Mongolian racing rat ( Meriones unguiculatus )
|JE Gray , 1825|
The gerbils (Gerbillinae) form a subfamily of the long-tailed mice and inhabit the deserts, semi-deserts, steppes and savannas of Africa and Asia. They are the largest group of rodents adapted to life in dry environments. Some gerbils, particularly the Mongolian racing rat , are often kept as pets.
Build and measurements
Gerbils often resemble the familiar Mongolian racing rat, but come in varying sizes from mouse-sized and slender to larger and compact. In contrast to the outwardly similar rats and mice, they usually have hairy tails and soles as well as somewhat elongated hind legs.
Smaller species of gerbil, such as the real gerbil, weigh 8 to 11 grams, have a body length of 6.2 to 7.5 centimeters and a tail length of 7.2 to 9.5 centimeters. The Indian gerbil, on the other hand, weighs 115 to 190 grams, and its body and tail measure 15 to 20 centimeters and 16 to 22 centimeters.
Fur, color and tail
The soft fur of the gerbils gives them a mouse-like appearance. That of the top is usually light yellow, light brown or grayish, that of the underside is white or cream-colored. On the one hand, this coloring reduces the risk of being eaten, since it corresponds to the color of the soil on which the animals live. Even within a species, regional populations adapt to the respective color of the subsoil. Animals on dark floors are dark brown, those on red floors have a reddish fur. On the other hand, the light underside reflects the heat of the surface better. The hairy soles of their feet also allow them to walk on hot floors.
The tail serves the gerbils as a balance aid during movement and they can use it to sweep sand over the entrance of their burrow to hide it. It also serves as protection against predators. The tassel at the tip of the tail distracts from the animal's body and the tail can fall off in whole or in part if it is grasped. A hairy tail protects the animals from dehydration.
Gerbils have large, dark eyes that are high on the head and give them a large field of vision. Their large middle ear is also typical , especially in species in desert landscapes. This enables hearing in the low frequency range and even sounds like the flapping of owls' wings can be heard.
Sound conduction apparatus
The gerbil's eardrum is either simple and consists of only one main part, or it is made up of several parts with an additional secondary eardrum . The main part is made up of two parts, which, as in other mammals, differ in the structure of the tissue : the large, tense pars tensa and the small, flaccid pars flaccida . However, the fabric of the side eardrum is similar as in the gerbils which the Pars tensa . It lies above this and thus occupies a similar position as the pars flaccida in most other species of mice. It is separated from the pars tensa by the border arch , a thin band of connective tissue between the anterior and posterior protruding bones . However, the protruding bones are only clearly pronounced in gerbils with a secondary eardrum. In the Cape short-tailed gerbil , the brewer gerbil , the Somali gerbil and the fat -tailed gerbil , the secondary eardrum is large, and it occurs in some species of the real gerbils and the racing rats .
In gerbils without a secondary eardrum, the lateral wall of the epitympanic recess is formed by a thin plate of bone. Two bony leaves approach each other from the anterior and posterior limbs of the anulus tympanicus and are usually separated from one another by a narrow, slit-like opening. This opening is widest at the free, lower end and becomes narrower towards the top. It is spanned by part of the pars flaccida . Occasionally the two bone leaves are fused together without gaps. In the Azeri racing rat , the bone leaf has a rounded opening in front of the notch in the border of the eardrum, the tympanic notch . In contrast, the lower part of its rear edge has grown together with this notch. If the bone leaf is enlarged, as in the Mongolian racing rat and the Libyan racing rat, its rear edge displaces the lower part of the notch and thus reduces its vertical extent. With a further enlargement of the bone sheet, so in some species of the real gerbil, the fat-tailed gerbil and the brewer gerbil , the notch and the side wall of the recess are almost completely displaced by the secondary eardrum. In all gerbils, however, the notch remains at least as a small gap on the rear, upper edge of the border of the secondary eardrum.
In the absence of the secondary eardrum, the head of the hammer is covered by the bony septum that fills the upper part of the ear canal . In contrast, if the secondary eardrum is present, the head is visible from the outside. It is either light with a straight upper edge or rather massive and teardrop-shaped with a protruding upper edge. The smaller hammer grip takes a vertical position.
