Naked mole-rat

Name and taxonomy

The German naturalist Eduard Rüppell described both the genus and the species in 1842. Heterocephalus is thought to mean different head, and glaber is Latin for smooth or bald. For most of its history the species sat in Bathyergidae, the family of the other African mole-rats. In 2014 Patterson and Upham proposed a family of its own, Heterocephalidae, on genetic evidence. Some databases, including the one behind this site, still file it under Bathyergidae.

Allen recognised two subspecies in 1939, but the taxon has not had a full revision. Genetic data show two geographic groups within Kenya. The chromosome number is 2n = 60. English names include sand puppy and desert mole rat. German speakers call it the Nacktmull and French speakers the rat-taupe nu, both meaning naked mole.

Evolution and relatives

The naked mole-rat belongs to the rodent group that includes the African mole-rats, or blesmols. Its fossil record reaches back about 4.3 million years, to the early Pliocene. It is the only living member of its genus and, in the newer scheme, of its family. Its closest relatives are the blesmols of the family Bathyergidae, which also dig with their teeth.

One of those relatives, the Damaraland mole-rat, is the only other mammal known to be eusocial. The two species probably evolved colony life separately, in answer to the same problem: scattered food in hard, dry soil. The blind mole-rats of the Middle East are not close relatives, though they also resist cancer, by a different route. Convergent evolution seems to have shaped several unrelated rodents into the same underground body plan.

Appearance

A typical worker measures 8 to 10 centimetres in head and body and weighs 30 to 35 grams. ADW records total lengths of 147 to 165 millimetres and weights from 30 to 80 g. A breeding queen is larger than her workers and often passes 50 g. The heaviest queens reach 80 g. Breeding males are also bigger than workers, but by a smaller margin.

The skin is pink to yellowish brown, wrinkled and almost bare. About 100 fine hairs on the body work like whiskers and guide the animal in the dark. Pups have dark spots that fade with age. The eyes are tiny, with thick lids, and sight is poor. The legs are short and thin, yet the animal runs backwards as fast as it runs forwards.

The front teeth stick out through the closed lips, so the mouth stays free of soil while it digs. About a quarter of the body's muscle works the jaws. There is no fat layer under the skin to hold in heat. Males and females of the worker caste look the same.

Habitat and range

The species lives in the drier tropical grasslands of the Horn of Africa. Its range covers central Somalia, central and eastern Ethiopia, and central and southern Kenya. ADW notes historical records from Uganda and Tanzania. GBIF holds occurrence records from 10 countries, which include zoo animals far from the native range.

Colonies dig in grassland and savanna at elevations between 1,100 and 3,000 metres. The tunnels run about 2 metres below the surface. One colony's tunnels can add up to 3 to 5 kilometres in total length. The only sign above ground is a scatter of small mounds where workers throw out soil. The animals almost never come to the surface, so the species is hard to see even where it is common. Wet soil stops digging, and colonies live on stored food until the ground dries.

Diet and feeding

The naked mole-rat eats the underground parts of plants: tubers, roots and bulbs. Some tubers weigh a thousand times as much as a worker. Workers eat the inside of a tuber and leave the skin, so the plant regrows and feeds the colony for months or years. Bacteria in the gut ferment the plant fibre into fatty acids the animal can absorb. It needs no drinking water, because its food supplies enough.

Food plants are scattered, so workers dig long tunnels to find them. A worker that finds a tuber gives a special call on the way home and waves the food about in the nest. Other workers follow the scent trail it left back to the find. Colonies store food in the nest for use when the soil is too wet to dig. The animals also eat their own droppings, which recovers nutrients and passes the queen's hormones around the colony. Near farms they will take sweet potatoes and maize.

Behaviour and social life

The naked mole-rat was the first mammal shown to be eusocial. One queen and one to three males breed, and every other animal works. Colonies average 75 to 80 animals, and some hold more than 300. Workers are not sterile in body, only in behaviour, and the queen suppresses the ovaries of the other females. Small workers forage, build the nest and tend pups, while large workers dig and act as soldiers.

Digging runs like an assembly line. The front animal cuts soil with its teeth, the others push it back, and the last one kicks it out of a mound. ADW reports that about 100 animals can build 400 to 500 mounds in a year, along tunnels up to 2.9 km long. Workers huddle to stay warm and move to shallow, sun-warmed tunnels when cold, or deeper ones when hot. Most animals have no daily rhythm and sleep in short bouts at any hour.

