Tenrec

The tenrecs or tanrecs (Tenrecidae) are a family of mammals living on Madagascar. The group is heterogeneous in shape and includes representatives similar to hedgehogs, shrews and otters. The not too large mammals are mainly nocturnal, live largely solitary lives and feed on invertebrates. They have adapted to different ways of life. Thus, among the tenrecs there are burrowing, ground-dwelling, tree-climbing and aquatic forms. As a result, the animals exploit different ecological niches that had remained unoccupied due to a lack of competitors among mammals on Madagascar. This diversity also reduces intra-species competition and has resulted in more than thirty species, some of which differ significantly in body structure due to their divergent lifestyles. Overall, the tenrecs thus represent a prime example of adaptive radiation. In addition, the individual tenreks sometimes exhibit characteristics that are rather unusual for other higher mammals. These include the strongly fluctuating body temperature and the occurrence of a torpor under tropical conditions, the ability of some hedgehog tenrecs to stridulate with the spines of their fur coat, as well as the sometimes extremely short generation succession and the high number of newborns documented especially in the greater tenrec.

The first mention of tenreks by western naturalists and travellers occurred as early as the middle of the 17th century. About 100 years later, several forms were already known, but all of them were spiny-haired tenreks. Initially, the tenrecs were therefore related to the European hedgehogs. The establishment as an independent group prevailed in the transition from the 18th to the 19th century. The characteristic appearance and the partly insectivorous diet were the reason for the classification of the tenrecs into the large but not uniform group of insectivores (Lipotyphla) in the course of the 19th and 20th century. By the end of the 20th century, however, molecular genetic studies showed that they, together with the golden mole rats (Chrysochloridae), form a separate mammalian group: the tenrecids (Afrosoricida). This group is in turn close to other original African mammals, which are grouped together under the name Afrotheria.

The origin of the Tenreks is unknown so far. On Madagascar there are no fossils older than the Holocene. There are also only a few remains from continental Africa, the oldest dating from the Eocene of southwest Africa. Other finds came to light in eastern Africa and date to the Lower Miocene. It is likely that the group evolved on the continent, the ancestors of the Malagasy tenrecs then arriving on the island some forty to twenty million years ago. However, it has been disputed among experts whether Madagascar was reached in one or several waves of colonization. Today's tenrecs are adapted to both the moist rainforests of the eastern and the drier landscapes of the western part of the island. Almost one-fifth of today's species are more or less threatened in their existence.

Features

Habitus

Tenrecs form a diverse group of smaller mammals. The head-torso length varies considerably, reaching 4.7 to 6.4 cm in the smallest species such as the Dwarf Tenrec (Microgale parvula) or the Gnome Tenrec (Microgale pusilla), and 26.5 to 39.0 cm in the largest species, the Great Tenrec. The weight varies accordingly from 2.6 to 2400 g. Most species show no particular sexual dimorphism. Two types of form can be distinguished externally. The smaller species, such as the rice treks (Oryzorictinae) and earth treks (Geogalinae), have a shrew-like, sometimes otter-like appearance with soft fur and an elongated head. The length of the tail is variable, there are species whose tail is only one third of the body length, in others it exceeds the rest of the body by more than twice the length. The second group of forms, the hedgehog-rakes (Tenrecinae), is characterized by a hedgehog-like appearance. Their tail is only a short stub and their fur consists of bristles and spines. In general, the coloration of the fur of the Tenrecinae varies from gray to brown to black, with the underside usually lighter in color. A conspicuous patterning of the fur is only developed in the striped tenrecs (Hemicentetes). The hands and feet of the tenrecs end in five toes each, an exception being the four-toed rice burro (Oryzorictes tetradactylus), which has only four-toed hands. The legs show no noticeable difference in length, the thumb and the big toe are not opposable. The head appears comparatively large and, as in the Great Tenrec, may account for a good third of the body length. The snout is generally long and mobile. The length of the ears varies from comparatively large to small depending on the ecological adaptation of the animals, the eyes usually remain small. The face has conspicuously long vibrissae.