The combination of a light or solid head and a single or multi-part eardrum enables the reconstruction of the phylogenetic relationships of some gerbil groups. Knowledge of the original or derived nature of the secondary eardrum and the light head is of the greatest importance. The original characteristic state of the mice, a hammer with a light head and horizontal hammer handle, is always accompanied by a secondary eardrum. It occurs in mice generalized with regard to the tympanic bladder and never in gerbils. The evolutionary transformation involves first shifting the hammer grip from a horizontal to a vertical position and then developing from a light to a massive head, and occasionally a regression to the light head. In voles , as in gerbils, the front part of the hammer has a rib-shaped element that always runs at the top of the hammer with a horizontal handle and always in the middle of the hammer with a solid head. For gerbils it can accordingly be assumed that in the hammer with a light head, a rib running at the top indicates the original feature state and a central rib indicates a derived feature. For example, some racing rats with heavily inflated tympanic bladders have a secondary eardrum and a hammer with a central rib and a light head. The mentioned characteristics of the hammer allow conclusions to be drawn about the subdivision of the short-tailed gerbil and the position of the bush- tailed gerbil within the “ typical gerbil ” (Gerbillini).
Gerbils have the myomorphic chewing apparatus that is typical of the mice . However, unlike the more generalized representatives of the mice, the anterior parts of the masticatory muscle dominate the temporal muscle . Corresponding modifications of the bony chewing apparatus are an enlargement of the zygomatic plate and the anterior part of the zygomatic arch , combined with a reduction in the temporal plate of the cerebral skull and a reduction in the muscle process of the lower jaw. The articular process, on the other hand, is high, the ascending branch of the mandible is narrow, and the posterior bulge between the articular process and the angular process is large. The jaw is therefore comparatively weak. The under-eye hole is often very narrow and never large.
The zygomatic plate consists of two clearly distinguishable parts. The front part is the elongated outgrowth of the plate forming a keel towards the front. It runs along the snout and is characteristic of the gerbil, but also occurs in some other species of mice. The posterior part is typical of all mice and is located near the insertion of the anterior zygomatic arch root. Among the recent gerbils, the brewer gerbil and the actual gerbil are characterized by the least developed zygomatic plate. In addition, two major, if not entirely different, further developments can be ascertained: In the case of the taterillines , in particular the bare-soled gerbils , the keel is greatly elongated, in the " higher gerbils " (Rhombomyina), especially the gerbil , the plate is on the other hand vertically enlarged. In some genera, such as the Somali gerbil , an upward expansion of the front part of the zygomatic arch leads to the formation of a conspicuous orbital shield . However, no clear differences can be found between the ancestral groups with regard to these characteristics. Only the extension of the keel can possibly be interpreted as a derived common feature of the taterillines. The two development trends of the zygomatic plate are related to a reshaping of the temporal plate of the brain skull, which is delimited above by parasagittal ridges : it is narrower in the taterillines with increasing keel elongation and expands in the most advanced "higher gerbils". The reshaping of the temporal plate reflects the parallel evolution of the chewing apparatus between “higher gerbils” and voles and can be assessed as a derived specialty of the “higher gerbils”.
The changes in the lower jaw are partly related to the development of the tympanic bladders and the change in the proportions of the masticatory muscles . It can be assumed that the inflation of the tympanic membranes is responsible for the deformation of the chewing apparatus. This connection is most noticeable in comparison to mice with generalized tympanic bladders: The ascending branch of the lower jaw in gerbils is more vertical and narrower, the posterior bulge between the articular process and the angular process is deeper, and the angular process is less vertical. These interrelated features were even found in gerbils with comparatively small tympanic bladders and can serve as an indirect indication of an inflation of the tympanic bladders. In particular, the recent gerbils seem to be distinguishable from the fossil myocricetodontins , which have a lower jaw typical of the mouse species. However, this connection is not particularly pronounced and in the fat-tailed gerbil with its inflated tympanic bladders the ascending branch of the lower jaw is very wide, but takes a more vertical position than in the " typical gerbils " (Gerbillini). Some other gerbils also have a peculiar structure of the lower jaw, each representing a derived specialty. In the Somali gerbil, the muscle process is completely absent. This is a unique feature within the mouse species and is associated with the narrowest temporal plate and the largest orbital shield. In the Brauer gerbil, the articular process is long and narrow and the angular process is very wide. The Mauritanian gerbil has a similar structure of the lower jaw as the brewer gerbil .
