Naked mole rat

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 genus Heterocephalus. It is one of only two known eusocial mammals (the other being the Damaraland mole rat) and has a highly unusual set of physical traits that enables it to thrive in a harsh, underground environment, including a lack of pain sensation in its skin and a very low metabolism.

Physical description

Typical individuals are 8–10 cm long and weigh 30–35 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.

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 using 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 can only regulate its body temperature in typical mammalian fashion, homeostasis, over a relatively narrow range of temperatures, outside of which it overheats or suffers hypothermia. They can overcome this via behavioral means to keep a constant temperature 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. When naked mole rats are exposed to acid or capsaicin, they feel no pain. When 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 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.

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 two or three miles in cumulative length.

Social structure and reproduction

The naked mole rat is one of the two species of mammals that exhibit eusociality. They have a complex social structure in which only one female (the queen) and one to three males reproduce, while the rest of the members of the colony function as workers. 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. Colonies range in size from 20 to 300 individuals, averaging 75 individuals.

This eusocial organisation social structure, similar to that found in ants, termites, and some bees and wasps, is very rare among mammals. The Damaraland Mole Rat (Cryptomys damarensis) is the only other eusocial mammal currently known.

The relationships between the queen and the breeding males may last for many years. A behaviour called reproductive suppression is believed to be the reason why the other females do not reproduce, meaning that the infertility in the working females is only temporary, and not genetic. 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.

Males and females are able to breed at one year of age. Gestation is about 70 days. A litter typically ranges from three to twelve pups, but may be as large as 28. The average litter size is 11. 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.

Diet

Naked mole rats feed primarily on very large tubers (weighing as much as 1000 times the body weight of a typical mole rat) that they find deep underground through their mining operations, but also eat their own feces (coprophagia). 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 help them digest the fibres.

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. The reason for their longevity is debated, but is thought to be related to the fact that they can shut down their metabolism during hard times, and so prevent oxidative damage. This has been summed up as "They're living their life in pulses." Because of their extraordinary longevity, an international effort was put into place to sequence the genome of the naked mole rat.

Cancer

Naked mole rats appear to have a high resistance to cancer; cancer has never been observed in them. The mechanism that stunts cancer is a gene called p16, known as an "over-crowding" gene, which prevents the creation of new cells once a group of cells reaches a certain size. Most mammals, including naked mole rats, have a gene called 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 creates a double-layered barrier that prevents the formation of cancer cells.

Conservation status

Naked mole rats are not threatened. Despite their tough living conditions, they are quite widespread and numerous in the drier regions of East Africa.

Translation of "Naked mole rat"

Bengali: নেকেড মোল ইঁদুর, Breton: Razh-goz noazh, Catalan: Farumfer, Czech: Rypoš lysý, German: Nacktmull, Spanish: Heterocephalus glaber, Esperanto: Heterocefalo, French: Rat-taupe nu, Italian: Heterocephalus glaber, Luxembourgish: Plakeg Maulefsrat, Hungarian: Csupasz turkáló, Dutch: Naakte molrat, Japanese: ハダカデバネズミ, Norwegian (Bokmål): Nakenrotte, Polish: Golec, Portuguese: Rato-toupeira-pelado, Russian: Голый землекоп, Simple English: Naked Mole Rat, Finnish: Kaljurotta, Ukrainian: Кротячий пацюк.