Oddities in Nature: The Naked Mole Rat

The naked mole rat is a strange little creature in nearly every way. Living its entire life in total darkness, this unusual rodent makes its home in underground tunnels below the grasslands of East Africa. It is part of a very special sort of family, one comprised of several dozen siblings and a single queen. In other words, this interesting rodent is part of a colony which has more in common with the social structure of insects than with that of mammals. Adapting to this dark, secluded world has resulted in a variety of unique traits which have molded this creature’s design into a form unlike that of any other mammal. From its hairless, wrinkled skin and underdeveloped eyes to its reptile-like thermoregulation and ghastly dentition, the naked mole rat is in a category of its own when it comes to peculiarity. Yet those very same adaptations are what allow the naked mole rat to survive effectively in its environment, allowing it to take advantage of its underground habitat while avoiding the many dangers present above its head in the African grasslands. However, there is more to the naked mole rat’s wonder than its appearance and survival skills, for within this rodent’s very genes may lie invaluable information for medical researchers. Indeed, this strange little mammal is full of surprises, and is nothing short of fascinating for those fortunate enough to study it.

The naked mole rat (Heterocephalus glaber) is a small animal, reaching 3-13 inches in body length (along with a 3 inch tail) and weighing up to 3 lbs.[1]While it does not possess fur (as its common name indicates), the naked mole rat does possess some sensory whiskers which allow it to move about in its underground habitat. These whiskers are located not only on its face, but also on its tail, since naked mole rats move throughout their tunnel systems both forwards and backwards. Hair is also present between the animal’s toes, allowing it to use its feet as brooms in order to sweep soil. Its bare skin is pink in color, as well as somewhat translucent on the animal’s underside and of a more light purplish brown on its back and tail. This countershading seems to disappear as the naked mole rat grows older. One of the naked mole rat’s most distinguishing features is its fearsome set of ever-growing incisors. This dentition is absolutely essential to the animal’s survival since they function as digging tools which allow the naked mole rat to expand the range of its colony’s tunnel system. Of course, this tough work also requires strong jaws, which naked mole rats certainly possess. In fact, 25% of an individual’s muscle mass is involved in jaw closure alone. The lips of these animals close behind their large incisors, preventing dirt from entering their mouths as they dig through the ground. While there is no sexual dimorphism between males and females of the species, the colony’s queen and her breeding males are larger than all the other colony members, with the queen being the longest individual. Once a female becomes a queen, she begins to grow larger, even if she is already an adult. This is a result of the distance between the vertebrae in her spine increasing, making for an odd transformation that would never have taken place had she never become the colony’s queen.[2]

The naked mole rat is unique among mammals due to its variety of unusual adaptations. Photograph Credit: Dr. Lloyd Glenn Ingles © California Academy of Sciences

Naked mole rats are unique among mammals in that they are unable to maintain a constant internal body temperature. This makes them the only poikilothermic mammals (poikilotherms are animals whose internal body temperatures fluctuate depending on ambient temperatures). This strange trait has been attributed to the lack of fur and minimal amounts of subcutaneous fat, which leads to high thermal conductance. In order to compensate for this, naked mole rats must regulate their body temperatures in the same way that many other poikilotherms, such as reptiles, do; moving to areas with higher or lower ambient temperatures so that their own body temperatures may increase or decrease as is necessary. In the case of the naked mole rat, this means migrating to shallower, warmer burrows in order to increase body temperature or descending to cooler burrows for the opposite effect.[3]

This African species occurs throughout much of Somalia, central Ethiopia, and northern and eastern Kenya, extending as far south as the eastern border of Tsavo West National Park and the town of Voi. The species appears to have a wider range than is presently known, as naked mole rats have also been observed in Djibouti. Naked mole rates are usually recorded at an altitudinal range of 400-1500 miles above sea level. The naked mole rat’s habitat is arid, characterized by high temperatures and low and irregular rainfall, generally averaging out at 200-400 mm/year. Naked mole rats are usually found in hard, consolidated, lateritic loams, although they can also live in fine sand, pure gypsum, and laterite. The colonies of these subterranean rodents normally average between 75 and 80 animals, though they can contain as many as 290 mole rats.[4]