Way of life
Water is usually released through the skin, breathing, urine, and feces. Most gerbils colonize dry regions with difficult climatic conditions and have an unfavorably large body surface in relation to their volume. They have developed properties adapted to this in order to keep water loss as low as possible and thereby reduce the need for fluids. They do not sweat and therefore cannot survive temperatures above 45 degrees Celsius for more than two hours. Most species are nocturnal and live during the day in burrows underground, the entrances of which are often blocked and which offer them constant temperatures between 20 and 25 degrees Celsius at a depth of around 50 centimeters. Some northern species also surface during the day, and southern sand mice also come to the surface in winter. At night their food, often just dry seeds and leaves, is moistened with dew and increases the moisture in the burrow when it is taken in to eat. The gerbil's digestive system deprives food of almost all water, the feces are dry, and the kidneys produce only a few drops of concentrated urine.
Gerbils feed primarily on plant material such as seeds, fruits, leaves, stems, roots and tubers. The nocturnal species of real gerbils look for seeds blown by the wind in the desert. The sand rats specialize in salty, succulent plants and the Indian gerbil needs fresh food all year round and often lives near irrigated fields. However, many species take what they can get and also consume insects , snails , reptiles, and even other rodents. Animals in the extremely dry deserts of southern Africa, in particular, mainly catch insects, while mountains of empty snail shells form before Gerbillus dasyurus is built.
As a precaution, the food is usually eaten in the building. Species in areas with cold winters store large stocks in their burrows, and large gerbils also build up to 1 meter wide and 3 meter long piles in front of their burrows.
While gerbil species are mostly solitary in hot deserts, more social species with permanent pairing and family structures live in areas with more food. The most complex are the social structures in animals of the Rhombomyina, which live in areas with cold winters. The gerbil and the Mongolian racing rat in particular live in large colonies made up of numerous subgroups.
Reproduction and Life Expectancy
Gerbil species living in savannahs throw up after the rainy season. In places where there is always fresh food, the females give birth two to three times throughout the year. Some desert species, on the other hand, only reproduce in the colder months. The gestation period is 21 to 28 days and a litter consists of 1 to 12 young animals, usually three to five. The offspring are born helpless, naked and blind and are dependent on their mother for two weeks. The puberty begins between the ages of two and six months and depends also on whether the animals may come to breed in the same season.
Life expectancy in nature is usually one to two years.
Gerbils are common in three main regions:
- In the savannahs of Africa , but also in the Namib and Kalahari , where temperatures in winter often fall below freezing point.
- In the hot deserts and semi-deserts of North Africa and the Middle East as well as the dry Horn of Africa .
- In the deserts, semi-deserts and steppes of Central Asia , where winter temperatures also drop well below freezing.
The individual genera are usually assigned to one of these three regions. In addition to deserts, steppes and savannahs, they also colonize cultivated land .
The gerbils form a to the long-tailed mice belonging lineage of mice-like . They can be clearly distinguished morphologically from other species of mice by a number of derived peculiarities . Molecular genetic studies of several genes of the mitochondria and the cell nucleus confirm their independence and indicate a closer relationship with the Old World mice and a sister group relationship with the Deomyins . They may have evolved from the fossil myocricetodontins .
In earlier systematics , the gerbils were often placed in the vicinity of Old World burrows with originally simple, cricetid molars or were regarded as related to the Madagascar rats and other mice endemic to Africa . The closer relationship with the myocricetodontins, i.e. mice with a more complicated, murid molar type was suggested because of the great similarity with regard to the pattern of the molar crowns in the most generalized gerbils and myocricetodon . However, the additional side humps of the mandibular molars that are present in most myocricetodontins were initially unknown in gerbils.
The internal systematics of gerbils follows Chevret and Dobigny (2005) and Pawlinow (2008) and is based on molecular genetic studies of several mitochondrial genes as well as morphological studies of the skull and teeth. Information on the distribution follows McKenna and Bell (1997) and Pawlinow (2008) and includes fossil finds.
