352.

Mexican Water Mouse

Rheomys mexicanus

French: Ichthyomys du Mexique / German: Mexiko-Wassermaus / Spanish: Ratén de agua de México

Other common names: Goodwin's Water Mouse, Mexican Ichthyomyine

Taxonomy. Rheomys (Neorheomys) mexicanus Goodwin, 1959, San José Lachiguiri, 4000 ft (= 1219 m), District of Miahuatlan, Oaxaca, Mexico.

This species is monotypic.

Distribution. Endemic to C & SE Oaxaca, Mexico.

Descriptive notes. Head-body 105-148 mm, tail 131-171 mm, ear 6-8 mm, hindfoot 32-42 mm. No specific data are available for body weight. The Mexican Water Mouseis a large species of Rheomys, characterized by light brownish fur; small ears concealed in fur of head; thick bicolored tail, longer than head-body length; and very large, whitish hindfeet, with fringes of long, stiff, white hair alongside toes. Pelage is long and lax, light brown to brownish orange dorsally, darkest on rump, and becoming slightly paler on shoulders and sides of body. Underparts, including under sides of foreand hindlimbs, are pale yellow-brown; upper sides of forefeet are dusky to base of toes; toes are whitish, soles of forefeet are flesh colored; tail is rich brown above and white below to roots of hair, with line of demarcation sharply defined; hair on underside oftail is longer than on upper side; and extreme tip of tail is white all round.

Habitat. Small shallow ponds and forested riparian areas at headwaters of streams, in particular small tributaries with closed tropical forest vegetation from near sea level to elevations of ¢.2200 m. The Mexican Water Mouse does not occur in larger rivers. The type series was taken in a rather slow, rocky stream passing through dry cactus country with some stands of oak ( Quercus, Fagaceae) and pine ( Pinus, Pinaceae).

Food and Feeding. Stomachs and intestinal tracts of specimens originally used to describe the Mexican Water Mouse contained a large assortment of legs, body casings (probably dominated by the family Hydrophilidae), wings, other parts of various beetles and other insects, and insect larvae. Stomach of one individual contained scales of a small fish. Captive individuals were photographed eating fish provided by collectors.

Breeding. The holotype, a female collected in May 1958, had two well-developed fetuses.

Activity patterns. Little information available, but one individual was seen diving to the bottom of a small pond (c.5 m wide, less than 50 cm deep) where it was searching, presumably for food, in the base of small rocks. When a capture attempt was made,it tried to escape by diving several times with intervals of breathing of 10-12 seconds and always staying away from the margins.

Movements, Home range and Social organization. No information.

Status and Conservation. Classified as Endangered on The IUCN Red List. The Mexican Water Mouse occurs in less than 5000 km?its distribution is severely fragmented, and there is ongoing decline in extent and quality ofits habitat. It requires pristine habitat, and any kind of water pollution is a threat. At least 28 Mexican Water Mice have been collected from Oaxacan localities, perhaps suggesting that it is not rare in these areas.

Bibliography. Briones-Salas & Sanchez-Cordero (2004), Briones-Salas et al. (2015), Caviedes-Solis et al. (2017), Goodwin (1959a, 1969), Martin-Regalado et al. (2016), Santos-Moreno et al. (2003), Timm et al. (2008), Voss (1988).