Naked Mole rat

There are many different kinds of mole rats. The best known is probably the naked mole rat, also known as the **sand puppy** or **desert mole rat** whose hairless, tubular, wrinkled body makes it appear a bit like a tiny walrus—or perhaps a bratwurst with teeth. The genus is classified as being in the family Bathyergidae or in its own family, Heterocephalidae. The naked mole-rat, the Damaraland mole-rat, and Mechow's mole-rat are eusocialmammals, the highest classification of sociality. It has a highly unusual set of physical traits that enable it to thrive in an otherwise harsh underground environment; it is the only mammalian thermoconformer, almost entirely ectothermic (cold-blooded) in how it regulates body temperature.

The mole rat lacks pain sensitivity in its skin, and has very low metabolic and respiratory rates. While formerly considered to belong to the same family as other African mole-rats, the Bathyergidae, more recent investigation places the naked mole-rat in a separate family, Heterocephalidae. The mole rat is also remarkable for its longevity and its resistance to cancer and oxygen deprivation.

## Description

Typical individuals are 8 to 10 cm (3 to 4 in) long and weigh 30 to 35 grams (1.1 to 1.2 oz). Queens are larger and may weigh well over 50 grams (1.8 oz), the largest reaching 80 grams (2.8 oz). They are well-adapted to their underground existence. Their eyes are quite small, and their visual acuity is poor. Their legs are thin and short; however, they are highly adept at moving underground and can move backward as fast as they can move forward. Their large, protruding teeth are used to dig and their lips are sealed just behind the teeth, preventing soil from filling their mouths while digging. About a quarter of their musculature is used in the closing of their jaws while they dig—about the same proportion that is utilized in the human leg. They have little hair (hence the common name) and wrinkled pink or yellowish skin. They lack an insulating layer in the skin. Its karyotype has 2n = 60.

## Physiology

### **Metabolism and Respiration**

The naked mole-rat is well adapted for the limited availability of oxygen within the tunnels of its typical habitat; its lungs are very small and its blood has a very strong affinity for oxygen, increasing the efficiency of oxygen uptake. It has a very low respiration and metabolic rate for an animal of its size, about 2/3 that of a similarly sized mouse, thus using oxygen minimally. In response to long periods of hunger, its metabolic rate can be reduced by up to 25 percent.

The naked mole-rat survives for at least 5 hours in air that contains only 5% oxygen; it does not show any significant signs of distress and continues normal activity. It can live in an atmosphere of 80% CO
2 and 20% oxygen. In zero-oxygen atmosphere, it can survive 18 minutes apparently without suffering any harm (but none survived a test of 30 minutes). During the anoxic period it loses consciousness, its heart rate drops from about 200 to 50 beats per minute, and breathing stops apart from sporadic breathing attempts. When deprived of oxygen the animal uses fructose in its anaerobic glycolysis, producing lactic acid. This pathway is not inhibited by acidosis as happens with glycolysis of glucose. As of 2017 it was not known how they survive acidosis without tissue damage.

### **Thermoregulation**

The naked mole-rat does not regulate its body temperature in typical mammalian fashion. They are thermoconformers rather than thermoregulators in that, unlike other mammals, body temperature tracks ambient temperatures. However, it has also been claimed that "the Naked Mole-Rat has a distinct temperature and activity rhythm that is not coupled to environmental conditions." The relationship between oxygen consumption and ambient temperature switches from a typical poikilothermic pattern to a homeothermic mode when temperature is at 28 °C or higher.[15] At lower temperatures, naked mole-rats can use behavioral thermoregulation. For example, cold naked mole-rats huddle together or seek shallow parts of the burrows that are warmed by the sun. Conversely, when they get too hot, naked mole-rats retreat to the deeper, cooler parts of the burrows.

### **Resistance to Cancer**

Naked mole-rats have a high resistance to tumours, although it is likely that they are not entirely immune to related disorders. A potential mechanism that averts cancer is an "over-crowding" gene, p16, which prevents cell division once individual cells come into contact (known as "contact inhibition"). The cells of most mammals, including naked mole-rats, undergo contact inhibition via the gene p27 which prevents cellular reproduction at a much higher cell density than p16 does. The combination of p16 and p27 in naked mole-rat cells is a double barrier to uncontrolled cell proliferation, one of the hallmarks of cancer.

