The northern voalavo (Voalavo gymnocaudus), also known as the naked-tailed voalavo or simply the voalavo, is a rodent in the family Nesomyidae found in the Northern Highlands of Madagascar. Discovered in 1994 and formally described in 1998, it is the type species of the genus Voalavo; its closest relative is the eastern voalavo of the Central Highlands. DNA sequencing suggests that it may be more closely related to Grandidier's tufted-tailed rat than to other species of the closely related genus Eliurus. The northern voalavo is found at above sea level in montane wet and dry forests in the Marojejy and Anjanaharibe-Sud massifs. Nocturnal and solitary, it lives mainly on the ground, but it can climb and probably eats plant matter. Despite having a small range, the species is classified as being of least concern because it lacks obvious threats and much of its range is within protected areas.
The northern voalavo is a small, mouse-like rodent with soft, grey fur that is only slightly darker above than below. The ears are short and rounded. The long tail appears mostly naked and lacks a distinct tuft, which is present in Eliurus. It differs from the eastern voalavo mainly in the values of some measurements. The skull is delicate, with a long, narrow rostrum (front part), narrow interorbital region (between the eyes), and no development of ridges on the braincase. The molars are relatively high-crowned (hypsodont). It has a body mass of .
Taxonomy
The rodent fauna of the Northern Highlands of Madagascar remained almost totally unstudied until the 1990s. A 1994 survey of the Anjanaharibe-Sud Reserve partially filled this gap and led to the discovery of two new species: Grandidier's tufted-tailed rat and northern voalavo, the first known and type species of the genus Voalavo. Both species were formally described in 1998 by Michael Carleton and Steven Goodman. The generic name, Voalavo, is a general Malagasy word for rodent, and the specific name, gymnocaudus, refers to the naked tail, which distinguishes the northern voalavo from the related tufted-tailed rats (Eliurus). In 2000, the species was also recorded from the nearby Marojejy National Park.
Meanwhile, in 1999, Sharon Jansa and colleagues published a molecular phylogenetic study of the Nesomyinae, the native Malagasy rodents, using the mitochondrial gene cytochrome b. Their results suggested that the current definitions of Eliurus and Voalavo may not be correct, because they found that northern voalavo and Grandidier's tufted-tailed rat are more closely related to each other than to the remaining species of Eliurus. However, the DNA of Petter's tufted-tailed rat, a possible close relative of northern voalavo, could not be sampled, so Jansa and colleagues recommended further evaluation of the problem. According to a 2003 report, data from nuclear genes also support the relationship between northern voalavo and Grandidier's tufted-tailed rat, but Petter's tufted-tailed rat remains genetically unstudied and the taxonomic issue has not been resolved.
A second species of Voalavo, eastern voalavo, was named in 2005 from central Madagascar. Morphological differences between the two are subtle but consistent, and the cytochrome b sequences of the two species differ by about 10%. In mammals, closely related species regularly differ by less than 5% in their cytochrome b sequences, and a divergence of more than 5% within a single species suggests the presence of cryptic species.
Description
External morphology
The northern voalavo is a small, mouse-like rodent. It differs from the very similar eastern voalavo mainly in some measurements, such as a greater tail length. It also resembles small species of Eliurus, but the fur is darker and there is no tail tuft. The fur is soft, short, and thick, and appears dark grey on most of the upperparts, but more brownish on the sides. On the back, the cover hairs, which form the main part of the fur, are three-colored: most of the hair is grey, followed by a narrow light buff band and a black tip. The longer guard hairs are black. The fur of the underparts is not different in overall color, but the individual hairs are gray for about three quarters of their length and white at the tips, except for those at the chin, which are white throughout.