Naked Mole-Rats

Underground Life

The naked mole-rat is a small, nearly hairless rodent that inhabits the dry grasslands of East Africa, in countries such as Kenya, Ethiopia, and Somalia, where it spends virtually its entire existence in subterranean tunnels that it excavates itself. An adult is usually about three or four inches long, approximately the length of a human finger, and it weighs only as much as a few coins. Its pink, wrinkled skin hangs loosely on its body, a scattering of thin hairs grows along its back, and long whiskers help it navigate through the darkness. Despite the name, it is neither a mole nor a true rat, since biologists classify it in a separate family of African rodents, and very few people have ever encountered one outside a laboratory or a well-equipped zoo. Its body is shaped like a tiny sausage, and its legs are short.

Naked mole-rats live in cooperative colonies, and a typical colony contains approximately seventy or eighty animals, although a few have been found with several hundred members. The most extraordinary characteristic of these communities is that only one female, called the queen, produces offspring, while the other females never reproduce and only a small number of males are selected as her mates. Scientists describe this arrangement as eusocial, meaning that a large group shares the labor of raising young while one or two individuals are responsible for all of the reproduction. Among mammals the system is exceptionally rare, and honeybees, ants, and termites are the more familiar creatures that organize their societies in the same way. A queen can give birth to ten or more pups at once, and some litters have contained more than twenty. The queen is the only one in the colony who has pups.

Every other member of the colony performs a specific task. Some mole-rats are diggers that lengthen the tunnels, others transport food to the nest, and the largest are defenders that protect the community from danger, such as snakes that occasionally wander into the passages. The diggers cooperate in an orderly line, much like a team passing buckets of water: the first animal gnaws at the soil, and the ones behind it kick the loosened dirt backward from one pair of feet to the next until it is expelled from the tunnel in a small fountain. The colony feeds primarily on enormous roots and tubers, which are swollen underground parts of plants, and the workers frequently consume only the inside of a tuber, leaving the plant alive so that it can regenerate and supply food again. One big tuber can feed a whole colony for a long time.

Strange Bodies

The most conspicuous feature of a naked mole-rat is its pair of long, orange front teeth, which protrude in front of its lips. Because the lips seal behind the teeth, the animal can gnaw through compacted soil without swallowing any dirt or filling its mouth with dust. The two teeth can also move apart and come together again, rather like a pair of chopsticks, which allows each tooth to be used independently for grasping objects and tearing off pieces of food. Its eyes are minuscule and its vision is poor, but sight is hardly necessary for an animal that lives in permanent darkness. Instead it depends on its whiskers and on sensitive hairs between its toes, and it is capable of scrambling backward through a narrow tunnel almost as swiftly as it travels forward. Its front teeth never stop growing.

Naked mole-rats are also unusual in the way they manage their body temperature. Most mammals, including people, maintain a constant internal temperature, but the naked mole-rat regulates its heat rather poorly and therefore depends heavily on the conditions surrounding it. Deep in the burrow the air remains warm and steady, so the animals generally cope very well. When they become chilly, they gather in a tight heap inside the nest to share warmth, and individuals sometimes creep into a shallow tunnel near the surface to absorb heat from the sun-warmed ground. The colony also assigns different regions of its tunnel system to different purposes, with one chamber reserved for sleeping and raising pups, another designated as a toilet, and a long network of passages devoted to searching for food. Every room has a job. Mole-rats like it warm.

Each naked mole-rat colony has its own distinctive manner of communicating. Researchers have recorded soft chirps that the animals exchange as greetings, and they have discovered that the chirps of one colony differ slightly from those of another, much like the local dialects that human communities develop. A mole-rat that hears the familiar call of its own colony will answer it, whereas the call of a stranger earns a considerably less friendly response. The queen's voice appears to be especially influential, because when a colony loses its queen the voices of its members gradually change, and the group slowly adopts the sound of a new leader. These animals produce many other noises as well, including trills and squeaks, and scientists have catalogued several dozen different types of calls. Many of them are quiet enough that a person standing nearby would never hear them.

A Long, Odd Life

Naked mole-rats survive for an astonishingly long time. A mouse, which is about the same size, usually lives for only two or three years, yet a naked mole-rat in a laboratory can live for thirty years or more, and a few individuals have been recorded at over thirty-five, which is longer than any other known rodent of comparable size. Even more remarkable, they appear to remain healthy and active throughout most of those years. Breeding queens do not seem to slow down, and researchers have observed that the likelihood of dying does not climb steeply with age, as it does in nearly every other mammal. Scientists are intensely curious about this phenomenon, because understanding how these creatures age so slowly might eventually teach us something valuable about human health. For an animal this small, that is a very long life. It is also a healthy one.

Another peculiar trait is that naked mole-rats very rarely develop cancer, a disease in which cells multiply uncontrollably. Cancer is common in mice, dogs, and people, but in several decades of careful investigation, researchers have found only a handful of tumors in naked mole-rats. Part of the explanation seems to be an unusual sugar molecule in the spaces between their cells, which makes the surrounding tissue thick and prevents cells from crowding together and dividing out of control. Researchers are studying the substance because the discovery could someday lead to new ways of protecting people. It is important to remember, however, that no scientist has produced a cure from this work yet, and the research remains at an early stage, although the animal has provided investigators with an extremely intriguing clue. Cancer is hard to beat, and this small animal may help.

Naked mole-rats are remarkably resilient in other respects as well. Their skin does not react to the sting of acid, and they appear to feel no burning from hot pepper, the spicy chemical that makes a person's mouth hurt. Scientists have traced these abilities to small differences in the nerves of the skin, which probably help the animals tolerate the stuffy air in their burrows, where carbon dioxide can accumulate to uncomfortable levels. They can also survive when oxygen is extremely scarce. In one laboratory experiment, naked mole-rats lived for about eighteen minutes without any oxygen at all, a period in which a mouse would have died within a few minutes, because their bodies switched to burning a kind of sugar as an emergency fuel. That is a long time without air. It is a trick that most mammals cannot do.

There is one more surprise concerning the queen. When a female becomes queen, her body actually grows longer, because the bones of her back spread apart, and she becomes the largest animal in the colony. If the queen dies, several of the females may compete to replace her, and the winner begins to breed while the others return to the ranks of the workers. A naked mole-rat colony can consequently continue for many years as a single, closely related family. Perhaps the most fascinating aspect of these odd animals is how many ordinary rules of mammal life they appear to break, since they cooperate like insects, resist pain and cancer, and survive for decades while living in total darkness inside a crowded, humid burrow. That is a lot to learn from an animal as small as your finger. Few other mammals live this way.