Subfamily Tylomyinae
vesper rats and climbing rats

Tylomyinae, vesper rats and climbing rats, is an arboreal New World cricetid subfamily with ten species in four genera: Nyctomys, Otonyctomys, Ototylomys, and Tylomys). The four genera are divided between two tribes. (Musser and Carleton, 2005; Nowak, 1999)

Tylomyines are distributed throughout Central America, from southern Mexico to Panama. (Nowak, 1999)

Tylomyines live in tropical evergreen and semideciduous forests, especially those in rocky areas, at elevations from sea level to 2,000 meters. (Hunt, Morris, and Best, 2004; Lawlor, 1982; Nowak, 1999)

In the past, the genera in this subfamily were often placed in a broadly defined Cricetinae, which included New World muroid rodents as well as palearctic hamsters (Miller and Gidley 1918, Ellerman 1941, Simpson 1945). Vorontsov (1959) placed them as part of the tribe Oryzomyini within Cricetinae. Other authors separated the New World muroids from Cricetinae, and gave them the subfamily name Hesperomyinae (Tullberg 1899, Chaline et al. 1977). Hooper and Musser (1964) broke the New World group down even further, recognizing that North American taxa and South American taxa had different penis morphologies. Based on their simple penis morphology, Ototylomys and Tylomys were thought to be related to the North American, or "neotomine-peromyscine" group (Hooper 1960), later known as the subfamily Neotominae (Reig 1980). Other authors suggested that Ototylomys, Tylomys, Nyctomys, and Otonyctomys may form a clade basal to neotomines and sigmodontines, the other major New World muroid group (Carleton 1980). Based on this hypothesis, Reig (1984) gave tylomyines subfamily status. Still, some authors retained tylomyines within a broadly-defined Sigmodontinae (Musser and Carleton 1993). The results of phylogenetic analysis based on the GHR, BRCA1, RAG1, and c-myc genes do not support a basal position for tylomyinae; instead, tylomyines seem to be sister to sigmodontines, with neotomines basal to both groups (Steppan et al. 2004). Further molecular work will no doubt help resolve relationships between the tylomyine genera and other muroid groups. (Carleton, 1980; Chaline, Mein, and Petter, 1977; Ellerman, 1941; Hooper and Musser, 1964; Hooper, 1960; Miller and Gidley, 1918; Musser and Carleton, 1993; Reig, 1980; Reig, 1984; Simpson, 1945; Steppan, Adkins, and Anderson, 2004; Tullberg, 1899; Vorontsov, 1959)

Tylomyines are medium to large-sized muroid rodents, ranging from 95 to 255 mm in length, with tails measuring 85 to 250 mm. Their tails are usually a bit longer than their bodies. They weigh 29 to 280 grams and, in some species, males are slightly heavier than females. Other species exhibit no detectable sexual dimorphism. The fur is either short or long, and is cinnamon, buff, tawny, gray, russet, or brown above and white below. The tail is either covered with long hairs and tufted at the tip, or it is nearly naked. The ears are nearly naked, and can be either short or long. There are long black whiskers, and some species have a dark ring around each eye. The eyes are quite large and the hind feet are modified for climbing. Tylomyines have two pairs of mammae in the inguinal region.

Tylomyines have brachydont, cuspidate molars, with the major cusps lying opposite one another. The cheek teeth bear well-developed mesolophs and mesolophids. The second upper molar has four roots, and the third lower molar is relatively large and has a crown pattern like that of the second lower molar.

Tylomyines have a cuneate interorbital region, with prominant supraorbital shelves that continue posteriorly as pronounced temporal ridges. The interparietal bone is large and contacts the squamosal, to which the tegmen tympani are united. The zygomatic plates are narrow and there is usually no dorsal notch. There is an alisphenoid strut, but no subsquamosal fenestra, and the postglenoid foramen is quite small. The mesopterygoid fossa is usually completely ossified, and the parapterygoid fossa is shallow and slender. If there are sphenopalatine vacuities, they are present as tiny slits.

The first rib of tylomyines attaches to only the first thoracic vertebra. There is an entepicondylar foramen in the humerus. The calcaneum has a wide, proximally-positioned trochlear process.

The tylomyine stomach is single-chambered, and there is no gall bladder. The caecum is long and complex. The glans penis is wide and short (though it is longer than the baculum), and it has large, well-spaced spines. (Nowak, 1999)

Captive tylomyines have been recorded living up to five years and five months. Lifespan in the wild is probably much shorter. (Nowak, 1999)

Tylomyines are almost exclusively arboreal, and their hind feet are modified for climbing. They build nests out of plant fibers and twigs on tree branches or on the ground among rocks. They are nocturnal. These rodents are solitary or live in small family groups, and strangers placed together in an enclosure will fight viciously. (Hunt, Morris, and Best, 2004; Nowak, 1999)

Tylomyines have large eyes and probably have good vision. They can be seen twitching their ears and vibrissae back and forth when investigating new objects. They make a variety of trills, squeaks, chirps, and grunts to communicate with one another during courtship, copulation, aggressive encounters, and while raising young. Young tylomyines are often very vocal, and they chirp when their nest is disturbed or when playing with littermates. (Nowak, 1999)

These rodents are primarily herbivorous; they consume seeds, fruits, and leaves. Occasionally they eat moths. (Hunt, Morris, and Best, 2004; Nowak, 1999)

Tylomyines are preyed upon by owls and snakes. They bite viciously, especially if disturbed with young. (Hunt, Morris, and Best, 2004; Lawlor, 1982)

Tylomyines are, for the most part, primary consumers, and they are food for secondary consumers such as snakes and owls. In addition, tylomyines are parasitized by laelapid and trombiculidmites, argasidticks, ceratophyllidfleas, and female sandflies. Tylomyines are susceptible to infestations of Trypanosoma cruzi.

Sympatric tylomyine species are potential competitors with each other, but they may avoid competition by foraging at different levels in the forest. (Hunt, Morris, and Best, 2004; Lawlor, 1982)

Tylomyines are resevoirs for cutaneous leishmaniasis (Leishmania mexicana). Also, they sometimes enter buildings and make their nests in unwelcome places. (Hunt, Morris, and Best, 2004; Lawlor, 1982)

There are no known positive effects of tylomyines on humans.

There are currently four tylomyine species on the IUCN's Red List of Threatened Species. Two of those species are critically endangered (Chiapan climbing rats, Tylomys bullaris, and Tumbala climbing rats, Tylomys tumbalensis), one is vulnerable (Panamanian climbing rats, Tylomys panamensis), and one is lower risk (fulvous-bellied climbing rats, Tylomys fulviventer). (IUCN, 2004)