Mexican water mouse  (Rheomys mexicanus)

Classified as Endangered (EN) on the IUCN Red List (1).

With its streamlined body, small, almost inconspicuous ears, and large, paddle-shaped hind feet, the Mexican water mouse is well adapted to its semi-aquatic lifestyle. The hind feet are also partially webbed and are fringed with hair, giving extra propulsion in the water, and the long, furred tail, which is longer than the length of the head and body, may also aid in swimming (2) (4) (5) (6). The short, dense, glossy fur is dark brown on the upperparts, often with a few longer, silvery outer hairs, and silvery white on the underparts, while the tail is also dark above and white below. The muzzle is quite blunt, with thin, stiff whiskers, and the eyes are tiny. Juveniles are reported to be slate grey in colour (4).

The Mexican water mouse is very similar in appearance to crab-eating rats of the genus Ichthyomys, but can be distinguished by its smaller size and by having four rather than five pads on the palm of the forefoot (2) (4) (5) (6).

The Mexican water mouse is endemic to the state of Oaxaca, Mexico, where it is known from only four sites (1) (2) (3) (4).

This semi-aquatic rodent inhabits tropical montane forest at the headwaters of freshwater streams, where it is restricted to small tributaries that are shaded by vegetation, and avoids larger rivers (1) (2) (4) (6). It has been recorded, for example, in pine-oak forest at elevations of around 2,000 metres (3), and appears to require pristine, non-polluted habitat (1).

Very little is known about the biology of the Mexican water mouse. Like other members of the genus, it has a semi-aquatic lifestyle, foraging for a range of aquatic insects, insect larvae, snails and other aquatic invertebrates, and possibly even small fish (2) (4) (6) (7). Other species have been observed to swim with only the head and the tip of the tail protruding above the water, and with the hind feet used for propulsion (5). Nothing is known about the reproductive behaviour of these rodents.

In general, water mice are considered very difficult to capture, and so the exact status of many species is unknown (4) (5). However, the Mexican water mouse is believed to be rare and to occupy a limited and severely fragmented range, and has been recorded from only four locations, making it particularly vulnerable (1) (2). The main threats to the species are deforestation, which is reported to be occurring throughout its range, and water pollution. The Mexican water mouse is reliant on areas of pristine habitat, and any kind of water pollution, including household pollution from the washing of clothes, is a potential threat to its survival (1).

The Mexican water mouse is classed as ‘Rare’ by the Mexican government (8), but there are not known to be any specific conservation measures in place for the species. More research is needed to better understand this little-known rodent, and to determine the status of its populations and its specialised habitat, before appropriate conservation measures can be put in place (1).

View information on this species at the UNEP World Conservation Monitoring Centre.

To find out more about the Mexican water mouse see:

• Emmons, L.H. (1997) Neotropical Rainforest Mammals: A Field Guide. Second Edition. University of Chicago Press, Chicago.
• Nowak, R.M. (1991) Walker’s Mammals of the World. The Johns Hopkins University Press, Baltimore and London.

• Lidicker Jr, W.Z. (1989) Rodents: A World Survey of Species of Conservation Concern. IUCN/SSC Rodent Specialist Group, IUCN, Gland. Available at:
  http://data.iucn.org/dbtw-wpd/edocs/SSC-OP-004.pdf