Skull and dentition characteristics

The skull of the Tenreks is altogether elongated and narrow as well as flat. It has an elongated, often cylindrically shaped and more or less pointed rostrum. The cranium, on the other hand, is broader, the greatest width usually occurring here at the joint pits for the lower jaw. Especially in hogfishes the rostrum exceeds the posterior part of the skull considerably in length and can take up to 60 % and more of the skull length. In contrast, the rostrum is shorter in the rice rays and accounts for about half of the skull length. The most striking feature is the unclosed zygomatic arch, which occurs in many insectivorous mammals. The area behind the eyes is often clearly retracted and elongated. In general morphology, the skulls of tenrecs vary little, but in detail they show a high degree of diversity, reflecting their sometimes different adaptations. For example, the rostrum can be very narrow as in the Striped Tenrecs or broader as in the Great Tenrec or the Great Hedgehog Tenrec, respectively. Also a crest is differently developed and occurs for example with the large Tenrek, is missing however with the strip-duck-rakes. Likewise, the skull is curved or flattened to different degrees and may be equipped with further bone bulges as muscle attachment points. In the embryonic stage, the skulls of all tenrecs largely resemble those of the lesser tenrecs (microgale).

The dentition is composed of 32 to 40 teeth, and the dental formula is: . The common feature of all Tenreks is the absence of the first premolar. With regard to the anterior teeth, the individual species and genera differ significantly from each other. In principle, the incisors are equipped with a pointed tooth crown, the occurrence of individual additional cusps in front of and behind the main tip varies between species. The canine can resemble the incisors, i.e. it can be incisiviform and very small, as for example in the Lesser Tenrecs or the Striped Tenrecs, or it can be more like a typical caninus (caniniform), as in the case of the latter it overhangs the incisors. Also the arrangement of the front teeth up to and including the premolars shows strong differences, since in many species in each case differently pronounced diastemata (interdental spaces) occur. In the construction of the premolars and molars the Tenreks differ less clearly from each other. They are also characterized by pointed cusps, of which the molars bear three each (tritubercular or tricuspid). The three main cusps, in relation to the maxillary teeth, include the paraconus, the metaconus, and the protoconus. The paraconus and metaconus are closely spaced, and the protoconus is reduced in size, not as much as in the golden shrews, but more so than in the otter shrews. There is also a V-shaped shear bar (ectoloph) on the chewing surface. Based on these features, the molars of tenrecs can be considered typically zalambdodont. Partially, there is a cingulum, an enamel bulge, on the tongue side.

Skeletal features

As in all Afrotheria, the spine of the Tenreks is characterized by an increased number of dorsal vertebrae (thoracic and lumbar). It consists of 7 cervical, 15 to 19 thoracic, 4 to 7 lumbar, 2 to 3 sacral and 8 to 52 caudal vertebrae. Thereby the hedgehog treks have the shortest tails with only 8 to 10 vertebrae, which results in a very short tail stump. The small long-tailed hogfish (Microgale longicaudata), on the other hand, has the highest number of caudal vertebrae among modern mammals, along with the long-tailed hogfish, and its tail often reaches more than twice the length of the rest of its body.

In general, the Tenrecs show a consistent skeletal structure, all have in common the presence of a clavicle and the tibia and fibula fused together in the lower region. In addition, as in many other higher mammals, the upper ankle joint is formed only by the tibia and talus. However, in terms of the position of the ankles, the upper hock joint is configured somewhat differently in hedgehog treks than in rice treks. This results in the former keeping their feet rotated outward at rest, whereas in the latter they are oriented parallel to the body axis. In addition, the sometimes quite different lifestyles of the Tenreks are reflected by various variations in the construction of the locomotor system. These are most visible in the forelimbs, while the hind limbs are less clearly affected. The shoulder blade of burrowing tenrecs (Oryzorictes) and striated tenrecs (Hemicentetes) tends to be narrow, whereas the shoulder blade of climbing tenrecs (Echinops) is short and broad. In addition, ground-dwelling tenrecs have a short and broad humerus with a rather laterally compressed condyle, the shape of which thus restricts rotational movements. The lower end of the joint (elbow joint) spreads strongly laterally, corresponding with an olecranon, the upper articular process of the ulna, which is also distinctly extended. This is where the forearm musculature mainly attaches. In tree-dwelling species, on the other hand, the humerus is rather long and narrow, the condyle, with its round shape, is designed for wide rotary movements, and the joints of the elbow are not so very conspicuously elongated. The skeleton of the hand and foot may be characterized by short and broad bones, as in burrowing species, or by longer and narrower ones, as in climbing ones. The former also possess single fusions of carpal bones, which reduces and stabilizes the mobility of the hand. In addition, the median ray is often elongated.

