Subfamily Rhizomyinae
(bamboo rats and mole rats)

Rhizomyinae, the bamboo rats, is a small Old World family of fossorial muroid rodents. There are four species of bamboo rats in two genera (Cannomys and Rhizomys). (Musser and Carleton, 2005)

Bamboo rats are distributed from Sumatra and the Malay Peninsula in the south through southern China, Nepal, and eastern India. (Carleton and Musser, 1984)

Rhizomyines live in bamboo thickets, forests, grassy areas, and gardens in hilly or mountainous regions, at elevations up to 4,000 meters. They burrow in a range of soil types, from loose and well-drained to hard and stony. (Carleton and Musser, 1984; Nevo, 1999; Nowak, 1999)

Rhizomyines have often been placed in a family separate from Spalacidae, the Rhizomyidae (Miller and Gidley 1918; Ellerman 1940, 1941; Simpson 1945; Chaline et al. 1977, Flynn 1990). Molecular evidence now lends support to the classification proposed by Tullberg in 1899: rhizomyines, spalacines, tachyoryctines, and myospalacines form a monophyletic group, and all belong to the family Spalacidae (Michaux et al. 2001, Jansa and Weksler 2004, Norris et al. 2004, Steppan et al. 2004). More molecular work is need to sort out the relationships between Rhizomyinae and other spalacids.

Another point of contention is whether Tachyoryctes is a rhizomyine genus or whether it warrants placement in its own subfamily, Tachyoryctinae. Most authors have favored the former scenario (Carleton and Musser 1984, Musser and Carleton 1993, Potapova and Vorontsov 2004), but Flynn (1990) recognized separate tachyoryctine and rhizomyine clades based on a number of morphological characteristics, and this is the classification we use here. It is estimated from fossil evidence that rhizomyines and tachyoryctines diverged about 17 million years ago, in the Miocene. (Carleton and Musser, 1984; Chaline, Mein, and Petter, 1977; Ellerman, 1940; Ellerman, 1941; Flynn, 1990; Jansa and Weksler, 2004; Michaux, Reyes, and Catzeflis, 2001; Miller and Gidley, 1918; Musser and Carleton, 1993; Musser and Carleton, 2005; Nevo, 1999; Norris et al., 2004; Nowak, 1999; Potapova and Vorontsov, 2004; Simpson, 1945; Steppan, Adkins, and Anderson, 2004; Tullberg, 1899)

EXTERNAL CHARACTERISTICS

Rhizomyines are powerfully-built rodents with chunky bodies and short limbs. Their head and body length ranges from 147 to 480 mm, and their tail length ranges from 50 to 200 mm. They weigh 500 to 4,000 grams. There is no sexual dimorphism in size. Their short tails are hairless and lack scales. Their eyes and and external ears are small but not covered by fur. Bamboo rat fur ranges from soft, thick, and silky to coarse and sparse. The fur is colored slate gray, pinkish-gray, brownish-gray, chestnut, or cinnamon, and is paler on the belly than on the back in most species. Bamboo rats dig with their broad incisors, which protrude in front of the lips, and with their long, robust claws. The largest claw is on the third digit of the forefeet.

INTERNAL CHARACTERISTICS

The rhizomyine dental formula is 1/1, 0/0, 0/0, 3/3 = 16. The molars are hypsodont, and the molar rows run in parallel. The molars have mesolophs and mesolophids, giving them a pentalophodont enamel pattern, and they are nearly equal in size. The alveoli of the upper incisors end above the roots of the first molars, obstructing the orbits. The incisive foramena are short, and the bony palate is relatively smooth. The heavy mandible has prominant capsular and coronoid processes. Wide, strong zygomatic arches and the prominent sagittal and lambdoidal crests provide the broad attachment surfaces for the powerful head and neck muscles necessary for digging with their jaws. Due to the lack of the ventral portion of the infraorbital foramen, the zygomatic plate is poorly demarcated. The infraorbital foramen contains the nasolacrimal canal. The anterior portion of the lateral masseter muscle has a broad origin on the side of the wide, short rostrum, instead of on the zygomatic plate. The area between the orbits is constricted and the frontals are compressed. There are no sphenofrontal, stalacerate, or entepicondylar foramena. The pterygoid fossa is deep and well-ossified. The external auditory meatus is tubular in shape, the auditory bullae are moderately inflated, and the malleus is constructed perpendicularly. The interparietal bone is tiny. The rhizomyine stomach has two chambers, and the cecum has a spiral valve. There is no stapedial artery, and the internal carotid artery provides circulation to the orbits. (Carleton and Musser, 1984; Nevo, 1999; Nowak, 1999)

The lifespan of bamboo rats is three to four years. (Nevo, 1999; Nowak, 1999)

Bamboo rats are perhaps the least fossorial of the spalacids. Although they spend much of their time underground, they forage on the surface at night from time to time and some even climb bamboo. However, they move slowly when walking on the surface, with an awkward waddling gait. Underground, bamboo rats dig with their chisel-like incisors and strong foreclaws, and use their heads and feet to remove loose soil. They construct deep burrow systems consisting of a nest chamber, foraging tunnels, and bolt holes into which they can flee if threatened by predators. One bamboo rat may have several burrows. Generally, they are sedentary, but some have been known to migrate with the seasonal availability of food. Rhizomyines are nocturnal or crepuscular and they are active year-round. For the most part, they are solitary and territorial, although females have sometimes been seen foraging with their young. (Carleton and Musser, 1984; Flynn, 1990; Nevo, 1999; Nowak, 1999)

Bamboo rats perceive the world using vision, touch, smell, taste, and hearing. Given their small eyes and the fact that they spend most of their lives underground in complete darkness, vision is probably the least important of these senses. They are known to make grunting and tooth-grinding noises, which may be a form of communication, and it is likely that they use scent to communicate, as most mammals do. (Nowak, 1999)

Bamboo rats are herbivores that feed on the roots and shoots of bamboo and other plants. They also eat seeds and fruit if available. Bamboo rats store excess food in underground chambers in their burrow systems. (Carleton and Musser, 1984; Nowak, 1999)

Bamboo rats fall prey to a variety of snakes, eagles, owls, and small mammalian carnivores. They probably avoid predation to some degree by staying hidden underground and only venturing forth under cover of darkness. Bamboo rats incorporate bolt holes into their burrow systems into which they can make a quick escape if caught out in the open. If cornered, they can be vicious and do not hesitate to rush at their attacker and attempt to bite. (Carleton and Musser, 1984; Nevo, 1999; Norris et al., 2004)

Because of their fossorial lifestyle, bamboo rats probably help to aerate the soil. They are important consumers of bamboo and other plants, and they are prey for a variety of avian and mammalian predators. Where they are sympatric, different species of bamboo rats are potential competitors, but they avoid competition by partitioning microhabitats (i.e., by specializing on different species of bamboo). (Nevo, 1999)

Bamboo rats sometimes raid and damage tapioca and sugarcane crops. (Nowak, 1999)

Bamboo rats in northern Thailand are hosts of Penicillium marneffei, a pathogenic fungus that infects humans, especially those with HIV. However, it is not thought that bamboo rats transmit the fungus directly to humans; rather, that humans and bamboo rats become infected from a common environmental source. Therefore, research on bamboo rats may prove helpful for controlling outbreaks of P. marneffei in humans. Besides being valuable for disease research, bamboo rats are hunted and used as food by native peoples. (Chariyalertsak et al., 1996; Chariyalertsak et al., 1997; Nowak, 1999)

Rhizomyines are still numerous in many areas, and none of the species in this subfamily are currently listed as threatened. (IUCN, 2004)