Endowed with pinkish-gray, wrinkly skin, scant hair, and long buck teeth, naked mole-rats (Heterocephalus glaber) aren’t likely to win any beauty contests. Some might refer to them as downright ugly, resembling an overcooked hotdog with teeth. Nonetheless, biologists and zoogoers are enchanted with these bizarre rodents.
Naked mole-rats spend virtually their entire lives in the total darkness of underground burrows. Ensconced in the arid soils of Africa, these three-inch-long creatures must continually dig tunnels in search of sporadic food supplies and evade the deadly jaws of snakes. Within this formidable environment, naked mole-rats have broken many mammalian rules and evolved an oddly insect-like social system.
Despite the fact that they burrow underground like moles and have rat-like tails, naked mole-rats are in fact neither moles nor rats. The majority of the species referred to as mole-rats belong instead to the family Bathyergidae and are more closely related to porcupines, chinchillas, and guinea pigs than to their namesakes. Thirty-seven other species, such as the Palestine mole-rat (Spalex ehrenbergi), are also referred to as mole-rats but share only superficial similarities to the Bathyergidae and to one another.
Mole-rats of the family Bathyergidae are endemic to sub-Saharan Africa, where they can be found just about anywhere there are plants with large underground roots and tubers—the staple of mole-rats’ diet. Naked mole-rats are limited to the horn of Africa, including parts of Somalia, Ethiopia, and Kenya. They are the only mole-rat species that lacks typical rodent fur. However, that’s not the only thing that makes them odd.
Much like ants, termites, and some bees and wasps, naked mole-rats are considered “eusocial,” or truly social. They live in large colonies, presided over by a queen, in which only the queen and a few select males breed while the rest of the colony—all members of the same family—work together to raise young and maintain the colony. Wild colonies range in size from 20 to 300 individuals, with an average colony consisting of 75 individuals. The Smithsonian’s National Zoo houses three colonies, the largest of which boasts more than 50 mole-rats. Visitors to the Small Mammal House can examine one of these colonies, which occupies a labyrinth of transparent tubes. The Zoo also displays a colony of Damaraland mole-rats (Cryptomys damarensis), which are also eusocial, and a Palestine mole-rat, which lives solitarily as it would in the wild.
Native Africans have long known of the existence of naked mole-rats, which they call sand puppies. Rarely making appearances above ground, naked mole-rats didn’t capture the attention of Western biologists until the mid-1970s, when pioneering researchers Jennifer Jarvis, now affiliated with the University of Cape Town, Paul Sherman from Cornell University, and Richard Alexander at the University of Michigan uncovered the rodents’ secret societies.
Naked mole-rat colonies are organized into strict hierarchical castes. At the top of the heap—second only to the ruling matriarch—is the queen’s harem of one to three males with whom she chooses to mate. Beneath these high-status breeders are soldiers—both male and female—who defend the colony against predators and foreign mole-rats. Odors distinguish friends from foes. To achieve a recognizable odor, naked mole-rats often roll about in the burrow’s toilet chamber, coating their body with the familiar scent of the colony’s feces and urine. Extremely xenophobic, naked mole-rats will fiercely attack unfamiliar intruders that may be encountered when one colony breaks into the burrow of another.
At the bottom of the social totem pole, and smallest in size, are the workers, who are responsible for maintaining the burrow, finding food, and caring for the queen and her pups. So dependent are naked mole-rats on their social lifestyle that individual zoo mole-rats kept in isolation will die. “The entire colony is like one big organism,” says David Kessler, an animal keeper at the National Zoo’s Small Mammal House.
Because only a few members of the colony produce all the young, and the queen typically mates with close relatives selected from within the colony, naked mole-rat colonies are highly inbred. In fact, colony members are so closely related that their DNA “fingerprints” are virtually identical. Even though individual workers may not reproduce, many of their same genes are carried and passed on by others in the colony. In this eusocial scheme, naked mole-rats that sacrifice the opportunity to reproduce pass down their genes indirectly by caring for their colony mates.
