The Luzon montane forest mouse (Apomys datae ) is a species of rodent in the family Muridae, from the genus Apomys.It occurs only in the Philippines, where it has been found on the large northern island Luzon (in the Cordillera Central and on the coast of Ilocos Norte). It is most closely related to the large Mindoro forest mouse, which occurs on Mindoro. There may be another related species in the Sierra Madre, but this species is yet undescribed. The Luzon montane forest mouse is a relatively large, ground-dwelling rat with a tail that is quite short for its genus.
The genus Apomys, of which the Luzon montane forest mouse is a member, can be identified by its small size, long tail, elongate, narrow hind feet, the presence of four abdominal mammary glands, and a large number of skull characteristics.
The Luzon montane forest mouse is a large, thickset species with a tail that is about the same length as the body. The soft, thick dorsal fur is dark brown, while the ventral side of the body is a cream white. The hind feet are partly brown in colour on the dorsal side, but otherwise white. The tail is brown on the dorsal side, and a cream white ventrally. The animal has a large skull with a square-shaped neurocranium. Some of its skull characteristics are so special that they set the animal apart from most every other species of Apomys. One specimen has a head-torso-length of 143 mm, a tail length of 144 mm and a hind feet length of 34 mm. The specimens captured by Sanborn on Mount Data have a mean skull length of 39.2 mm (37.0 to 40.6 mm with a standard deviation of 1.1 mm). The Luzon montane forest mouse's holotype has a skull length of 39.9 mm; that of A. major has a length of 38.4 mm.
The animal has a karyotype of 2n=44 and FN=54, which consists of five pairs of four-armed chromosomes and sixteen pairs of two-armed (telocentric) chromosomes. The X-chromosome is a large, and the Y-chromosome a small telocentric chromosome. This karyotype has much in common with that of the least forest mouse (Apomys musculus ), while at the same time being very different from the karyotypes of species form other parts of the Philippines (the karyotype of the large Mindoro forest mouse, on a side note, is unknown).
