The large Mindoro forest mouse (Apomys gracilirostris ) is a species of rodent in the family Muridae, from the genus Apomys. It is found only in the Philippines. Its natural habitat is subtropical or tropical moist montane forests. It is a large mouse with large feet, a long tail and an elongated snout which is morphologically unique within its genus. It is covered in soft fur which is mostly dark brown in colour. Its closest relative is thought to be the Luzon montane forest mouse, based on genetic and morphological similarities.
The genus Apomys, of which the large Mindoro forest mouse is a member, can be identified by its small size, a long tail, elongate, narrow hind feet, the presence of four abdominal mammary glands, and a large number of skull characteristics. Within this genus, the large Mindoro forest mouse has several specific identifying features, the most important of which are its very thin and short upper incisors, long lower incisors and long snout. The tail and feet are also relatively long, even within the genus Apomys. The characteristics of the incisors are so peculiar that when Ruedas discovered the species, he initially thought he had found a new genus.
The fur of the large Mindoro forest mouse is soft and smooth. The hairs of the dorsal fur are light gray near the root, and dark brown at the tip. Between the normal hairs are many black tactile hairs. This makes the dorsal fur appear dark brown, although recently captured animals have a dark blue-green hue. Some animals are darker around the midline of the back, and all are a lighter colour at the flanks. The colour of the ventral fur varies between animals, with it being a yellow-brown colour in some, slightly lighter coloured than the dorsal fur in others, still others having gray hairs with brown or silver-coloured tips in the ventral fur, and finally some animals showing no difference between the dorsal and ventral fur at all.
The tail is usually an even, dark colour, but in some animals the ventral side is somewhat darker. In some animals the tail ends in a white tip of 2 to 10 mm in length. The tail has fourteen scales per centimeter. The long, dark hind feet end in long claws (about 4 mm). The claws on the front feet are about 3 mm in length. The animal has seven cervical vertebrae, thirteen thoracic vertebrae, six lumbar vertebrae, three sacral vertebrae and thirty-two to thirty-five caudal vertebrae.
The large Mindoro forest mouse has been found at three different sites in the municipality of San Teodoro, on the North flank of Mount Halcon. These sites are located at altitudes between 1,255 and 1,900 m (4,117 and 6,234 ft). There is a good possibility, however, that the large Mindoro forest mouse also occurs on other mountains on Mindoro, especially on the northern part of the island, which is home to large areas of undamaged cloud forest.
The lowest forests in which the large Mindoro forest mouse has been found, at some 1,250 m (4,100 ft) above sea level, are mostly between 14 and 16 m (46 and 52 ft) in height, with the very tallest reaching 20 m (66 ft). The dominant species of tree are from the genera Leptospermum flavescens, Tristaniopsis and Lithocarpus, from the families Myrtaceae and Fagaceae. These very humid forests have a dense understory vegetation, which consists, amongst other plants, of mosses, tree ferns and species of Pandanus. At higher altitudes (between 1,600 and 1,950 m or 5,250 and 6,400 ft) the forest is dominated by bamboo, while the only tree species present is Agathis philippinensis. Here, vegetation reaches a height between 7 and 10 m (23 and 33 ft). There are many smaller plants present, such as ferns, orchids and species of Pandanus. The forest floor is covered with a layer of leaves with a thickness between 6 and 8 cm (2.4 and 3.1 in).
Nothing is known with any certainty about the behaviour of the large Mindoro forest mouse. The long snout, small upper incisors and long lower incisors suggest that the animal feeds on soft invertebrates, but this is not certain. Furthermore, the long tail and elongated hind feet with well-developed claws point towards a climbing lifestyle.
Very little is also known about the animal's procreation habits. A female specimen was found to carry three embryos. Another female had a swollen uterus, and testicles were visible in several of the male specimens.
