaroi, the parietal bone is usually limited to the roof of the braincase and does not extend to the sides.
The incisive foramina, which perforate the front part of the palate, do not extend between the molars. The palate ends beyond the third molars and is perforated by posterolateral palatal pits there. Behind it, the roof of the mesopterygoid fossa is perforated by poorly developed sphenopalatine vacuities. The auditory bullae, which house the inner ear, are large. Usually, the mastoid bone lacks openings, which are present in T. talamancae. The pattern of the arteries in the head is primitive, as indicated by the condition of various foramina and grooves in the skull.
of Transandinomys bolivaris from Cerro Azul, Panama
The mandible looks chunky and has a long condyloid process at its back; that of T. talamancae is more slender. The capsular process, a projection at the back of the jaw which houses the root of the lower incisor, is poorly developed.
The incisors are large and ungrooved. Their enamel is orange, but paler on the lowers. The orientation of the upper incisors is opisthodont, with the cutting edge oriented backwards. The molars are brachydont and have two rows of main cusps separated by deep valleys and complemented by a network of crests and smaller cusps. The first upper molar is broader than in T. talamancae. As in this species, but unlike in many other rice rats, including H. alfaroi, the mesoflexus on the second upper molar, which separates the paracone from the mesoloph, is not divided in two by an enamel bridge. 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, it is similar to T. talamancae but unlike H. alfaroi. Each of the upper molars has three roots and each of the lowers has two.

Distribution, ecology, and behavior

Transandinomys bolivaris is an uncommon species. Its known distribution extends from northeastern Honduras, on the Caribbean seaboard, through eastern Nicaragua, Costa Rica, and Panama, into coastal western Colombia and northwestern and west-central Ecuador. It has been found near sea level and the upper altitudinal records are at nearly in Panama and in Ecuador. This distribution coincides with that of the humid Transandean forest. It generally occurs in areas with mean temperatures above and annual rainfall of, prefers mid-elevation forests, and often occurs near water. The actual range of this species may be expected to extend further north and west, perhaps into Veracruz, southern Mexico, and western Venezuela, where it has not yet been recorded. Omar Linares mentioned a possible record from the Lake Maracaibo region of northwestern Venezuela in 1998. Its range is similar to that of various other rainforest animals, including the semiplumbeous hawk, the rice rats Sigmodontomys alfari and S. aphrastus, the spiny rats Proechimys semispinosus and Hoplomys gymnurus, and the opossum Marmosa zeledoni. T. talamancae and H. alfaroi are often found in the same localities as T. bolivaris, but also occur in other areas.
Little is known of its biology. The species mainly lives on the ground, but some young animals have been taken in vegetation, up to above the ground. It is usually captured "under logs, around the roots of large trees, or among rocks along streams." Two females with four embryos each have been caught in Panama in June, one with two in Nicaragua in September, and one with four in Costa Rica in December. A very young specimen was trapped in Costa Rica in March. One pregnant female was herself still in juvenile fur. Four species of mites have been found on T. bolivaris in Panama, two chiggers, and two fleas.

Conservation status

The 2009 IUCN Red List lists T. bolivaris as "Least Concern", as it is a widely distributed species with a presumably large population that is found in numerous protected areas. However, habitat destruction by deforestation may pose a threat.

Literature cited