In 2013, scientists reported that the reason naked mole-rats do not get cancer can be attributed to an "extremely high-molecular-mass hyaluronan" (HMW-HA) (a natural sugary substance), which is over "five times larger" than that in cancer-prone humans and cancer-susceptible laboratory animals. The scientific report was published a month later as the cover story of the journal *Nature*. A few months later, the same University of Rochester research team announced that naked mole-rats have ribosomes that produce extremely error-free proteins. Because of both of these discoveries, the journal *Science* named the naked mole-rat "Vertebrate of the Year" for 2013.

In 2016, a report was published that recorded the first ever discovered malignancies in two naked mole-rats, in two individuals. However, both naked mole-rats were captive-born at zoos, and hence lived in an environment with 21% atmospheric oxygen compared to their natural 2–9%, which may have promoted tumorigenesis. The Golan Heights blind mole-rat (*Spalax golani*) and the Judean Mountains blind mole-rat (*Spalax judaei*) are also resistant to cancer, but by a different mechanism.

### **Longevity**

The naked mole-rat is also of interest because it is extraordinarily long-lived for a rodent of its size (up to 32 years) and holds the record for the longest living rodent. Naked mole-rats are highly resistant to cancer and maintain healthy vascular function longer in their lifespan than shorter-living rats. The reason for their longevity is debated, but is thought to be related to their ability to substantially reduce their metabolism during hard times, and so prevent aging-induced damage from oxidative stress. This has been referred to as "living their life in pulses". Their longevity has also been attributed to “protein stability.” Because of their extraordinary longevity, an international effort was put into place to sequence the genome of the naked mole-rat. A draft genome was made available in 2011 with an improved version released in 2014. Further transcriptome sequencing revealed genes related to mitochondria and oxidation reduction processes to have high expression levels in the naked mole-rat when compared to mice, which may contribute to their longevity.

The DNA repair transcriptomes of the liver of humans, naked mole rats and mice were compared. The maximum lifespans of humans, naked mole rats, and mice are respectively ~120, 30 and 3 years. The longer-lived species, humans and naked mole rats, expressed DNA repair genes, including core genes in several DNA repair pathways, at a higher level than did mice. In addition, several DNA repair pathways in humans and naked mole rats were up-regulated compared with mice. These findings suggest that increased DNA repair facilitates greater longevity, and also are consistent with the DNA damage theory of aging.

### **Size**

Reproducing females become the dominant female, usually, by founding new colonies, fighting for the dominant position, or taking over once the reproducing female dies. These reproducing females tend to have longer bodies than that of their non-reproducing counterparts of the same skull width. Interestingly enough, the measurements of females before they became reproductive and after show significant increases in body size. It is believed that this trait does not occur due to pre-existing morphological differences but to the actual attainment of the dominant female position. As with the reproductive females, the reproductive males also appear to be bigger in size than their non-reproducing counterparts but not as much so as in the case of the females. These males also have visible outlines of the testes through the skin of their abdomens. Unlike the females, there are usually multiple reproducing males.

## Ecology and Behavior

### **Distribution and Habitat**

The naked mole-rat is native to the drier parts of the tropical grasslands of East Africa, predominantly southern Ethiopia, Kenya, and Somalia. Clusters averaging 75 to 80 individuals live together in complex systems of burrows in arid African deserts. The tunnel systems built by naked mole-rats can stretch up to three to five kilometres (2–3 mi) in cumulative length.

### **Colony Behavior**

Naked mole rats are rodents, but they live in communities like those of many insects. Several dozen rats live together in colonies led by one dominant rat—the queen. As in some insect species, the queen is the only naked mole rat female to breed and bear young.

Worker animals dig the burrows that the whole clan inhabits, using their prominent teeth and snouts. They also gather the roots and bulbs for the colony to eat. Other rats tend to the queen. Most other types of mole rats live on their own or in small families. Though mole rats spend most of their time excavating and foraging in their burrows, they occasionally emerge to search for seeds or other plants.

### **Diet**

Naked mole-rats feed primarily on very large tubers (weighing as much as a thousand times the body weight of a typical mole-rat) that they find deep underground through their mining operations. A single tuber can provide a colony with a long-term source of food—lasting for months, or even years, as they eat the inside but leave the outside, allowing the tuber to regenerate. Symbiotic bacteria in their intestines ferment the fibres, allowing otherwise indigestible cellulose to be turned into volatile fatty acids. Naked mole-rats sometimes also eat their own feces. This may be part of their eusocial behavior and a means of sharing hormones from the queen.

### **Predators**

Naked mole rats are primarily preyed upon by snakes especially the Rufous Beaked Snake and Kenyan Sand Boa as well as various raptors. They are at their most vulnerable when constructing mounds and ejecting soil to the surface.