The bossy queen rules her brood with brute force, often venturing from her nest to check up on subordinates. Researchers Paul Sherman and Kern Reeve from Cornell observed that if the queen discovers more food is needed, a tunnel has caved in, or danger is near she will shove the workers and soldiers around, prodding them into action. The queen also seems to use prolonged nose-to-nose shoving to prevent other naked mole-rats from breeding. Researchers Chris Faulkes of Queen Mary & Westfield College, London, and Frank Clarke of the University of Aberdeen discovered that the queen gets particularly pushy when she is close to breeding or when she senses that a non-breeder is about to become reproductively active. The researchers speculate that shoving may cause stress and inhibit hormone release, which in turn prevents ovulation in females and leads to lower testosterone and sperm count in males. Whatever the mechanism, the vast majority of naked mole-rats are doomed to a life of celibacy.
Undistracted by courtship and parenthood, workers focus most of their energy on one activity—digging. Their large, sharp incisors make ideal digging tools. The four front teeth—two on the top and two on the bottom—are actually located outside the mouth. To keep from swallowing unwanted dirt, hairy lips close behind the teeth, as do skin folds at the sides of the mouth.
Powerful jaw muscles add steam-shovel power to the digging apparatus. Twenty-five percent of a naked mole-rat’s muscle mass is devoted to closing the jaw, the same proportion of muscle that humans have in each leg. By contrast, only one percent of a human’s muscle mass is located in the jaw. Similarly, a large proportion of the naked mole-rat’s brain—one third of the cortex—is devoted to processing information from the area around the teeth.
The incisors can be moved independently, spread apart, or moved together like chopsticks. To compensate for the wear and tear of digging, naked mole-rats’ incisors grow continuously throughout their lives. At the Zoo, mole-rats are kept in clear plastic tubes and chambers that can’t be dug through (although that doesn’t stop the mole-rats from trying). Each day the keepers fill the tubes with wood chips and food for the mole-rats to gnaw on. Mole-rats also grind their teeth in their sleep. Occasionally, a Zoo mole-rat’s incisors will grow to an unwieldy size. “When that happens, we just trim their teeth with nail clippers,” Kessler explains.
When working together to dig tunnels in the wild, naked mole-rats line up nose to tail and operate like a conveyor belt. A digger mole-rat at the front uses its teeth to break through new soil. Behind the digger, sweepers use their feet and the fine hairs between their toes to whisk the dirt backwards. At the back of the line a “volcanoer” kicks the dirt up onto the surface of the ground, creating a distinctive, volcano-shaped mole hill about the height of a ballpoint pen.
Working as a team, naked mole-rats make for extremely efficient excavators. Robert Brett of the Zoological Society of London once observed a naked mole-rat colony dig a mile-long tunnel in less than three months. In all, a typical underground colony of about 80 animals can cover the area of twenty football fields. If all the tunnels were laid out in a straight line, they would extend to several miles in length. Branching, interconnected networks of tunnels link up nest areas, toilet chambers, and food sources. Major ‘highway’ tunnels are wide enough for two naked mole-rats to travel side by side and include turnouts for the animals to back into and change directions. Smaller side tunnels are less than two inches across, but tight squeezes aren’t a problem. With skin so loose that they can wriggle halfway around inside of it, naked mole-rats easily get through tight spots.
Their burrows are dark and stuffy places, and naked mole-rats have evolved to survive in these low-oxygen, high-carbon-dioxide environments. Their metabolic rate is less than half that of a typical rodent, and their specially adapted hemoglobin efficiently captures oxygen for the blood stream. A lower metabolism also may help prevent naked mole-rats from overheating during vigorous digging. Unlike other mammals, naked mole-rats aren’t capable of physiologically regulating their body temperature. They lack sweat glands and have no insulating layer of fat beneath their thin skin. More like reptiles, their body temperature fluctuates with the temperature of their environment. Fortunately, the temperature in underground burrows remains relatively constant at a toasty 82º to 89ºF. To slow heat loss, naked mole-rats sleep together in one big huddle of naked bodies. Individuals sometimes warm up by basking in tunnels just beneath the surface that have been heated by the sun. Sherman speculates that warmed mole-rats may return to the nest and share their heat with others, like living hot-water bottles.
