Mole-rats are small, fossorial rodents, which means they spend their entire lives underground in a sealed burrow system. Native to Africa, these little animals are found from the southernmost tip of the continent to about 10 degrees north of the equator. Mole-rats make up the family Bathyergidae, which includes at least 14 species in five genera (not to be confused with an unrelated family Spalacidae, containing a single genus living in eastern Europe and the eastern Mediterranean region). As a group, bath-yergid mole-rats have the greatest diversity of both body size and social structure of any subterranean rodent. The species in three genera (Bathyergus, Georychus, and Heliophobius ) are completely solitary. The species in the other two genera (Cryptomys and Heterocephalus ) are social. The most social of all is also the smallest: weighing in at around 0.8 oz (23 g), the naked mole-rat (Heterocephalus glaber ). It lives in highly cooperative groups, sharing food, living quarters, and care of the young. At the other end of the spectrum is the solitary Cape dune mole-rat, in which adult males may weigh up to 63 oz (1.8 kg).
Mole-rats are well-adapted to life underground. Their eyes are much reduced, as is the visual center in the brain, suggesting that sight does not play much of a role in their dark, subterranean environment. In fact, it is not clear whether some mole-rat species can perceive light at all; most keep their eyes closed while going about their underground business, opening them only when alarmed. Their ears are also tiny, but their hearing is acute. Mole-rats communicate using a wide array of chirps, trills, and other vocalizations, as well as by drumming with their hind feet
on the floor of the burrow. Their senses of smell and touch are also well developed, and are used to help identify and communicate with one another.
First-time viewers may find the mole-rat’s appearance a bit odd: they have short limbs, a cylindrically shaped body, and very loose skin with numerous folds. Long hairs, called vibrissae, stand out from the skin of the head and body, providing sensory information. The incisor teeth protrude prominently from the mouth, and the lips close behind the teeth to keep out dust and soil. Some mole-rat species actually dig their tunnels using their incisor teeth; other species dig using strong front legs armed with sturdy claws. Most mole-rats have a short tail, but naked mole-rats have a tail up to half of their body length, used to help guide the animal as it runs backwards along tunnels. Perhaps because of their tail, naked mole-rats can navigate as quickly and easily in the reverse direction as forward; if two animals meet in a tunnel, one of them may back up some distance before coming to a place it can turn around and face forward again.
Large, prominent teeth are also important in food-gathering, as the primary food source of mole-rats is underground roots, tubers, and corms, which can be quite tough. Mole-rats occasionally eat above ground shoots that are pulled underground from below; the animals almost never venture into the open air above. Tunnels used for food-collecting are dug up to the level where food is found; the main burrow system may lie several feet deeper in the ground. Collected food items are stored in a special food chamber, located off the main burrow.
In addition to a food chamber, the burrow system also features chambers used for nesting and as a communal toilet. In addition, mole-rats dig deep, blind-ended tunnels they may use to escape enemies or to cool themselves; these tunnels may also function as drains in the event of flooding. Normally, the burrow system is not open to the surface, but is tightly sealed to provide protection against weather, extremes of temperature, and predators. Openings are necessary during tunnel excavation, in which a digging mole-rat loosens the soil with teeth or forelegs, pushes it beneath its body, and kicks it behind. When enough soil has accumulated in this way, the mole-rat backs up in the tunnel, pushing the soil behind it; the soil is directed out a side tunnel to the surface, where it is kicked out. With some species, the soil becomes compacted in being pushed on its way out, and may be seen emerging from the ground in a compacted core, like toothpaste being squeezed from the tube. With the naked mole-rat, the dry soil is vigorously ejected in a fine spray, creating the characteristic soil “volcanoes” that erupt at the excavation hole. An open hole, however, is an invitation to predators. Mole snakes have been observed to enter these excavation tunnels, and larger predators (herons, storks, skunks and weasels, for instance) may wait by a fresh opening for the digger to return with its next load of soil.
Little is known about courtship and mating in these animals, or about how new burrow systems become established. Individuals of solitary species will defend a territory, fighting viciously with any others it may encounter (perhaps under a rich food patch). Male-female pairs will tolerate each other in the same burrow for a day or so to accomplish mating, but they soon separate. The solitary female cares for the young until weaning, at about eight weeks; after this, antagonism and aggression build and the young disperse.
Biologists are especially interested in the highly social naked mole-rat, because it is one of the rare mammals that exhibits eusociality. Eusocial animals, such as many bees and wasps, live in colonies where only one or a very few individuals produce all the offspring, and the rest serve as sterile helpers; thus, we observe a division of labor. In naked mole-rats, one large female, known as the queen, is the only female to undergo reproductive cycling (i.e., to have an active estrus cycle) and produce offspring. In addition, a colony generally contains only 1-3 breeding males; reproduction in all other individuals is effectively suppressed, apparently by means of aggressive behavior and olfactory cues from the breeding adults, who appear to carefully sniff and monitor the physiological condition of the others. Non-reproductive adults carry out the tasks of tunnel digging and maintenance, foraging for and storing food, and caring for the young, which of course are not their own. If the queen should die, violent fights may erupt as potential successors try to assert themselves over their competitors. Ferocious defensive behavior has also been observed when the burrow systems of two separate colonies become linked by a common opening; mole-rats will fight to the death in defense of their home burrow.
Eusociality presents a problem to evolutionary theorists, which was recognized by Charles Darwin. How could an animal evolve by means of natural selection (in which organisms with particular traits survive and reproduce better than their competitors), if they forsake their own reproduction and devote themselves to helping others reproduce? How can a trait spread, if its bearer fails to produce offspring? Darwin’s answer was that if an individual contributes enough to the reproduction of closely related individuals who carry but do not necessarily express the trait, it can spread by natural selection. This is known as inclusive fitness. This explanation appears to work for naked mole-rats, in which the non-reproductive workers help the related queen produce offspring, who go on to recreate the same social structure at home or in another burrow. Evidently, this system suits them well; perhaps this kind of social group can collect more food than a single animal foraging alone. Perhaps the danger of traveling above ground means staying at home is more desirable than emigrating; suppressed reproduction may be necessary to keep colony numbers down and avoid starvation. It may be that their highly social population structure has enabled naked mole-rats to inhabit the hot, arid regions in Africa, where the solitary species are not found.
Bennett, Nigel C., and Chris G. Faulkes. African Mole-rats: Ecology and Eusociality. Cambridge: Cambridge University Press, 2000.
Sherman, Paul W., Jennifer U.M. Jarvis, and Richard D. Alexander, eds. The Biology of the Naked Mole-rat. Princeton, NJ: Princeton University Press, 1991.
Milius, Susan. “Naked and Not: Two Species of Mole-rats Run Complex Societies Underground.” Science News 166 (June 24, 2006): 394–396.