The mustelids are a rather diverse group and the seven Bornean species are divided into four subfamilies. The subfamily Mustelinae includes the Yellow-throated Marten, Martes flavigula, and Malay Weasel, Mustela nudipes, both slender, agile mammals adapted to hunting small vertebrate animals, as well as invertebrates. The subfamily Melinae is represented by the Ferret-badger, Melogale personata, stockily built, with a short tail and long muzzle. The Teledu, Mydaus javanensis, is beleived to be closely allied to the skunk subfamily. Mephitinae, of North America. The otters, subfamily Lutrinae, are easily distinguished by their semi-aquatic existence, webbed or partially webbed feet and broad muzzle. The presence of otters may be detected by piles of faeces, known as spraints, consisting of the hard parts of crustaceans, molluscs or fish, deposited on open places near to water courses. All large otters found in Borneo are usually allocated to the genus Lutra, although the Smooth Otter, L. perspicillata, is sometimes placed in the genus Lutrogale. Mustelids appear to be closely related to the Viverridae. They can usually be distinguished by having 34 to 38 teeth, whereas all viverrids (except the Banded Linsang, Prionodon linsang) have a total 40 teeth. Mustelids also tend to have stockier limbs and broader feet, and are generally more strictly carnivorous.
Eurasian Otter (Lutra lutra)
Measurement: HB 673, T 417, HF 128 (1 specimen from P. Langkawi, off Peninsular Malaysia). Skull: cbl about 97-106 (mainland Asia).
Identification: Upperparts rather dark brown, with paler chin and upper throat. Fur dense, consisting of short hairs and longer guard hairs producing a somewhat grizzled appearance. Rhinarium hairless. Two skins of an otter which may be this species from the Kelabit uplands in Sarawak are darker than Eurasian Otters from elsewhere in Asia. Skulls of both specimens missing, but those of mainland specimens appear intermediate between those of the Smooth and Oriental Small-clawed Otters, L. perspicillata and Aonyx cinerea, unlike the long flattened skull of the Hairy-nosed Otter, L. sumatrana.
Similar species: the Hairy-nosed and Smooth Otters appear to live primarily in coastal and flatland habitats, and may have a paler, sleeker coat.
Ecology and Habitat: On mainland Asia, diet includes fish, other small vertebrates and crustaceans. Most Asian records, as well as the two Sarawak specimens, are from mountainous areas.
Distribution: Borneo: No definite records, but two skins (without skulls) from the Kelabit uplands in Sarawak may belong to this species.
Hairy-Nosed Otter (Lutra sumatrana)
Measurements: HB 575, T 375, 385, HF 103, 107 (2 specimens). Skull: cbl 91-101, io 14.4-14.8, mt 31.5-31.6 (3 specimens).
Identification: Entirely brown, except lips, chin and upper throat which are whitish. Fur rather rough but short. Rhinarium (moist part of nose) covered in hair. Tail flattened oval in cross section. Feet fully webbed between the digits. Claws prominent. Penis of adult male not visible externally. Contact call between otters a single syllabic chirp. Adult females call to cubs with a staccato chatter.
Similar species: the large otters are very similar and can be positively distinguished only by close inspection of the nose and fur, or the skull. In this species, the skull is flatter than that of the Smooth Otter, L. perspicillata and has smaller teeth.
Ecology and Habitat: Occurs in coastal areas and larger inland river systems, including associated lakes and streams. Diet includes fish. May be seen solitary or in groups of up to at least six. Pairing of male and female may be limited to the breeding period.
Distribution: Borneo: Recorded from scattered localities in most areas including the Tg. Sirik and Kuching areas in Sarawak.
Smooth Otter [Lutra (Lutrogale) perspicillata]
Measurements: HB 522-750, T 355-450, HF-140 (1 immature Sabah and several mature Thailand specimens). Skull: cbl 120.8, io 18.5, mt 40.6 (1 Sabah specimen).
Identification: Upperparts brown; underparts buffy. Throat and sides of neck creamy coloured. Fur short, smooth and sleek. Nose hairless. Tail flattened on underside. Feet large and webbed up to last joint on digits. Claws prominent. Penis of adult male protrudes beyond body wall. Contact call "wiuk".
Similar species: the Hairy-nosed Otter, L. sumatrana, has smaller footprints, smaller teeth and greater contrast between the dark upperparts and whitish throat. The European Otter, L. lutra, has a coarser, two-layered coat of fur and, if present, probably differs in habitat.
Ecology and Habitat: Unknown in Borneo, but elsewhere lives on the coast or inland in extensive flatlands. Several individuals may cooperate in fishing. Males and females are believed to pair permanently.
Distribution: Recorded only from S. Mugang in Sarawak (locality unknown; probably in the Bau region, south-west of Kuching).
Oriental Small-Clawed Otter [Aonyx (Amblonyx) cinerea]
Meassurements: HB 360-460, T 225-310, HF 85-95 (7 specimens). Skull: cbl 80.5-84.6, mt 24.2-26.0 (6 specimens).
Identification: Upperparts dark brown or greyish-brown; underparts slightly paler. Chin, throat, cheeks and sides of neck buffy. Digits only partially webbed. Claws short, not extending beyond the end of the digits. Contact call is 'wiuk'; various other calls are also made.
Similar species: the other Bornean otters are larger and have prominent claws.
Ecology and Habitat: Diurnal. Diet includes crabs, other crustaceans and molluscs. Occurs in many habitats where there is permanent water and some tree cover, including the coast, large rivers, small streams in the hills, ponds and lakes. Solitary individuals sometimes encountered, but often in groups. Males and females appear to pair permanently.
Distribution: Borneo: A.c.cinerea. Known from all regions of Sarawak. Up to 1300 m on Usun Apau.