Naked mole rat
|Naked mole rat|
Naked mole rat ( heterocephalus glaber )
|Scientific name of the family|
|Landry , 1957|
|Scientific name of the genus|
|Rüppell , 1842|
|Scientific name of the species|
|Rüppell , 1842|
The naked mole rat ( Heterocephalus glaber ) is the only species of the genus Heterocephalus and the family Heterocephalidae within the order of rodents . It was long classified in the family of sand graves (Bathyergidae), with which it still forms a common superfamily today.
Naked mole rats live in large underground structures in the semi-deserts of East Africa, especially in southern Ethiopia , Kenya and Somalia . The species lives in large colonies in eusociality and with this state formation shows behavior that is very rare in mammals. Another recent eusocial species is the Damara gray mullet ( Fukomys damarensis ) in southern Africa.
Naked mole rats have a body five to fifteen centimeters long. They weigh about 30 to 50 grams. They owe their name to the fact that their small and very fine hairs, with the exception of a few sensory hairs ( vibrissae ), are barely noticeable and they therefore appear naked. This is seen as an adaptation to their way of life in underground colonies . Because of the low hairiness, parasites can also settle and spread more poorly on their bodies. The skin is loose and wrinkled on the body and is brown-pink in color, with the back being slightly darker than the belly. On the one hand, the wrinkled skin protects the internal organs when the animals push their way through very narrow corridors, and on the other hand it enables rapid movement in the corridor systems, whereby the animals can move backwards as quickly as forwards. Other features are the pronounced incisors, which are very easy to recognize. With them the naked mole rat can cut and chop its food.
The vibrissae (whiskers) are mainly found on the face, where the whiskers are the longest with up to two centimeters, and also on the front and rear legs, the eyelids and the mouth area. The head of the animals is conical with a flattened snout, only the region around the very small eyes is rounded. The eyes are also covered by a thick eyelid , so that only small slits remain. The visual perception is very low; however, the eyes are sensitive to drafts. The ear openings are small and the animals have no external auricles . The nostrils are close together in a horseshoe-shaped area above the incisor teeth , they are covered by a fold of skin.
The animals also have strikingly large incisor teeth that they can use like excavator shovels. For this purpose, the animals have very strong masticatory muscles , which determine the shape of the head and make up about 25 percent of the animal's total muscle mass. The incisor teeth sit in front of the skull, in front of the lip. They can be used by the naked mole rat to burrow through the hard desert floor. While the animals dig with their teeth, the oral cavity is closed by skin folds to prevent dirt from entering. The teeth are worn at the tip, but at the same time they are also sharpened by rubbing against the excavation material. The incisor teeth grow lifelong. As an additional specialty, naked mole rats can also move their incisor teeth individually. In addition, the animals have three molars in each arch, resulting in a dentition formula of 3 · 0 · 0 · 1 | 1 · 0 · 0 · 3 yields.
The animals use up to 18 different sounds to communicate with each other, some of which are reminiscent of birds chirping.
In addition to the anatomical features already described, the naked mole rat also have a number of physiological adaptations to their habitat.
Breathing plays an important role , as there is hardly any oxygen in the animals' burrows (hypoxic habitat). The lungs of the animals are very small. On the other hand, their hemoglobin has a very high oxygen affinity, which means that naked mole rats can very efficiently absorb oxygen into the blood. Added to this is the very low breathing and metabolic rate for animals of their size, which keeps oxygen consumption to a minimum. In longer periods of hunger, such as during the dry season , this rate is reduced by another 25 percent.
Naked mole rats are considered to be of the same temperature , but compared to other mammals, the characteristics of a cold-blooded animal are particularly striking. They adjust their body temperature in the range between 12 ° C and 32 ° C to the ambient temperature. This helps to save energy with the different temperatures in their cave systems.
While the warmth dominates in the higher corridors, the naked mole rat find cooling in the lower and colder corridors. The thin skin and the also very thin layer of fat allow a very quick temperature adjustment. "Group cuddling" in the nest serves an additional, external regulation of body heat: several animals press close together.
The diet of the naked mole rat consists of very fibrous plant tubers, which usually do not have a very high nutritional value. In order to be able to use this food optimally, the animals have symbiotic bacteria, especially in the appendix , the caecum, which break down the food and make nutrients available. Much like rabbits , naked mole rats eat food that has been digested and excreted a second time in order to use it more efficiently ( coprophagia ).
The animals also ingest minerals through food and bones that they find during their digging activity. These minerals can be used in the animals through vitamin D 3 -independent processes, since the vitamin is not formed in animals that live without sunlight.
