Earless Water Rat - Characteristics

Characteristics

Of all muroids, the Earless Water Rat is adapted best to a life in water. It has extremely long hindfeet, the toes of which are webbed completely, strongly reduced forelegs, absent or invisible ears, very small eyes, and a long tail with a row of hairs at the downside. That row starts at each side of the beginning of the tail as a long white row of hairs; these two rows merge at about 50 mm from the beginning of the tail and the row goes on to the end of the tail. In all these characters, it resembles the Web-footed Water Shrew (Nectogale elegans), a good example of convergent evolution.

The back is greyish brown, and the belly is white. The fur is soft and water-proof. The top half of the tail is light grey, and the bottom half is white. The forefeet and claws are very small, but the hindfeet are very large. The upper lip contains a row of short, strong brushes, which may be used for rasping. The external ear (the pinna) is reduced to a small oval that does not or hardly protract above the fur. It is possible that the ear canal can be closed. The brain is rather large, like in many aquatic animals. The nasals are relatively small, just like the palate and the molars. The bullae are small. The rostrum is narrow. The Earless Water Rat is a medium-sized rat, about as large as its close relative Baiyankamys. The head-body length is 175 to 200 mm (6.9 - 7.9 inch) (based on four specimens), tail length is 212 to 260 mm (8.3 - 10.2 inch) (4), hind foot length is 44 to 53 mm (1.7 - 2.1 inch) (4), ear length is 1 to 4,5 mm (0.2 - 7.9 inch) (2), and weight 165 g (5.8 ounce avoirdupois) (1). Females have 0+2=4 mammae (no thoracic and two inguinal pairs), the same number as most other Australasian rodents.

The Earless Water Rat and Baiyankamys are related because they share the following characters: tail much longer than head-body length; soft, thick, greyish dorsal coat; long, narrow rostrum with a narrow top; very narrow canines; very narrow mesopterygoid fossae; narrow zygomatic arches with a high squamosal root. B. habbema also has the reduced external ears of the Earless Water Rat.