Fork-marked lemurs or fork-crowned lemurs are strepsirrhine primates; the four species comprise the genus Phaner. Like all lemurs, they are native to Madagascar, where they are found only in the west, north, and east sides of the island. They are named for the two black stripes which run up from the eyes, converge on the top of the head, and run down the back as a single black stripe. They were originally placed in the genus Lemur in 1839, later moved between the genera Cheirogaleus and Microcebus, and given their own genus in 1870 by John Edward Gray. Only one species (Phaner furcifer) was recognized, until three subspecies described in 1991 were promoted to species status in 2001. New species may yet be identified, particularly in northeast Madagascar.
Fork-marked lemurs are among the least studied of all lemurs and are some of the largest members of the family Cheirogaleidae, weighing around 350 grams (12 oz) or more. They are the most phylogenetically distinct of the cheirogaleids, and considered a sister group to the rest of the family. Aside from their dorsal forked stripe, they have dark rings around their eyes, and large membranous ears. Males have a scent gland on their throat, but only use it during social grooming, not for marking territory. Instead, they are very vocal, making repeated calls at the beginning and end of the night. Like the other members of their family, they are nocturnal, and sleep in tree holes and nests during the day. Monogamous pairing is typical for fork-marked lemurs, and females are dominant. Females are thought to have only one offspring every two years or more.
These species live in a wide variety of habitats, ranging from dry deciduous forests to rainforests, and run quadrupedally across branches. Their diet consists primarily of tree gum and other exudates, though they may obtain some of their protein and nitrogen by hunting small arthropods later at night. All four species are endangered. Their populations are in decline due to habitat destruction. Like all lemurs, they are protected against commercial trade under CITES Appendix I.
Fork-marked lemurs are found in the west, north, and east of Madagascar, but their distribution is discontinuous. Their habitat ranges from dry deciduous forests on the western coast of the island to rainforest in the east. They are also commonly found in secondary forest, but not in areas lacking continuous forest cover. They are most common in the west of the island. Fork-marked lemurs are not found in the southern spiny forests in the dry southern part of the island, and only recently have been reported from the southeastern rainforest at Andohahela National Park, though this has not been confirmed. A team led by E. E. Louis Jr. has suggested that undescribed varieties may also exist elsewhere on the island.
The Masoala fork-marked lemur is found on the Masoala Peninsula in the northeast of the island, while the Amber Mountain fork-marked lemur is located in the far north of the island, particularly at Amber Mountain National Park. Pariente's fork-marked lemur is found in the Sambirano region in the northwest, and the pale fork-marked lemur is in the west of the island.