Naked mole rats do not drink. They get the water they need from their food. Since this also is a high salt content, mole rats have very efficient kidney that a urine with a maximum concentration up to 1.5 moles of salt can produce per kilogram of urine, which corresponds to 87.5 grams per liter.
In order to absorb their own stable smell, which differs from that of other colonies, naked mole rats roll around in their excrements in the communal toilet cave.
According to Sherman and Jarvis, naked mole rats can live to be significantly older than 15 years. Both describe the litter of a 21.7 year old female, other authors have shown an age of more than 28 years. This means that naked mole rats outperform other rodents such as mice, rats and hamsters many times over. While the latter usually die (or are eaten) from cancer and compensate for this with a high reproductive rate, the gene repair system and protein stability in nude mole rats are evidently much better developed.
A study from 2013 suggests a special version of the glycosaminoglycan hyaluronic acid , which is about five times larger than in humans or rodents and interacts with the cell differentiation cluster CD44 , which allows an early detection of cancer cells (and an increased contact inhibition) as the cause . The extracellular chain-like glycosaminoglycan hyaluronic acid is also broken down much more slowly than in other animal species or humans.
The blood protein alpha2-macroglobulin (A2M) and the like has long been used as a further cause of the cancer protection of the naked mole rat . a. researched at the University of Leipzig. Presumably, it disrupts the signaling pathways that cause healthy cells to turn into cancer cells. It also occurs in humans, but here its concentration decreases continuously from the age of 20. In the case of the naked mole rat, however, it remains consistently high throughout life. Since this also promises to be cancer protection for humans, the A2M of naked mole rats and humans were compared.
According to Birkenmeier, there are also indications of an influence of the intestinal flora, which is similar to that of the hundred-year-old Japanese on the Okinawa archipelago.
Insensitivity to pain
Naked mole rats have a reduced pain sensation. They are the only known species of mammal whose skin lacks substance P. This molecule, consisting of eleven amino acids, is involved in the perception of pain in a way that has not yet been conclusively explained. Naked mole rats perceive stings, heat or acid, but do not perceive them as painful. However, if the animals are made to produce substance P by introducing a gene , their sensitivity to pain increases significantly. Results like these brought substance P antagonists into the focus of scientific research, for example for pain therapy .
distribution and habitat
The naked mole rat lives as an endemic exclusively in the dry and hot semi-deserts of East Africa. The distribution area extends over most of Somalia, the central part of Ethiopia and a large area of eastern and northern Kenya. The southernmost distribution limit runs in the area of the border of the Tsavo West National Park and the city of Voi .
The habitat of the naked mole rat is characterized by a dry semi-desert climate with 200 to 400 millimeters of precipitation per year. The soil is usually a very hard lateritic red earth, but colonies can also be found in pure sand or in real laterite soils.
Way of life
Naked mole rats live in colonies of 20 to 300 animals. The organization of these colonies shows some peculiarities that are otherwise observed in particular in insects and are referred to as eusociality in their entirety . It is characterized by a strict, highly specialized division of labor that is tied to the age of the individual.
Young naked mole rats look after their younger siblings. When they get older, they work as graves ("workers") and are responsible for expanding the corridor system. They work together like on an assembly line. Larger and older animals ("soldiers") are at the exits of the burrow and guard them, for example against their main enemy, the reddish beak-nosed snake . They also throw out the material brought in by the workers from the corridors (“volcanoes”).
Each colony is dominated by a single queen who is the only female fertile and gives birth approximately every 70–80 days. The queen is larger than the other animals and mates with one to three males from the colony, which age surprisingly quickly after entering the mating phase. In order to suckle the up to 27 offspring of a litter, the queen has visible teats on which they suckle for 4 weeks. In order to accommodate the many unborn young animals in her body, she has an arched spine; However, in order for it to remain mobile in the narrow passages, it grows in length during pregnancy. In contrast to the gray mullet, which also show highly socialized behavior, naked mole rats are extremely aggressive and often attack their infertile daughters and sons.
The reasons for the sterility of the offspring have not yet been adequately researched. Some older speculations assume hormonal suppression by the queen, but these assumptions could be refuted by simple experiments. Another explanation is that due to the constant stress that the daughters have to endure, their ovaries do not reach maturity and they therefore remain sterile . Why there are no matings among the sterile workers in nude mole rats can be explained by the fact that mammals have mating times that are tied to the estrous phase in the sexual cycle of the females and can therefore only start when the females have fully developed ovaries and thus also have a sex cycle .
If a naked mole rat queen dies, her stress suppression is missing, which means that several workers become fertile and then fight each other. In a laboratory situation in the 1980s, a "war of succession" lasted 16 months and killed nine rodents. As a rule, the female who can give birth to young the fastest wins.
