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. 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. 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, It is dark brown above and lighter below. The hindfeet are long and have the three central digits longer than the two outer ones. 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. 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.
Skull and teeth (cheekbone) is heavy. The
nasal and
premaxillary bones extend about as far backward.
T. bolivaris is similar, but has stronger beading The
parietal bone is usually limited to the roof of the braincase and does not extend to its side, as it does in most
T. bolivaris. The
interparietal bone, part of the roof of the braincase, is large. The
posterolateral palatal pits, which perforate the palate near the third molars, are small, and may or may not be recessed into a
fossa. The
sphenopalatine vacuities (openings in the roof of the
mesopterygoid fossa, behind the palate) are also small, The pattern of grooves and
foramina (openings) in the skull indicates that the circulation of the arteries of the head in
T. talamancae follows the primitive pattern, as in most similar species but unlike in
Hylaeamys. The
coronoid process (a
process in the back part of the bone) is small and the
capsular process, which houses the root of the lower
incisor, are small. 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. The upper incisor is
opisthodont, with the cutting edge oriented backward. As usual in oryzomyines, the molars are
brachydont (low-crowned) and
bunodont (with the cusps higher than the connecting crests). The first upper molar is narrower than in
T. bolivaris. 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. 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.
Male reproductive anatomy (lower jaw) from Gatun, Panama, seen from the labial (outer) side As is characteristic of Sigmodontinae,
Transandinomys talamancae has a complex penis, with the distal (far) end of the
baculum (penis bone) ending in a structure consisting of three digits. As in most oryzomyines, the central digit is larger than the two at the sides. The outer surface of the penis is mostly
covered by small spines, but there is a broad band of nonspinous tissue. Some features of the accessory glands in the male genital region vary among oryzomyines. In
Transandinomys talamancae, a single pair of
preputial glands is present at the penis. As is usual for sigmodontines, there are two pairs of
ventral prostate glands and a single pair of
anterior and
dorsal prostate glands. Part of the end of the
vesicular gland is irregularly folded, not smooth as in most oryzomyines.
Karyotype The
karyotype in
T. talamancae is variable. Samples from two different localities in Venezuela have 34
chromosomes and a
fundamental number of 64 chromosomal arms (2n = 34, FN = 64). Four specimens from another Venezuelan locality each have a different karyotype, with the number of chromosomes ranging from 40 to 42 and the fundamental number from 66 to perhaps 67. The
autosomes (non-
sex chromosomes) of the 2n = 34 karyotype all have two major arms, but the 2n = 40–42 karyotypes include several
acrocentric autosomes, which only have one major arm. The 2n = 34 karyotype includes two large
submetacentric pairs, which have two long arms but one distinctly longer than the other, and one pair of
subtelocentric chromosomes, with a long and a much shorter arm, but the 2n = 40–42 karyotypes lack the submetacentrics and have another pair of subtelocentrics. Both
Robertsonian translocations (fusions of the long arm of one chromosome with the long arm of another and the short arm of the one with the short arm of the other) and
pericentric inversions (reversals of part of a chromosome that includes the
centromere) are needed to explain the difference between the two groups. Musser and colleagues, in discussing these karyotypes, assumed that the 2n = 40–42 sample was from within a
hybrid zone between two karyotypic morphs. 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. In contrast, a sample from south of the Gulf had 2n = 54, FN = 60, including 23 pairs of acrocentrics and four pairs of
metacentrics (with two equally long arms). Musser and colleagues termed the difference between the two Ecuadorian forms "impressive" Both
T. bolivaris and
H. alfaroi have more chromosomes and arms, at 2n = 58, FN = 80 and 2n = 60–62, FN = 100–104 respectively.
Hylaeamys megacephalus has 2n = 54, FN = 58–62 and the similar
Hylaeamys perenensis has 2n = 52, FN = 62; these karyotypes resemble that of southern Ecuadorean
T. talamancae. ==Distribution and habitat==