Golden mole

The golden moles (Chrysochloridae) are a family of mammals living in southern and partly in central Africa. The animals resemble the moles in their physique and also lead a similar underground way of life, but are not closely related to them.

The body shows special adaptations to a burrowing way of life. It is spindle- to diamond-shaped, externally visible ears and a tail are missing, the eyes are hidden under the fur. The forelimbs are particularly well adapted for digging. They have a short and strong build, large digging claws are formed on the hands. The forearm has a third bone, the number of finger and carpal bones is reduced by characteristic adhesions. A characteristic feature of the golden mole rat is also the silky fur, which sometimes shines metallically in certain light conditions.

Golden mole rats inhabit open to desert regions and closed forests, they can be found in lowlands as well as in highlands. Due to their burrowing way of life, they are habitat specialists that sometimes only inhabit narrowly defined habitats that are mostly bound to loose subsoil. There is only little information available about the way of life of these animals. They are nocturnal and solitary.

The diet consists mainly of invertebrates such as earthworms and insects. The animals detect their prey with the help of vibrations produced by their prey, which they are enabled to do by their sometimes greatly enlarged ossicles. A striking feature is the unstable body temperature, which adapts to the environment and therefore has an energy-saving function. Reproduction has been little studied so far, one to three young are born per litter, which remain as nestlings in the underground burrow. The individual development is probably very slow.

The phylogeny of the family is only sparsely documented. However, it goes back to the Middle Eocene about 47 million years ago. Fossil finds include, with exceptions, only single skull and dentition remains. Golden moles have been known since the 18th century, but they were initially equated with moles. It was not until the turn of the 19th century that they were separated from the group, with the naming of the family dating to 1825.

Due to similar lifestyle and comparable body characteristics, the golden moles were considered to be members of the order of insectivores (Lipotyphla) until the end of the 20th century. More recent molecular genetic studies place them, along with the tenrecs, in an entirely separate mammalian group, the Tenrecidae (Afrosoricida) within the Afrotheria. Currently, the family comprises ten genera with 21 species, of which more than two-thirds are endangered to varying degrees.

Description

Habitus

The golden moles are relatively small representatives of the mammals. Among the smallest forms is the desert golden mole rat (Eremitalpa granti) with a head-torso length of 7.6 to 8.5 cm, while the giant golden mole rat (Chrysospalax trevelyani) grows to a length of 20.8 to 23.5 cm. The body weight varies accordingly between 15 and 40 g respectively between 410 and 500 g. However, most representatives have total lengths of 9 to 14 cm and weights of 20 to 100 g. The individual species show strong similarities in the physique, but differ partly considerably with regard to their fur colouring, which despite the family name of the golden mole-rats does not only appear golden-yellow coloured, but also varies between black, grey or brownish shades. The physique of the golden mole rat is adapted to a burrowing, subterranean lifestyle. Externally, it resembles that of moles, with which golden moles are not closely related. The compact trunk shows a spindle-shaped to diamond-shaped structure, a tail is not visible externally. The nose is covered by a coarse, leathery padding, which is used as a support when digging. The nostrils are located in a fold of skin on the underside of the tip of the nose. Like many other burrowing mammals, golden moles have no external ear pinnae, and the eyes are also covered by fur. Accentuating bright colored spots often appear on the face and especially around the eyes. The limbs are short and stout and tend to be located below the torso, unlike moles in which they are more laterally attached. The forelimbs have four toes - the outer fifth ray is reduced - and are transformed into digging tools. The claw on the middle finger (ray III) is greatly enlarged and may be longer than the forearm. On the first and second fingers (rays I and II), the gold mules each bear a shorter, pointed claw, but its length varies with the species. Only the desert golden mole rat also has a small, claw-like nail on the distinctively shortened fourth finger (ray IV). The hind legs end in five toes, which are equipped with small sharp claws. The individual toes are connected by small membranes, and the sole of the foot is hairless. Females possess two pairs of teats.