After millions of years of living in the dark, the naked mole-rat’s eyes have shrunk to the point that they can hardly see. They often run through tunnels with their tiny eyes closed. In fact, the eyes may be more useful as sensors of moving air currents. For example, when a predator breaks through a sealed burrow entrance, a blast of fresh air is sent down the tunnel, alerting mole-rats to the predator’s presence. Naked mole-rats are also very sensitive to vibrations in the ground that may warn of nearby danger. Whiskers on their faces and tails brush along tunnel walls, guiding them in much the same way that a person may use her hands to feel along the wall of a darkened room. Since they aren’t looking where they’re going anyway, naked mole-rats can run just as fast backwards as forwards.
Predators, such as the rufous-beaked snake (Rhamphiophis oxyrhynchus rostratus), are attracted to the smell of freshly dug soil and will slither into burrows through mole hills in search of a rodent meal. Soldier mole-rats fight back with their teeth and attempt to block the entrance with dirt. If all else fails, a soldier will directly attack the snake, sometimes sacrificing its own life while others escape.
The ultimate goal of all the digging is the discovery of a tasty root or tuber to share with the colony. Many plants in arid climates develop fleshy tubers underground that can grow larger than a soccer ball. Naked mole-rats will eat out the succulent center and leave the outer skin intact. This allows the plant to regenerate, providing a reliable source of food. Naked mole-rats don’t drink any water and must obtain all their hydration from the plants that they eat. Their high-cellulose diet is also rather hard to digest, and their stomachs and intestines are inhabited by lots of microscopic organisms—bacteria, fungi, and protozoa—that help break down the vegetable matter. Naked mole-rats also re-ingest their own feces in order to maximize the amount of nutrients they get from their food.
Selfless by nature, naked mole-rats happily share their food with colony-mates. Biologist Paul Sherman studied such foraging behavior in his laboratory at Cornell University. He observed that when an individual mole-rat discovers a new food source it will grab a chunk and scurry back to the nest, chirping all the way. Upon arrival at the nest, the food scout will wave the morsel aloft for all to smell. This chirp-and-wave maneuver sets off a small stampede, as workers race to collect more food and bring it back to feed the whole colony. Sherman believes the rodents are following a chemical trail or signature scent left by the scouting mole-rat.
With all her food and security needs looked after by her minions, the queen is able to devote much of her time and energy to producing and caring for pups. When a female becomes queen she actually grows in length by increasing the space between the vertebrae of her backbone. The queen’s elongated body allows her to carry large litters during her pregnancy while still fitting through the narrow tunnels of the burrow. Naked mole-rats are unique among mammals for their prolific reproduction. The typical litter consists of 12 pups, but can be as large as 27. Gestation takes just ten to 11 weeks, and queens may have four or five litters each year. Hundreds of pups have been born at the National Zoo since the first colony arrived ten years ago; many of these are transferred to other zoos around the nation.
The ordered world of naked mole-rats rapidly disintegrates into chaos when a colony’s queen weakens or dies. High-ranking females—typically larger solider mole-rats—gain weight and begin fighting for ascension to the throne, sometimes to the death. They shove, bite, and fence with their large incisors. The battle in the burrow may go on for weeks or even months before one female vanquishes all her adversaries and emerges as the new queen.
Scientists once thought that the only way a new colony was established was when a large colony split apart. However, recent research by biologist Justin O’Riain of Cape Town University discovered that, within a colony, there may be a few naked mole-rats that are fatter and lazier than the rest and, ironically, seem to have the urge to travel. Both males and females of this type may eventually leave the colony, traveling by night up to a mile away in search of a partner to mate with and establish a new colony...