The queen is the most active animal in the colony and shoves workers to make them work harder. She eats first, and a strict ranking runs from her down to the pups. When she dies, the larger females fight, sometimes to the death, until one wins. Colonies attack strangers, and a wild colony may invade a neighbour and carry off its pups as future workers. A few fat, restless animals called dispersers leave to found new colonies with unrelated mates.

Communication and senses

Life in total darkness leaves sight almost useless, so the animal relies on touch, smell and hearing. The sensitive body hairs and whiskers read the tunnel walls. An animal sweeps its head and tail from side to side as it moves to feel its way. Each colony has its own smell, which fades within days if an animal is taken away.

The species has at least 18 different calls. Soft chirps pass between animals and help set who is dominant. A worker that meets a predator runs home chirping an alarm. A forager announces a new tuber with a special call and a scent trail. The queen's shoves and the shared droppings are also signals, carrying her rank and her hormones. The skin lacks the nerve chemical substance P, so acid and chilli cause no pain and histamine causes no itch.

Reproduction and life cycle

Only the queen breeds, with one to three chosen males, and the bond can last for years. When a female becomes queen, the gaps between her vertebrae widen and her body grows longer to carry pups. In the wild a colony usually raises one litter a year, if it survives. In captivity the queen breeds all year, with a new litter about every 80 days, up to five a year.

Pregnancy lasts about 70 days. A litter holds 3 to 12 pups on average, and the record is 28 in the wild and 27 in captivity. That is the largest litter of any mammal. A queen has only 11 to 12 teats, so pups take turns to nurse. Newborn pups are blind, and each weighs about 2 grams. The queen nurses them for about a month, and workers then feed them faecal pap until they can eat tubers.

Pups are weaned at about 36 days. Workers shove the young about after a few weeks, which is thought to prepare them for disturbance. Females can breed at 228 days and males at a year, but almost none ever do. Queens live 13 to 18 years and age more slowly than workers. The species can live more than 37 years, and ADW gives wild ages up to 30 years. Unlike almost every other mammal, its risk of death does not rise with age.

Predators and threats

Snakes are the main predators, above all the rufous beaked snake and the Kenyan sand boa. A snake can slip along the tunnels, and one has been found with three mole-rats in its stomach. Honey badgers dig into colonies, and raptors take animals at the surface. The most dangerous moment is when a worker kicks soil out of a mound, which biologists call volcanoing. Large workers stand guard and fight intruders, and soldiers face a snake head on.

Skin folds harbour parasites, and the gut can carry the protozoan Eimeria muris. Extreme weather and farm chemicals also cut short wild lives. Cancer is very rare, and the first two cases, found in 2016, were in zoo animals breathing far more oxygen than a burrow holds. No human activity threatens the species as a whole.

Population and conservation

The IUCN lists the naked mole-rat as Least Concern with a stable population trend. Nobody has counted the species, but it is widespread and numerous across its range. Its remote, dry home sees little development, so its habitat is largely intact. Colonies live in several national parks in Kenya, and zoos hold many more.

The animal is, in general, a research species rather than a conservation case. Its draft genome appeared in 2011 and an improved version in 2014. Laboratories keep colonies to study ageing, cancer, pain and survival without oxygen. In 2023 scientists moved its hyaluronan gene into mice, and the mice lived about 4.4 percent longer. The species may one day matter more to human medicine than to any ecosystem.

Relationship with humans

Few people in its range ever see one, because it lives its whole life underground. Where farms spread into its habitat it can eat sweet potatoes and maize. ADW counts it as a draw for ecotourism in Kenya. Zoos in many countries display colonies in glass tunnels, and the animal is a favourite with children.

Science is probably where the species matters most. In 2013 the journal Science named it Vertebrate of the Year after a team at Rochester found its cancer defence. Its cells make a form of hyaluronan five times larger than the human form, and its ribosomes make very few faulty proteins. It runs on about 70% of a mouse's metabolic rate and can cut that by a quarter during hunger. It lives for five hours in air with only 5% oxygen, and 18 minutes with none. These findings are likely to make it a model for stroke, heart attack and ageing research.

In culture and myth

No traditional folklore about the naked mole-rat is on record, which is likely because it is so seldom seen. Its fame is modern and comes from television and science news. In the Disney series Kim Possible, the sidekick Ron Stoppable keeps a naked mole-rat named Rufus. Rufus probably did more than any biologist to make the species known to a generation of children.

Science writers reach for jokes to describe it. National Geographic compares it to a small walrus, or to a sausage that grew teeth. Someone even entered the name Toothy McNudist among its vernacular names on GBIF. The nickname sand puppy is older and gentler. In popular science culture the animal appears to be a symbol of long life and of the hope that ageing can be slowed.