Naked mole-rats

The naked mole rat (Heterocephalus glaber) also known as the sand puppy or desert mole rat, is a burrowing rodent native to parts of East Africa and the only species currently classified in the genus Heterocephalus. It was once thought to be one of only two eusocial mammals (the other being the Damaraland mole rat) but this classification is controversial owing to disputed definitions of 'eusociality' as well as the existence of other mammals that satisfy the original definition of Wilson (1971). It has a highly unusual set of physical traits that enables it to thrive in an otherwise harsh, underground environment, including being the only mammalian thermoconformer, a lack of pain sensation in its skin and a very low metabolism.

Description
Typical individuals are 8 to 10 cm long and weigh 30 to 35 g. Queens are larger and may weigh well over 50 g, the largest reaching 80 g. 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 to prevent soil from filling their mouths while digging. They have little hair (hence the common name) and wrinkled pink or yellowish skin.

Its karyotype has 2n = 60.

Physiology
The naked mole rat is well adapted for the limited availability of oxygen within the tunnels that are its 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 mouse of the same size, thus uses oxygen minimally. In long periods of hunger, such as a drought, its metabolic rate can be reduced by up to 25 percent.

The naked mole rat does not regulate its body temperature in typical mammalian fashion, homeostasis. They are thermoconformers rather than thermoregulators in that, unlike other mammals, body temperature tracks ambient temperatures. The relationship between oxygen consumption and ambient temperature, however, switches from a typical poikilothermic pattern to a homeothermic mode at 28 °C. At lower temperatures, they use behavioural thermoregulation, as when cold, naked mole rats huddle together or bask in the shallow, more sun-warmed parts of their burrow systems. Conversely, when they get too hot, they retreat to the deeper, cooler parts of their tunnel system.

The skin of naked mole rats lacks a key neurotransmitter called substance P that is responsible in mammals for sending pain signals to the central nervous system. The naked mole rats feel no pain when they are exposed to acid or capsaicin. When they are injected with substance P, however, the pain signaling works as it does in other mammals, but only with capsaicin and not with the acids. This is proposed to be an adaptation to the animal living in high levels of carbon dioxide due to poorly ventilated living spaces, which would cause acid to build up in their body tissues.

Naked mole rats' substance P deficiency has also been tied to their lack of the histamine-induced itching and scratching behavior typical of rodents.

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.

Roles
The naked mole rat is the first mammal discovered to exhibit eusociality. This eusocial structure is similar to that found in ants, termites, and some bees and wasps. Only one female (the queen) and one to three males reproduce, while the rest of the members of the colony function as workers. The queen and breeding males are able to breed at one year of age. Workers are sterile, with the smaller focusing on gathering food and maintaining the nest, while larger workers are more reactive in case of attack. Contrastingly to the norm among mammals, there is no evidence for polygynous mating in the species.

Arguably, the Damaraland mole rat (Cryptomys damarensis) is the only other eusocial mammal currently known.

Queen and gestation
The relationships between the queen and the breeding males may last for many years; other females are temporarily sterile. Queens live from 13 to 18 years, and are extremely hostile to other females behaving like queens, or producing hormones for becoming queens. When the queen dies, another female takes her place, sometimes after a violent struggle with her competitors. Once established, the new queen stretches the space between the vertebrae in her backbone to become longer and ready to bear pups.

Gestation is about 70 days. A litter typically ranges from three to twelve pups, but may be as large as twenty-eight. The average litter size is eleven. In the wild, naked mole-rats usually breed once a year, if the litter survives. In captivity, they breed all year long and can produce a litter every 80 days. The young are born blind and weigh about 2 g. The queen nurses them for the first month; after which the other members of the colony feed them feces until they are old enough to eat solid food.

Workers
Smaller workers focus on acquiring food and maintaining tunnels, while the larger workers are more reactive in case of attacks. As in certain bee species, the workers are divided along a continuum of different worker-caste behaviors instead of discrete groups. Some function primarily as tunnellers, expanding the large network of tunnels within the burrow system, and some primarily as soldiers, protecting the group from outside predators. Workers are sterile when there is no new reproductive role to fill.

Colonies
Colonies range in size from 20 to 300 individuals, with an average of 75.

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, but also eat their own feces. 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 previously indigestible cellulose to be turned into volatile fatty acids.

Longevity
The naked mole rat is also of interest because it is extraordinarily long-lived for a rodent of its size (up to 28 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.

Resistance to cancer
Naked mole rats appear to have a high resistance to cancer; cancer has never been observed in them. A potential mechanism that averts cancer is an "over-crowding" gene, p16, which prevents cell division once a group of cells reaches a certain size. Most mammals, including naked mole rats, have the gene p27 which does a similar task, but prevents cellular reproduction at a much later point than p16 does. The combination of p16 and p27 in naked mole rats cells is a double barrier to cell proliferation. Hypersensitivity to contact inhibition may be the reason for the cancer resistance of the naked mole rat.

Blind mole rats Spalax golani and Spalax judaei also appear to be immune to cancer but by a different mechanism.

Conservation status
Naked mole rats are not threatened. They are widespread and numerous in the drier regions of East Africa.

In popular culture

 * Ron Stoppable, lead male character in the TV series Kim Possible, has a pet naked mole rat named Rufus.


 * Mo Willems' picture book Naked Mole Rat Gets Dressed features a naked mole rat named Wilbur who loves to wear clothes.


 * An exaggerated form of mole rats appear in the video game series Fallout, with their characteristics demented from excessive exposure to radioactive developments.