Colombian forest mouse

Chilomys instans is very similar to Chilomys fumeus in size and appearance. Both are small sigmodontines, with a head-and-body length of 72 to 102 mm (2.8 to 4.0 in) and a tail length of 102 to 137 mm (4.0 to 5.4 in). The ears are medium-sized and clad in short hairs, and the body fur is woolly and short. The dorsal surface is dark grey to greyish-brown and the underparts are a similar colour. The hind feet are narrow, and the slender tail is clad with short hairs, and often has a white tip. There are three pairs of mammary glands. The chief differences between the two species lies in the morphology of the skull.

The Colombian forest mouse is native to South America. It occurs in mountainous areas of Venezuela, Colombia and Ecuador, in humid forests at altitudes of between 1,000 and 3,700 m (3,300 and 12,100 ft).

Little is known of the behaviour. It seems to be nocturnal and lives on the ground where it creates runways among the mosses of the forest floor. Its diet includes insects, worms and plant material, and three new species of beetle have been identified from the stomach contents of this mouse in Colombia. The larger woodland Oldfield mouse (Thomasomys hylophilus) shares its range.