Soft Tissue Anatomy

Tenrecs have five types of muscles in the front of the snout, which attach to the zygomatic bone and the pre-eye region and extend to the tip of the nose with the help of tendons. They enable the animals to move their elongated nose in a variety of ways. Parallel to the adaptations of the skeleton to the different ways of life, there were also changes in the musculature. While ground-dwelling animals retained a relatively primitive arrangement of musculature, in the water-dwelling water duck and in the burrowing striped duck the semimembranosus muscle of the hind leg in particular developed enormously; this supports the knee in bending the leg and thus ensures powerful movement in the water or when burrowing. A similar thing occurred in the arms, where there was enlargement of some muscles such as the teres major muscle and the triceps brachii muscle in digging animals such as the striped duck treks and the rice burrowers. At the same time, there are differences in arm and shoulder muscles between the two genera, such as in the size ratio of the triceps brachii muscle to the supraspinatus muscle. It is likely that the adaptations to the different lifestyles within the subfamilies of the Tenreks occurred independently of each other, resulting in slightly varying modes of locomotion.

The tongue has so far only been examined more closely in the lesser hogfish. It is about 2.0 cm long and 0.5 to 0.7 cm wide. Four different types of papillae are formed on the surface: conical and filiform mechanical as well as mushroom-shaped and indented taste papillae. The filiform papillae have only one pointed process in the anterior portion of the tongue. The number increases posteriorly towards the root of the tongue, so that the papillae here appear crown-like. The mushroom-shaped taste papillae are distributed in four rows each at the edges of the upper surface of the tongue, while three indented taste papillae occur at the root of the tongue and form a triangle. Salivary glands penetrate the entire surface of the tongue, with the exception of the tip of the tongue.

A striking feature of Tenreks is the cloaca, a common outlet for the sexual, digestive and excretory organs. The cloaca has a bowl shape, and the entrance of the digestive tract is regulated by a sphincter. The digestive tract lacks the appendix. The testes are located in the abdominal cavity in many species, but in the lesser trevally and the paddy trevally they are advanced into the pelvis. The size of the paired testes varies between 0.07 and 0.56 g in the rice deer, which corresponds to 0.8 to 2% of the body mass. The penis is very thin and distinctly long, reaching a good 40% of the head-torso length in the Lesser Long-tailed Tenrec and a good 70% in the Greater Tenrec, penile spines are not developed. The uterus of the females is always two-horned (Uterus bicornis), the utero-vaginal-canal, that resembles a long and thin as well as spiral-like wound tube and is adapted so to the form of the penis, appears conspicuously. Unlike otter shrews, tenrecs have a haemochorial placenta.

The brain is among the most simply built of all higher mammals. It weighs 420 to 2490 mg, which is on average about the same size as the brain in the shrews, but considerably less than in the proboscideans, which are about the same size. The encephalization quotient varies from 0.3 to 1.0, being significantly higher in the small soft-haired tenrecs than in the larger hedgehog tenrecs. Neocortex and striatum are very small, whereas the olfactory bulb is enlarged.

Black-head-trek (Hemicentetes nigriceps), a representative of the hedgehog-good Tenreks with bristly fur


Skeleton of the Great Hedgehog (Setifer setosus)


Skull of the Great Tenrec (Tenrec ecaudatus)


Actual Reiswühler (Oryzoryctes hova), a representative of the spitzmausartigen Tenreks with soft fur

Distribution and habitat

Today, the Tenreks are endemic to Madagascar. Only the Great Tenrec is also found in the Mascarene Islands, Seychelles and the Comoros archipelago, but in all cases it has probably been introduced there by humans. The animals inhabit a variety of habitats on Madagascar, these include the moist rainforests of the eastern and central parts of the island, the dry forests of the western and the thornbush savannahs of the southwestern. Most, especially smaller, representatives occur in more narrowly defined habitats. Three ecotypes can be distinguished:

• Inhabitants of humid landscapes: approximately 74 % of all Tenrekarten, above all a majority of the representatives of the Kleintenreks, furthermore the representatives of the type Nesogale, the Reistenreks and the Streifentenreks;
• Inhabitants of dry landscapes: approximately 19% of all Tenrekarten, so some representatives of the Kleintenreks (Jenkins-Kleintenrek, Nasolo-Kleintenrek, Kurzschwanz-Kleintenrek, Grandidier-Kleintenrek), the Erdtenrek and the small Igeltenrek;
• Inhabitants of both landscape-types: in principle, only the big Igeltenrek and the big Tenrek occur on the entire island Madagascar.

Tenrecs are found both at sea level and at high mountain elevations up to 2500m. In many habitats more than one species occurs. For example, up to 18 species have been recorded at the Tsaratanana massif in the north, while there are still 16 species in the forest area of Tsinjoarivo. The frequent co-occurrence indicates a strong ecological and trophic differentiation within the family.