These crowded, carbon dioxide-rich living conditions require a special biology on the part of the mole rat, and scientists have, indeed, discovered some interesting traits possessed by these animals. Research on naked mole rats revealed that they are unable to feel burning pain from acids or foods like chili peppers. This results from the lack of a key messenger that carries the burning pain signal through the nervous system. When researchers reactivated the neurotransmitter on one foot of test rats, they were able to elicit a response from the animals to chili pepper exposure, though they still did not react to contact with acids. Further research revealed that naked mole rats are actually completely oblivious to acids, a trait which assists the animals in their natural living conditions.[5]

Photograph Credit: Trisha Shears

In a naked mole rat colony, every individual has a specific responsibility. The queen leads the colony and serves as its only breeding female, while only a small group of males is allowed to mate with her. All of the other mole rats serve as either soldiers or workers. Soldiers protect the colony from outside threats, such as predators. Should a snake, for instance, enter the colony’s tunnel system in search of an easy meal, the mole rats respond with an alarm call. This alerts the colony’s soldiers, which soon make their way to the location of the threat. These soldiers are not to be ignored, as they can use their powerful jaws and large, sharp teeth to deliver painful injuries. Several of these mole rats may even pile on top of each other in order to block a tunnel passage, presenting the predator with an impenetrable wall of gnashing teeth. Workers, on the other hand, tend to less tense activities, such as scouting for food, digging the tunnels, and providing care for the queen’s offspring. They also bring food to the queen, as she is often quite busy nursing her pups, which can number up to 27 at a time.[6]

While each mole rat’s job is interesting to observe, the queen is possibly the most fascinating member of the colony. As the only individual allowed to breed, she keeps her underlings in check by sociologically suppressing them through aggressive behavior; it is not strange to observe the queen biting and pushing other mole rats as she inspects the tunnels in order to remind them that she is in control. She mates with only a small number of males in relationships which can remain stable for many years. These mating sessions can take place quite often, as the queen can produce up to five litters every year. Each of these litters can consist of up to 27 pups, though on average the queen will produce litters of about twelve pups. She nurses these young for about four weeks (the queen possesses a total of twelve nipples in order to maintain her many offspring), though the young mole rats may begin eating solid food as soon as they are two weeks of age. The pups may also demand the workers to provide them with feces to eat, which provides nutrition and inoculates their digestive systems with gut bacteria. It makes for a rather disturbing diet, but the bacteria assist in the breaking down of roots and tubers (their main food), which are difficult to digest. At the age of three to four weeks, these pups begin performing work behaviors such as sweeping or digging. Although maturation rate is variable for the species, juveniles are generally capable of reproduction by the age of one year. Though they relied on their mother’s early on in their lives, the young mole rats soon find themselves on the receiving end of the same aggression the queen displays with other colony members.[7] A queen needs to be this way, as her position is never secure. This is due to the fact that no naked mole rat is born into this position; they must fight for the right to be queens. These battles tend to be bloody, as females are willing to fight to the death for control of the colony. Even upon succeeding in becoming the colony’s leader, the queen must continue to fight off females if she is to remain in charge. It is easy to see that, despite the struggles her underlings go through, the queen is not without her own challenges.[8]

These strange mammals live in colonies which have more in common with the social structures of insects than they do with those of other mammals. Photograph Credit: Dr. Lloyd Glenn Ingles © California Academy of Sciences