- Tribus Desmodilliscini Pavlinov, 1982
- Tribus Gerbillurini Pavlinov, 1982
- † Protatera Jaeger, 1977 , in North Africa, Southwest Europe and South Asia
- † Debruijnimys Castillo & Agusti, 1996 , in southwest Europe
- † Abudhabia de Bruijn & Whybrow, 1994 , in South Asia
- Indian gerbil ( Tatera lataste, 1882 ), in southern Asia
- Barefoot gerbils ( Gerbilliscus Thomas, 1897 ), in Africa
- Namib gerbils ( Gerbillurus Shortridge, 1942 ), in southern Africa
- Cape short-tailed gerbil ( Desmodillus Thomas & Schwann, 1904 ), in southern Africa
- Tribus Ammodillini Pavlinov, 1981
- Somali gerbil ( Ammodillus thomas , 1904 ), in the Horn of Africa
- Tribus Taterillini Chaline, Mein & Petter, 1977
- Small bare-sole gerbils ( Taterillus Thomas, 1910 ), in Africa
- Tribus Gerbillini Gray, 1825 ("typical gerbils")
- Untertribus Gerbillina Gray, 1825 ("lower gerbils")
- Bush-tailed gerbil ( Sekeetamys Ellerman, 1947 ), in Egypt and Southwest Asia
- Untertribus Rhombomyina Heptner, 1933 ("higher gerbils")
- † Mascaramys Tong, 1986 , in Northern Africa
- Sand rats ( Psammomys Cretzschmar, 1828 ), in western Asia and northern Africa
- † Pliorhombomys Fokanov, 1976 , in Turkmenistan
- Great gerbil ( Rhombomys Wagner, 1841 ), native to Asia
- Przewalski racing rat ( Brachiones Thomas, 1925 ), in China
- Racing rats ( Meriones Illiger, 1811 ), in northern Africa, western Asia and eastern Europe
The exact number of species is still unknown. Visible differences within the genera are often very subtle and manifest themselves in the color of fur and claws, in the length of the tail or in the absence or presence of a tail tassel. Even assigning a species to a genus is sometimes hardly possible without comparing chromosomes , proteins or molecules .
Gerbils and humans
Several species of gerbil are endangered by human encroachment on their habitat, and some are even threatened with extinction . Most of the animals live in barely inhabited areas, others are sometimes viewed by humans as pests because, especially in winter, they plunder the fields and damage the infrastructure through their burial activity. Farmers therefore fight the animals with poison gas or plow up their building systems. As hosts of fleas , they spread diseases such as the plague and are themselves also carriers of dangerous leishmaniasis . Their sweet meat, on the other hand, is considered a delicacy. Many species are used by humans as laboratory animals in research or kept as pets .
- Pascale Chevret, Gauthier Dobigny: Systematics and evolution of the subfamily Gerbillinae (Mammalia, Rodentia, Muridae) . In: Molecular Phylogenetics and Evolution . tape 35 , no. 3 , 2005, ISSN 1055-7903 , p. 674-688 , doi : 10.1016 / j.ympev.2005.01.001 .
- John Reeves Ellerman: The Families and Genera of Living Rodents. Volume II. Family Muridae . British Museum (Natural History), London 1941 (690 pages).
- Douglas M. Lay: The anatomy, physiology, functional significance and evolution of specialized hearing organs of gerbilline rodents . In: Journal of Morphology . tape 138 , 1972, ISSN 0362-2525 , pp. 41-120 , doi : 10.1002 / jmor.1051380103 .
- Malcolm C. McKenna, Susan K. Bell: Classification of Mammals Above the Species Level . Columbia University Press, New York 1997, ISBN 0-231-11012-X (631 pages).
- Guy G. Musser, Michael D. Carleton: Superfamily Muroidea . In: Don E. Wilson, DeeAnn M. Reeder (Eds.): Mammal Species of the World. A Taxonomic and Geographic Reference . 3. Edition. Johns Hopkins University Press, Baltimore 2005, ISBN 0-8018-8221-4 , pp. 894-1531 .
- Igor Jakowlewitsch Pawlinow: A review of phylogeny and classification of Gerbillinae (Mammalia: Rodentia) . In: Soologicheskie issledovanija . No. 9 , 2008, ISSN 1025-532X , p. 1-68 .
- Duane A. Schlitter, Greta Ågren: gerbils . In: David W. Macdonald (ed.): The great encyclopedia of mammals . Könemann (Tandem-Verlag), Königswinter 2004, ISBN 3-8331-1006-6 , p. 652–655 (German translation of the English original edition from 2001).
- Schlitter and Ågren 2004  (p. 652).
- Schlitter and Ågren 2004  (p. 653).
- Pawlinow, 2008 (pp. 19–20, Fig. 8)
- Lay, 1972 (pp. 50–52)
- Pavlinow, 2008 (pp. 9-10)
- Ellerman, 1941 ( p. 497 )
- Pawlinow, 2008 (p. 10, Fig. 1)
- Pavlinow, 2008 (p. 10)
- Pawlinow, 2008 (pp. 10–11, Fig. 2)
- Schlitter and Ågren 2004  (p. 654).
- Schlitter and Ågren 2004  (pp. 654–655).
- Musser and Carleton, 2005 (" Gerbillinae ", pp. 1211–1212)
- Pavlinow, 2008 (p. 25)
- Schlitter and Ågren 2004  (p. 655).