Heterocephalus glaber (Mammalia: Rodentia), the naked mole-rat, is a fossorial rodent endemic to the arid regions of eastern Africa. Its current distribution spans Ethiopia, Kenya, and Somalia (Jarvis and Sherman, 2002). Heterocephalus is a monotypic genus belonging to the clade Bathyergidae, the African mole-rats (Honeycutt et al., 1991). Currently there are 14 bathyergid species composing 5 genera (Jarvis and Sherman, 2002). The fossil record of Heterocephalus sp. extends into the early Miocene of Uganda (Jarvis and Sherman, 2002; Faulkes et al., 2004) with an unnamed specimen resembling H. glaber dated at a minimum of 17.8 mya (Bishop, 1962; 1969).

Naked mole-rats are the smallest of the bathyergids with an average weight of around 34 g (Brett, 1991a). Their burrowing lifestyle is reflected in their morphology, with cylindrical “torpedo-shaped” bodies and short limbs. The skull is dorsoventrally flattened and the eyes are minute with thick protective eyelids (Jarvis and Sherman, 2002). Unlike many other fossorial mammals, H. glaber does retain an external pinna (ear flap), albeit reduced. The dental formula is typically i 1/1, c 0/0, p 0/0, m 3/3 with the procumbent incisors being most prominent. The incisors are ever-growing and are the primary means of digging, with the limbs used only for pushing loosened earth (Nowak, 1991). Both the zygomatic arch and the angle of the mandible extend laterally to accommodate the massive jaw musculature required to perform his method of burrowing (see figures below). The manus (front foot) is broad with 5 clawed digits and all feet possess tufts of fine hairs that are used in a sweeping motion to move tilled earth (Nowak, 1991).

Despite their common name, naked mole-rats are not entirely hairless; many sensory hairs cover the body and vibrissae (whiskers) are prominent around the snout. A lack of thick pelage, however, reveals the naked mole-rat’s loose pinkish skin. This wrinkling of the integument permits the body to move within the skin, a useful ability while traversing tight tunnels (Jarvis and Sherman, 2002).

Naked mole-rats are unusual among eutherians in that they cannot regulate internal body temperature, and are effectively poikilothermic (dependent on ambient temperatures) (Bennett and Faulkes, 2000). This phenomenon has been attributed to the lack of fur and minimal amounts of subcutaneous fat, which lead to high thermal conductance (Bennett and Faulkes, 2000). To compensate, H. glaber migrate to shallower, warmer burrows to increase body temperature and conversely travel deeper to cooler burrows to decrease body temperature (Jarvis and Sherman, 2002).

Naked mole-rats are primarily herbivorous, and their diets consist mainly of high fiber roots and tubers, which also serve as their only source of water (Brett, 1991b; Jarvis and Sherman, 2002). These food sources are accessed through foraging burrows within an elaborate subterranean system. The complexity of the tunnel systems depends on rainfall and food availability and can extend for kilometers underground (Brett, 1991b; Jarvis and Sherman, 2002). Wild individuals are active 24 hours a day and exhibit no known circadian rhythm cycles (Jarvis and Sherman, 2002).

Naked mole-rats are eusocial mammals (having advanced social structure with divisions of reproductive labor) and live in colonies ranging in size from 25 to almost 300 individuals (Brett, 1991a). This social structure has many advantages not limited to protection, thermoregulation (huddling), and the division of labor, including the development of “digging chains.” These chains start with a single individual that gnaws into the closed end of the burrow and then using the limbs, transfers the dirt to a subsequent individual. This continues throughout the length of the tunnel until reaching the ground’s surface, where the last individual is responsible for ejecting the tilled earth. This method of excavation results in the formation of unique volcano-shaped mounds (Brett, 1991a; Jarvis and Sherman, 2002; Nowak, 1991).

Within the hierarchy of H. glaber colonies, breeding is restricted to a single dominant female (Brett, 1991a). This “queen” is reproductively active year-round, giving birth to 4-5 litters averaging 11.3 pups per litter (Jarvis and Bennett, 1991; Jarvis and Sherman, 2002). Other females within the colony are not sterile, but simply sociologically suppressed by the dominant queen (Jarvis and Sherman, 2002). For a detailed comparison of reproductive strategies within Bathyergidae see Jarvis and Bennett, 1991.