Skull and dentition characteristics

The head is conical to teardrop-shaped, the greatest width is in the area of the skull. The rostrum is usually elongated. The ratio of the greatest width of the skull to the greatest length varies from species to species. In long and narrow skulls the greatest width of the skull reaches only 60% of the greatest length of the skull, in short and broad skulls the corresponding value is up to 96%. Among the most narrow-skinned forms are the copper golden mole rats (Amblysomus), Arends' golden mole rats (Carpitalpa), the giant golden mole rats (Chrysospalax) or the representatives of the genera Neamblysomus and Chlorotalpa. In contrast, the Cape Goldmouths (Chrysochloris), the Desert Goldmouth or the members of the genus Cryptochloris belong to the broad skinned ones. The rostrum also varies in strength and can be narrow (with a palate width of about 28% of the largest skull length) or wide (with a corresponding value of up to 35%). Conspicuously absent in the Goldmulls are constrictions in the region of the eyes. The zygomatic arches are completely closed, but the zygomatic bone is receded and the arch consists of a bony process of the maxilla which articulates with the temporal bone. As a peculiarity, a joint connection between the lower jaw and the hyoid bone is found in the golden mullen. The articulation is between the angular process of the lower jaw and the stylohyal, which is part of the suspensory apparatus of the hyoid bone. A similar articulation is not known from other higher mammals. It possibly assists the tongue in chewing food.

Another conspicuous feature is found in the area of the middle ear. In some species the ossicles are strongly enlarged. This concerns especially the hammer (Malleus), whose head can be ball-like inflated or club-like extended. The enlargements are sometimes enormous: in the desert golden mole rat (body weight about 40 g) the malleus can reach a weight of 70 mg, in the giant golden mole rat (body weight about 500 g) up to 185 mg (in comparison: in humans with an average body weight of 70 kg the malleus weighs 28 mg). In some cases, the increase in size of the malleus is accompanied by an increase in bone density, which reaches a value of 2.44 g/cm³ in the desert golden mole rat. This represents one of the highest values in terrestrial mammals and is exceeded only by some whales and manatees. To accommodate the inflated malleus, a further sinus is formed on the upper side of the tympanic cavity, which in some species is externally visible as a bony bladder bulging above the temporal fossa behind the orbit. The bony bubble is called the temporal bulla (roughly "temporal bubble") because of its location on the temporal bone. In addition, the cochlea has a complex structure, possessing at least four convolutions with a total combined angle of rotation of 1200°, which is more than in numerous other subterranean mammals. The special features of both the ossicles and the cochlea are related to the auditory perception of golden moles. Golden mole rats can perceive very low frequency sounds of less than a few hundred hertz, and they are also able to pick up seismic oscillations and vibrations, for example from the movements of prey. Transmission is by bone conduction, which is triggered by the highly asymmetrical shape of the malleus due to the enlarged head and an associated shift in the centre of gravity. The ability is better developed in animals with an enlarged malleus than in those with a normally built one.

The golden moles have a slightly reduced dentition with 36 to 40 teeth. The anterior dentition is complete, only in the posterior some molars are receded. The dental formula is therefore: . The anterior-most incisor in the maxillary dentition and the second incisor in the mandible are enlarged, and the remaining incisors have a canine-like and small shape. The anterior premolar can be variably shaped - sectorial (with sharp cutting edges), bicuspid or tricuspid (with two or three cusps) types occur. The molars themselves have conspicuously high (hypsodont) crowns and a tricuspid occlusal pattern. The three main cusps represent the paraconus, the metaconus, and the protoconus (based on maxillary teeth). The protoconus is often, but not in all species, greatly reduced in size, and the metaconus and paraconus are closely spaced - the latter forming the main cusp of the grinders. In addition, a V-shaped shear bar (ectoloph) runs across the tooth surface, the tip of which is the paraconus. Based on the overall configuration, the tooth structure can be considered typically zalambdodont. On the mandibular molars, in some species a talonid is formed, a low-lying projection of the occlusal surface into which the protoconus of the upper molars engages when the dentition closes. Where the rearmost molar occurs, it is predominantly very small and either has the shape of the other grinders or resembles a post. However, the feature may be strongly overprinted by the chewing of the teeth.

Skeletal features

The spine is composed of 7 cervical, 16 thoracic, 3 lumbar, 5 sacral and 9 caudal vertebrae. The cervical spine is curved downwards. The spinous processes of the first twelve thoracic vertebrae are vertical, those of the posterior thoracic and lumbar vertebrae point backward. Of the sixteen pairs of ribs, eight are free-standing. The anterior ones, which are connected to the sternum, and the sternum itself are directed inward, creating space for the complex front leg muscles.

