Bornean Water Shrew

Chimarrogale phaeura, the Bornean Water Shrew, is a small semi-aquatic shrew endemic to the island of Borneo. It inhabits clear, fast-flowing montane and hill streams within primary tropical rainforest. Its remarkable adaptation to an aquatic lifestyle among shrews makes it a rare and specialized insectivore. As an endemic and habitat-restricted species, it is a useful indicator of freshwater ecosystem health in Bornean forests.

Borneo (Malaysia, Indonesia, Brunei)

Endemic to the island of Borneo, with records from Sabah and Sarawak (Malaysia), Kalimantan (Indonesia), and possibly Brunei; distribution is patchy and tied to suitable forested stream habitats.

Altitude and depth

Approximately 300–1800 meters above sea level, mainly in hill and lower montane forest zones.

Found along clear, fast-flowing rocky streams and rivers within lowland to montane tropical rainforest; requires well-oxygenated, unpolluted water and dense riparian vegetation cover.

Adaptations to its environment

Possesses dense, water-repellent fur that traps air for insulation and buoyancy, along with fringed feet and tail that aid swimming and diving. Its small eyes and sensitive whiskers (vibrissae) help it navigate and locate prey in fast-moving, turbid water.

Classified as Endangered (EN) by the IUCN due to its restricted range, habitat specificity, and ongoing decline in suitable stream habitat quality caused by deforestation and water pollution.

Estimated population

No precise population estimate exists, but the population is considered small, fragmented, and declining due to continued habitat loss across its Bornean range.

Primary threats include deforestation and logging (legal and illegal), oil palm plantation expansion, stream siltation, and water pollution from agricultural runoff and mining activities.

Some populations occur within protected areas such as national parks and forest reserves in Sabah and Sarawak; broader Bornean rainforest conservation initiatives indirectly benefit this species.

Recommended actions

Recommended actions include protecting riparian forest buffers, expanding protected stream habitats, monitoring water quality, and conducting further surveys to clarify distribution and population status.

The fur is dense, soft, and dark brownish-grey to almost black (‘phaeura’ meaning dusky), with a slightly paler underside. The tail and hind feet bear stiff fringing hairs that assist in swimming.

Size

Head-body length approximately 70–90 mm, with a tail of similar or slightly shorter length (about 60–80 mm).

Weight

Estimated body weight around 15–35 grams, though precise data are limited.

Distinctive features

Distinguished from other Chimarrogale species by its darker, more uniformly blackish-brown pelage and specific skull and dental characteristics; endemic to Borneo unlike more widespread congeners.

Believed to be solitary and territorial, as is typical for most shrews, with limited direct social interaction outside of mating encounters.

When it is active

Primarily nocturnal and crepuscular, foraging along stream banks and in the water during low-light hours to avoid predators and reduce competition.

Diet

Carnivorous insectivore feeding mainly on aquatic invertebrates such as insect larvae, crustaceans, and small fish or amphibians found in and near streams.

How it feeds and hunts

Hunts by diving and swimming in fast-flowing streams, using tactile whiskers and possibly echolocation-like water pressure sensing to detect prey among rocks and debris.

Communication

Likely relies on scent marking and ultrasonic or high-pitched vocalizations typical of shrews, alongside tactile communication via whiskers in aquatic environments.

Interactions with other species

Potential predators include larger carnivorous mammals, snakes, and birds of prey; may compete with other small aquatic insectivores for stream invertebrate resources.

Role in its ecosystem

Plays a role as a specialized predator of aquatic invertebrates, helping regulate stream insect populations and serving as an indicator species for clean, undisturbed forest streams.

Details of reproduction are poorly documented, but likely resembles other Chimarrogale species with multiple litters per year, short gestation periods, and small litter sizes typical of soricid shrews.

Raising the young

Presumed maternal care only, with females raising altricial young in a nest near the stream until weaning, though specific data for this species are lacking.

Maturity and lifespan

Specific data are lacking, but based on related shrews, sexual maturity is likely reached within a few months, with a wild lifespan of around 1–2 years.

Classification

Class Mammalia > Order Eulipotyphla > Family Soricidae > Genus Chimarrogale > Species Chimarrogale phaeura

Scientific synonyms

Crossogale phaeura; sometimes previously grouped under Chimarrogale platycephala

Recent studies

Recent taxonomic reviews of Bornean Chimarrogale species have refined species boundaries using morphological and limited molecular data, though comprehensive genetic studies remain scarce.

Compared with similar species

Similar to other Southeast Asian water shrews like Chimarrogale platycephala and Chimarrogale hantu, but distinguished by darker fur coloration and Bornean endemism.

This shrew is one of the very few semi-aquatic mammals adapted to fast mountain streams in Borneo; its water-repellent fur allows it to dive and swim almost like a miniature otter, and it is so rarely observed that much of its biology remains a mystery to science.

Where and how to see it

Best observed at dusk or night along clear, undisturbed forest streams in Sabah's highland reserves such as Crocker Range or Kinabalu Park, requiring patience and quiet observation near rocky stream banks.

Risks to humans

No notable risk to humans.