Naked mole-rats are the longest-lived rodent species, reaching 28 years of age in captivity (Buffenstein, 2005) and 17 years in the wild (Andziak et al., 2005). Because of this attribute, H. glaber has been utilized as a model for human senescence (aging) in recent studies (see Buffenstein, 2005; Andziak et al., 2005; Ungvari et al., 2008).

Additional Information on the Skull

Click on the thumbnails below for labeled images of the skull in standard anatomical views.

Lateral view	Dorsal view	Ventral view

This male specimen was collected from Wacomba Country, Ucase, Kenya, Africa. It was made available to The University of Texas High-Resolution X-ray CT Facility by Ms. Jeri Rodgers of The University of Texas at Austin, Department of Geological Sciences. Funding for scanning was provided by Ms. Rodgers. Funding for image processing was provided by The Jackson School of Geosciences.

This specimen was scanned by Matthew Colbert on 30 May 2007 along the coronal axis for a total of 1050 slices. Each 1024 x 1024 pixel slice is 0.0224 mm thick, with an interslice spacing of 0.0224 mm and a field of reconstruction of 21 mm.

Literature

Andziak, B.A., O’Connor, T.P., and Buffenstein, R. 2005. Low glutathione peroxidase activity in the longest living rodent species known; antioxidants do not explain the 8-fold difference in longevity between naked mole-rats and mice. Mechanisms of Ageing Development 126:1206–1212.

Bennett, N.C., and Faulkes, C.G. 2000. African mole-rats: ecology and eusociality. Cambridge University Press, United Kingdom, 273 pp.

Bishop, W.W. 1962. The mammalian fauna and geomorphological relations of the Napak volcanics, Karamoja. Uganda Geological Survey, Records 1957–58, pp. 1–18.

Bishop, W.W., Miller, J.A., and Fitch, F.J. 1969. New potassium-argon age determinations relevant to the Miocene fossil mammal sequence in East Africa. American Journal of Science 267:669–699.

Brett, R.A. 1991a. The population structure of naked mole-rat colonies. pp. 97–136 in The Biology of the Naked Mole-rat (P.W. Sherman, J. U. M. Jarvis, and R. D. Alexander, eds.). Princeton University Press, New Jersey.

Brett, R.A. 1991b. The ecology of naked mole-rat colonies: burrowing, food and limiting factors. pp. 137–184 in The Biology of the Naked Mole-rat (P. W. Sherman, J. U. M. Jarvis, and R. D. Alexander, eds.). Princeton University Press, New Jersey.

Buffenstein, R. 2005. The naked mole-rat: a new long-living model for human aging research. Journal of Gerontology: Biological Sciences 60A:1369-1377.

Honeycutt, R.L., Edwards, S.V., and Schlitter, D.A. 1991a. Systematics and evolution of the family Bathyergidae. pp. 45-65 In The Biology of the Naked Mole-Rat (P.W. Sherman, J.U.M. Jarvis, and R.D. Alexander, eds.). Princeton University Press, New Jersey.

Jarvis, J.U.M., and Bennett, N.C. 1991. Ecology and behavior of the family Bathyergidae. pp. 66–96 in The Biology of the Naked Mole-rat (P. W. Sherman, J. U. M. Jarvis, and R. D. Alexander, eds.). Princeton University Press, New Jersey.

Jarvis, J.U.M. and Sherman, P.W. 2002. Heterocephalus glaber. Mammalian Species 706:1-9.

Nowak, R.M. 1991. Walker’s Mammals of the World Volume I, Fifth Edition. Johns Hopkins University Press, Baltimore. 642 pp.

Sherman, P.W., Jarvis, J.U.M., and Alexander, R.D. 1991 (eds.). The Biology of the Naked Mole-rat. Princeton University Press, New Jersey, 518 pp.

Ungvari, Z., Buffenstein, R., Austad, S.N., Podlutsky, A., Kaley, G., and Csiszar, A. 2008. Oxidative stress in vascular senescence: lessons from successfully aging species. Frontiers in Biological Science 13:5056-70.

Links

Heterocephalus glaber page on the Animal Diversity Web (University of Michigan Museum of Zoology)

Heterocephalus glaber Wikipedia entry

Front page image.