Especially the anterior musculoskeletal system shows special adaptations to the burrowing lifestyle. The entire shoulder girdle is displaced forward so that the shoulders are approximately at the level of the rear section of the head. The body's center of gravity, thus shifted forward, allows the animals to perform a powerful forward movement when digging. The shoulder blade and sternum are strikingly elongated; the former runs roughly parallel to the spine and is also exceptionally narrow. The greatly enlarged shoulder bones serve as the attachment point for the massive shoulder muscles. A clavicle is present, but lacks the typical curved shape. Due to its forward position on the body, it absorbs some energy when digging. The humerus is conspicuous for its short compact shape and twisted shaft. Along this runs a massive deltopectoral groin, to which parts of the arm and shoulder muscles attach. The elbow joint is strongly asymmetrical in shape; here the laterally strongly projecting inner (medial) bony prominence of the humerus (epicondylus medialis) stands out. As a result, the width of the lower joint end of the humerus can be 65% or more of the total bone length. The degree of formation of the epicondylus medialis is related to the intensity of digging activity of each species. Similarly, the olecranon, the superior articular process of the ulna, is strongly extended and conspicuously curved. It accounts for about 35% of the total bone length. The arm extensor muscles of the forearm attach to both articular processes, but both bony prominences also prevent excessive lateral movements of the arm. The formation of a rod-shaped bone, which to a certain extent represents a third forearm bone, should be emphasized. It probably developed from the ossification of a tendon of a forearm muscle, possibly the flexor digitorum profundus muscle, which moves the middle fingers. For this reason it is often called the "flexor" bone. The "flexor" bone lies below the ulna and is about the size of the radius. The number of carpal bones and phalanges is reduced. The carpal bones are characterized by a flattened shape, which limits the hand's ability to rotate. Of the four rays of the hand, the first, second, and fourth each consist of a metacarpal bone and two phalanges, the anterior phalanx of each being a result of the fusion of the first and second phalanges. The hypertrophied third ray represents a fusion of the metacarpal bone and the first two phalanges, and is therefore sometimes called the "triplex" bone. Another characteristic formation is present on the second ray of the finger. Here, the corresponding metacarpal bone is fused with the greater polygonal bone and the lesser polygonal bone to form the so-called trapezium-trapezoid-metacarpal-II complex (also called "ttm" or "totem" bone). The individual bone fusions form in the course of embryonic development. The last phalanx of each finger is enlarged and split at the end, indicating the existence of the claw. An exception is the first finger (thumb), whose last limb is not notched.

Compared to the front limbs, only a few characteristic features are found on the rear ones. The tibia and fibula are fused together above and in the lower third. Between them there is a wide space due to the distinct shaft curvatures of both bones. Likewise, the first two phalanges of rays II to IV are fused together, so that all five foot rays have only two phalanges each.

Fur

The coat consists of the outer coat and a dense undercoat. It is mostly silky and soft. The guard hairs are directed backwards and water-repellent. The individual hairs of the top coat grow between 7 and 21 mm long, the diameter is 78 to 190 μm. They are narrower at the base than in the upper third. In cross-section, they often have a flat to compressed shape in the upper third and a roundish shape at the base, the hair scales show a wavy arrangement, and the medulla has a lattice-like structure. In some species a metallic sheen occurs, ranging from reddish to yellowish, greenish, brownish to silver. The shine originates in the flattened, upper third of the hairs, where very flat scales are formed in several layers. The light thus hits a large flat surface for reflection and is refracted several times due to the superimposed layers of scales. Iridescent colors in living things often offer an advantage in attracting mates during reproduction; however, this can be ruled out in golden mole rats due to their blind nature. It is possible that the color iridescence is a side effect that resulted from the flattening and greater layering of the hair surfaces. What caused these hair changes is unknown, but they may be related to underground locomotion.

Soft Tissue Anatomy

The digestive tract is simple and tubular. It has a weight of 4.9 g and a length of 43.9 cm in medium-sized species such as the Hottentot's golden mole rat (Amblysomus hottentotus). In the giant golden mole rat, it weighs 24.5 g. The stomach occupies a total of 11 to 20% of the length of the digestive tract, the latter itself accounting for between 9 and 12% of the total body mass. As in many other insectivorous mammals, it lacks a caecum, making it almost impossible to separate the large intestine from the small intestine. The area of the gastric portal (pylorus) extends very widely. The entire intestinal section is covered with filiform processes (microvilli). The urogenital system terminates in a single body orifice, the cloaca, as in cloacal animals. In males, the testes are located in the abdominal cavity, and their combined weight is about 23 mg. The penis is relatively short, measuring only 3 to 4 mm, and penile spines are absent. The acrosome of the spermatozoa is regressed, but small barbs are formed on the head. The females have a two-horned uterus (uterus bicornis). The cloaca and the testes, which are located inside the body, represent similarities with the tenrecs and can be taken as an indication of the relationship between the two taxa. The kidneys are simple in construction and possess a relatively large medulla and large medullary cones that extend into the ureter. Both suggest that the kidney can highly concentrate urine. The brain reaches a volume of 700 to 736 mg in medium-sized species, which is only about half the size of comparably sized proboscideans.