One might wonder how these strange, reclusive animals could ever be of any major use to mankind. Yet scientists who have analyzed the naked mole rat’s genetic code claim that the species possesses inner traits that could aid medical research. The species’ lifespan is quite long for a rodent, consisting of up to 30 years in captivity, which could assist in finding methods of slowing down aging, and their ability to survive on low levels of oxygen in their tunnels makes them useful for research on protecting the brain from oxygen deprivation from strokes. Furthermore, their numbness to certain burning sensations could also prove valuable to medical research. Yet probably their most impressive trait is their immunity to cancer, which has made them an essential part of the search for the disease’s prevention. This fascinated researcher Vadim Gladyshev of Harvard Medical School when he first learned of the animals. Gladyshev saw that for scientists’ understanding of naked mole rats to truly progress, biologists would first need to know all of its genes, so he quickly assembled an international team to do just that. “We decided, ‘OK, let’s go full force and just sequence the genome,’” says Gladyshev. “Because the genome is really critical in order to understand the biology of that animal.” The team used the DNA of a four-year-old mole rat provided by biologist Thomas Park, who keeps many of the rodents in his lab at the University of Illinois, Chicago. The job took about a year and a half, and the findings have been very interesting. For instance, due to the naked mole rat’s poikilothermic nature, the team looked for genes that assist in thermoregulation. What they discovered was a gene which changed specifically in naked mole rats, not in other mammals.[9] They also went on to find changes in genes possibly linked to lifespan, body hair, and cancer.[10] With the discovery of these interesting genes, scientists can now try to figure out what exactly they do. “It’s up to the labs to take these findings and to take these analyses and go to the lab and test them experimentally,” says Joao Pedro de Magalhaes, a biologist at the University of Liverpool in England who is part of another team that has also been working on decoding the naked mole rat’s genetic code. He calls this new analysis “an excellent and hugely important work” for establishing the naked mole rat as “the first model of resistance to chronic diseases of aging.”[11] It is quite clear that the naked mole rat has become an unexpected source of hope for people, and, ironically, it contains this vital genetic information due to the very lifestyle which many people find to be strange or repulsive. Nature is simply full of surprises.

It is very unlikely that there is any other animal out there quite like the naked mole rat. This odd little mammal, resembling a walking sausage with sharp teeth, is difficult to ignore. Everything about it is just extraordinary, though perhaps this should be expected of an animal that lives its entire life underground, traveling through dark tunnels and shoveling its way through dirt using its teeth. Yet even more amazing is the role this little creature is playing in the world of medical science. Possessing what appears to be the right genetic information for assisting scientists in the search for the cure for cancer and other ailments, this fascinating rodent has become exceptionally important for many people, whether they know it or not. Whether one observes its appearance and behavior or marvels at the possibilities stemming from its genetic code, the naked mole rat is an amazing and unusual creature, a genuine oddity of nature.

[1] “Naked Mole Rat.” National Geographic. http://animals.nationalgeographic.com/animals/mammals/naked-mole-rat/ (accessed July 20, 2012).

[2] Ciszek, Deborah. “Heterocephalus glaber.” Animal Diversity Web. http://animaldiversity.ummz.umich.edu/site/accounts/information/Heterocephalus_glaber.html (accessed July 20, 2012).

[3] “Heterocephalus glaber, Naked Mole-rat.” Digimorph. http://www.digimorph.org/specimens/Heterocephalus_glaber/ (accessed July 20, 2012).

[4] “Heterocephalus glaber.” IUCN Red List. http://www.iucnredlist.org/apps/redlist/details/9987/0 (accessed July 20, 2012).

[5] Roach, John. “Naked Mole Rats Unable to Feel Burning Pain.” National Geographic. http://news.nationalgeographic.com/news/2008/02/080201-rat-pain.html (accessed July 20, 2012).

[6] “Naked Mole-rat .” www.sandiegozoo.org. http://www.sandiegozoo.org/animalbytes/t-naked_mole-rat.html (accessed July 20, 2012).

[7] Ciszek, Deborah. “Heterocephalus glaber.” Animal Diversity Web. http://animaldiversity.ummz.umich.edu/site/accounts/information/Heterocephalus_glaber.html (accessed July 20, 2012).

[8] “Naked Mole-rat .” www.sandiegozoo.org. http://www.sandiegozoo.org/animalbytes/t-naked_mole-rat.html (accessed July 20, 2012).

[9] Greenfieldboyce, Nell. “Naked Mole Rat’s Genetic Code Laid Bare.” NPR. http://www.npr.org/2011/10/13/141313080/-re-exposed-naked-mole-rat-genome-sequenced (accessed July 20, 2012).

[10] Kim, Eun Bae, Xiaodong Fang, Alexey A. Fushan et al. “Genome Sequencing Reveals Insights into Physiology and Longevity of the Naked Mole Rat.” Nature.com 479 (2011), http://www.nature.com/nature/journal/v479/n7372/full/nature10533.html (accessed July 20, 2012).

[11] Greenfieldboyce, Nell. “Naked Mole Rat’s Genetic Code Laid Bare.” NPR. http://www.npr.org/2011/10/13/141313080/-re-exposed-naked-mole-rat-genome-sequenced (accessed July 20, 2012).