Voalavo is a small rodent resembling a mouse with gray fur. Species of the genus are among the smallest known nesomyines, close in size only to
Monticolomys koopmani. In terms of external morphology,
Voalavo is barely different from
Eliurus; fur coloration patterns, general morphology of the feet, and number of
mammae (six) are all the same in both genera. However, all species of
Eliurus have a pronounced tuft of elongated hairs at the tip of the tail, a feature that is absent in
Voalavo, although the latter does have slightly longer hairs near the tip. The tail is longer than the head and body. Relative tail length in northern voalavo (136% of head and body length) is comparable to that of the longest-tailed species of
Eliurus, Grandidier's tufted-tailed rat and Petter's tufted-tailed rat, but
V. antsahabensis has a somewhat shorter tail. Furthermore, the
pads of the feet are larger in
Eliurus, and specifically, the
thenar pad (located at the middle of the
tarsus) is circular and fairly small in
Voalavo, but longer and larger in
Eliurus. On the chest,
Voalavo species have a gland that produces a sweet-swelling musk in breeding males; this gland is absent in
Eliurus. Unlike all other nesomyines but
Brachyuromys,
Voalavo lacks an
entepicondylar foramen, an opening on the
humerus (upper forelimb bone). The skull of
Voalavo also resembles that of
Eliurus, with a long facial skeleton, an hourglass-shaped
interorbital region (between the eyes), and a smooth interorbital region and
braincase, without ridges or shelves. Other shared characteristics include an essentially featureless bony palate, without many pits and ridges, and a broad
mesopterygoid fossa (the opening behind the palate). In other characteristics,
Voalavo resembles some but not all species of
Eliurus. For example, the length of the
incisive foramina matches the maximum seen in
Eliurus species (in this case, in
Major's tufted-tailed rat and
White-tipped tufted-tailed rat). The back margin of the incisive foramen is rounded in eastern voalavo, but angular in northern voalavo. The two species also differ in the shape of the
suture (dividing line) between the
maxillary and
palatine bones, which is straight in eastern voalavo, but more curved in northern voalavo. The
capsular process, a projection at the back of the
mandible (lower jaw) that houses the root of the lower
incisor, is indistinct in
Voalavo, a feature it shares with Grandidier's tufted-tailed rat, Major's tufted-tailed rat, and Petter's tufted-tailed rat, but not the other species of
Eliurus. Other features of the skull distinguish the two genera. The
tegmen tympani, the roof of the
tympanic cavity, is much reduced in
Voalavo relative to
Eliurus. The
subsquamosal fenestrae, openings in the
squamosal bone at the back of the skull, are larger in
Voalavo than in
Eliurus. The
zygomatic plate, a plate at the sides of the skull that roots the front part of the
zygomatic arches (cheekbones), is narrower in
Voalavo, and lacks a clear
zygomatic notch (a notch formed by a projection at the front of the zygomatic plate), which is present in
Eliurus. Among nesomyines, only
Brachytarsomys has a more reduced zygomatic notch. Like
Eliurus,
Voalavo has moderately high-crowned (
hypsodont)
molars with crowns that consist not of discrete
cusps, but of transverse laminae (plates) that generally lack longitudinal connections. However,
Eliurus molars are slightly more hypsodont than those of
Voalavo. The third upper and lower molars are smaller relative to the second molars in
Voalavo than in
Eliurus. Perhaps as a consequence, the upper third molar lacks discrete laminae in
Voalavo, and the lower third molar has only two laminae (three in
Eliurus). There are three roots under each upper molar and two under each lower. ==Distribution and ecology==