The eyes of golden mole rats are degenerated and hidden under the skin. In adult animals, however, an eyeball is still present, which grows to about 0.5 mm long and 0.4 mm deep. Similarly, the conjunctival sac and lacrimal glands are developed, as are the lacrimal ducts, but the lens, iris, and all ocular musculature are absent. It is possible that the lacrimal apparatus has the same function in golden mole rats as in other mammals, keeping the conjunctival tissue free from foreign bodies. On the whole, the eye is more retarded than comparably in the moles, but not quite so much as in the pouched moles. The development of the eye in the golden mole-rats begins in the embryonic stage, as in the other mammals, but there is hardly any further development or growth. At least five muscles are involved in the construction of the nose, some of which attach to the zygomatic arch and extend to the tip of the nose. Despite the short and broad shape of the nose in golden mole rats, it is thus very mobile and can function as a tactile organ with which the animals search for food and dig.

Mandibles of the Congo golden mole rat (Huetia leucorhina; A) and of Arends' golden mole rat (Carpitalpa arendsi; B). The arrow in B indicates the talonid on the fourth premolar in Arends' goldmull, which is present throughout the lower dentition but absent from the premolars and molars in Congo goldmull.


Upper (A, B) and lateral (C) skull views of the Yellow Golden Mole (Calcochloris obtusirostris; A), Stuhlmann's Golden Mole (Chrysochloris stuhlmanni; B) and Desert Golden Mole (Eremitalpa granti; C). The arrow in B refers to the "temporal bladder", the arrow in C to the enlarged head of the mull. In A the "temporal vesicle" is absent, in C the cavity in the middle ear is enlarged and contains the expanded head of the hammer.

Distribution and habitat

The golden mole rats are exclusively native to sub-Saharan Africa, with the main focus of their range being in southern Africa. More than half of the species are endemic to the Republic of South Africa; individual representatives also inhabit areas in Namibia, Lesotho and Mozambique. Outside southern Africa, only three species have been recorded so far: the Congo golden mole-rat (Huetia leucorhina) in central Africa, Stuhlmann's golden mole-rat (Chrysochloris stuhlmanni) in central and eastern Africa, and the Somalia golden mole-rat (Huetia tytonis) in northeastern Africa. Because of their subterranean lifestyle, the goldmouths are habitat specialists. As a result, their habitats are highly fragmented or narrowly restricted. Often, the individual species can only be found at a few, locally restricted sites - very few representatives are known from a larger distribution area. Other golden mole rats such as the Somalia golden mole rat, but also Visagie's golden mole rat (Chrysochloris visagiei), De Winton's golden mole rat (Cryptochloris wintoni) or Van Zyl's golden mole rat (Cryptochloris zyli) have only been recorded from a few specimens.

The different species of golden mole rats can be divided into two distinct ecological groups:

• Inhabitants of dry semi-desert until partly desert-like regions, to it the desert-gold-mull (Eremitalpa), the representatives of the Cape-gold-mulls (Chrysochloris) of the southern Africa and the types of the type Cryptochloris belong
• Inhabitants of open grasslands, savannahs and forests, these include the copper goldmouths (Amblysomus), Arends' goldmouth (Carpitalpa), the giant goldmouths (Chrysospalax) and representatives of the genera Neamblysomus, Calcochloris and Chlorotalpa.

The animals are distributed both in coastal lowlands and in high mountains at altitudes of up to 4000 m. Basic requirements for the presence of golden mole rats are a sufficient food supply and loose, penetrable soils. Rocky landscapes and rivers are the limits of their distribution. The adaptability to human-altered landscapes varies among species. If some forms occur in a common region, as is the case with the Fynbos golden mole-rat (Amblysomus corriae) and Duthie's golden mole-rat (Chlorotalpa duthieae) as well as the Highveld golden mole-rat (Amblysomus septentrionalis) and Sclater's golden mole-rat (Chlorotalpa sclateri), there are usually different biotope requirements.