The causes of the social behavior are not fully understood. The so-called cooperation theory sees this as an adaptation to the spotty food in the animals' habitat. In order to find food in the hard soil of the semi-desert, as many animals as possible must go in search of food and share the food sources found. A more recent hypothesis assumes that the length of the gestation period in naked mole rats and in general in the Bathyergidae was the cause of the development of eusociality in these animals. A single female is not able to eat enough fat reserves for the long gestation and breastfeeding period, and because of her large body size, it is difficult to walk through the underground passages and search for food during this time. It is therefore dependent on the help of the partner and later the boy. The gestation period is such a conservative characteristic that the social structure adapts to it rather than the other way around. According to this theory, eusociality developed via the intermediate stage of monogamy .
According to more recent findings, it is only when the naked mole rat queen eats the faeces that contain estrogen that the other animals take care of the young.
Naked mole rats stick together as a colony. If workers get into the territory of another colony during their digs, bloody fights ensue.
The oldest known fossils, which can safely be said to be direct ancestors of the naked mole rat, date from the Miocene 11 to 25 million years ago and were found in 1986 near the city of Karamoja in Uganda . There is also a fossil from the Kakesio Plain in Tanzania that is around 4.3 million years old . These two fossil finds did not have their own species names, unlike the finds of H. quenstedti from the Pliocene in Laetoli , H. atikoi from Omo , Ethiopia, and H. jaegeri from Olduvai , Tanzania. However, it has not been conclusively clarified whether these were actually other species.
Systematics of research history
The naked mole rat is classified as an independent species within the monotypic genus Heterocephalus . The first description of the naked mole rat under the scientific name Heterocephalus glaber (in German about 'Glatter Andersköpfiger') was made in 1842 by the German biologist Eduard Rüppell on the basis of specimens that were caught near the city of Shewa in Ethiopia. The species status of the animals was subsequently questioned and several researchers assumed that the animals described must be the naked young of larger, fur-bearing rodents. In the following years, however, other nudibranch species from different regions were described by Oldfield Thomas , which were recognized as synonyms by Glover Morrill Allen in 1930. It refers to:
- Heterocephalus phillipsi Thomas 1885 from Gerlogobi, Central Somaliland (today Geregube , Ethiopia )
- Heterocephalus ansorgei Thomas 1903, found between Ngomeni and Kjinani , British East Africa (now Kenya)
- Fornaria phillipsi Thomas 1903 from Mogadishu , Italian Somaliland (now Somalia )
- Heterocephalus dunni Thomas 1909 from Wardairi, Central Somaliland, (today Warder , Ethiopia)
- Heterocephalus stygius Allen in 1912 from the Guaso Nyiro River in British East Africa
Until then, research into the naked mole rat was limited to a description of anatomy and faunistics . The British evolutionary philosopher Alfred Russel Wallace called the naked mole rat "an extraordinarily ugly species" and based on this he coined the term Darwinist impasse . It was not until 1980 that South African Jennifer Jarvis discovered the colonial way of life of animals. Together with Richard Alexander she explained the eusociality of animals, which made them the focus of a number of researchers.
For the first extensive research in the laboratory, 2 colonies with 108 specimens were caught and placed in a system of up to 10 m long glass and Plexiglas tubes with shoe boxes as living containers. The diet was problem-free, since naked mole rats are not picky, a. with carrots, bananas and dog biscuits. Today, Jennifer Jarvis and Paul Sherman, as well as the German zoologists Hynek Burda and Thomas B. Hildebrandt , are mainly working on animal research.
Phylogenetic systematics of the sand graves
The naked mole rat has long been classified in the family of sand graves (Bathyergidae), with which it still forms a common superfamily today. The species represents the most basic group and at the same time the sister species of this family group. On the basis of molecular biological analyzes, a clear genetic distance to the other species of the sand burrows has been determined several times.
Due to the genetic distance and the long time separation of the two taxa , however, the species has been assigned to a separate family Heterocephalidae in current works.
Due to its ugliness , which many people perceive subjectively , the naked mole rat has already found its way into popular culture. Among other things, he was used as a mascot on a radio show for the NRJ Group . Songs and comics with the naked mole rat were also created. There is a naked mole rat named Rufus in the Disney animated series Kim Possible .
The English-language science magazine Discover used a retouched nude mole rat photo for an April Fool's joke in 1995 . In a serious article, it reported on an animal allegedly newly discovered in Antarctica, the hot-headed ice borer . These naked mole rat-like animals should be able to melt their way through the ice in groups from below with a special head outgrowth in order to catch prey (mostly penguins). Discover magazine received more letters to the editor for this article than for any other article ever published.
In a production of the operetta Die Molratte courting or: Liebesspiel am Nile by Stefan Frey , the naked mole rat are artistically processed.
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