The island continent of Madagascar was part of a large land mass that broke away from Africa some 165 million years ago. As tectonic plates shifted subtly on the Earth's surface, the land mass moved out slowly into the Indian Ocean. Other chunks of land later floated off to leave the island the size and shape we now know, situated 400 km off the Mozambique coast for the last 80 millions years.
At that time the island was probably a mix of habitats much as it is today: thick rain forest on the east coast, deciduous forests in the west, deserts with spiny succulent plants in the south-west, and amid the forests of the high plateau a mosaic of grasslands grazed by giant tortoises and walked upon by 3-4 metre tall elephant birds. Mammals had evolved from therapsid reptiles and were spreading across Africa, but none had appeared on Madagascar, as its reptiles were from a different stock.
What happened next is only conjecture, but sometime around 60 million years ago a small mammal - perhaps no more than 5 or 6 g in weight with a primitive body plan and physiology - was washed out to sea from Africa. Perhaps it was on a log that had fallen into a river from the coastal forest of what is now Kenya. Currents and winds moved the mammal across the channel until it arrived on Madagascar. Perhaps the founder was joined by others; perhaps it was a pregnant female. Whatever the case, the animals multiplied. And then evolution kicked in!
The mammal was an early tenrec; the island it had arrived on probably had no other mammals and so this early lineage evolved over generations to adapt its body shape to its environment. As a result of a process called "adaptive radiation"(made famous by Darwin's finches on Galapagos) new species appeared, each physically suited for its niche, free of competition.
Very few other mammals ever made the same journey. Eventually rodents, a mongoose-like carnivore and a primitive primate crossed the channel and gave rise to species found nowhere else on Earth. A pygmy hippopotamus also crossed but Madagascar never saw cats, dogs or large herbivores.
Most tenrecs died out on mainland Africa and are known only from fossil records; all except one small lineage that evolved to fill a specialized aquatic niche - the otter shrews (see section “Tenrecs in Africa – the otter shrews”). However, tenrecs still inhabit Madagascar today in an abundance and diversity not seen in any other mammalian family.
Because of their evolutionary history, tenrecs are incredibly diverse. Many have become morphologically similar to other mammals elsewhere in the world adapted to the same niche - a phenomenon known as "convergent evolution". The larger species adapted for foraging on the ground and eating diverse prey evolved defensive spines and look, at least externally, just like hedgehogs. Smaller species found on the forest floor or climbing trees look very much like shrews; and species that have adapted to burrowing under the leaf litter could easily be mistaken for a type of mole. The aquatic tenrec looks and acts like a very large water shrew or a very small otter.
But then there are species not just from another time, but also from another world. Streaked tenrecs (Hemicentetes) are so unique nothing like them ever evolved elsewhere. Their black and pale striped body is covered in spines, with a head crest of quills that can be erected. When irritated the animal makes head butting movements, trying to leave the barbed spines in the nose of its aggressor. A patch of spines on the back form what is known as a stridulating organ - the spines can rub together and produce a type of ultrasound that keeps the family groups together. Tongue clicks made by the animals are thought to be a type of echolocation, perhaps used for hunting prey.
In spite of their many adaptations, tenrecs still exhibit a number of characteristics which make them distinct from other small mammals and which were probably typical of the earliest mammals. Such traits include nocturnal activity patterns, small body size, the retention of a cloaca as a common uro-genital opening, abdominal testes, poor eyesight and a dependence on their sense of smell and hearing. They are also considered primitive physiologically, since all species have relatively low body temperatures and metabolic rates relative to their body size, and several species enter torpor regularly.
Box 1 - Tenrec species
Biologists divide tenrecs (Family Tenrecidae) into four sub-families:
The TENRECINAE are spiny tenrecs. The largest species is the tailless tenrec, Tenrec ecaudatus, which weighs up to 1 kg. It is as large as a rabbit, and is less spinescent than the other species. Other spiny tenrecs are the two species of hedgehog tenrec (Setifer setosus and Echinops telfairi) and the two species of streaked tenrec (Hemicentetes semispinosus and H. nigriceps)
The ORYZORICTINAE are furred tenrecs. There are two mole tenrecs (Oryzorictes hova and O. tetradactylus) with large forepaws and reduced eyes to allow them to burrow efficiently. The aquatic tenrec (Limnogale mergulus) is the only species on Madagascar adapted to life in rivers and streams. Its webbed feet and rudder-like tail allow it to hunt insect larvae and crustaceans in the water. There are some 20 species of shrew tenrec (Microgale) currently recognised, but seven new species have been described in the last 14 years so there is every chance that the full diversity of the group is not yet known. These tenrecs range in size from the small dark M. parvula (3 g) to M. talazaci (up to 37 g). Most live on the forest floor, but long-tailed species such as M. longicaudata and M. principula climb trees.
The GEOGALINAE is a recently recognised sub-family, comprising the single species, Geogale aurita (the large-eared tenrec). It is a small species (about 7g) adapted for life in the arid south-west and specialised in a termite diet.
The POTAMOGALINAE are otter shrews which are found on mainland Africa. Many scientists now consider these animals as tenrecs. The giant otter shrew, Potamogale velox, is widespread in the streams and rivers of central African forests, but the other two species have restricted distributions. The Nimba otter shrew, Micropotamogale lamottei, is found only in a small area around Mount Nimba on the borders of Ivory Coast, Liberia and Guinea, and the Ruwenzori otter shrew, M. ruwenzorii, is found only between Uganda and eastern DRC. Habitat loss, mining and fish traps threaten otter shrews across their range. (See section “Tenrecs in Africa – the otter shrews”).
Tenrecs are probably most closely related to golden moles (CHRYSOCHLORIDAE). Along with golden moles, scientists now consider tenrecs part of the Afrotheria, a grouping of African mammals with evolutionary connections that also contains the aardvark, sengis (or elephant- shrews), hyraxes, elephants and sea cows.
Tenrecs are generally found in forest habitats. Most species occur in the eastern rain forests, but a handful (e.g. Geogale, Echinops) are adapted to the arid spiny desert in the south-west of Madagascar. The aquatic tenrec (Limnogale mergulus) requires clear, running freshwater. Some species - such as tailless tenrecs (Tenrec ecaudatus) and streaked tenrecs (Hemicentetes) - appear able to adapt easily to man-induced disturbance, and can survive in secondary forest or agricultural land. Mole tenrecs (Oryzorictes) have been found in rice fields.
Tenrec diet is based on invertebrates. Insects and their larvae are the most commonly consumed prey items. However, many of the larger species (from Talazac's shrew tenrec – Microgale talazaci - to the tailless tenrec – Tenrec ecaudatus) sometimes take small vertebrates such as amphibians. Two species have become very specialized: streaked tenrecs eat mostly soft-bodied invertebrates, with an apparent preference for earthworms; large-eared tenrecs (Geogale aurita) prefer termites they find inside dead wood. The aquatic tenrec feeds on a range of prey in its freshwater habitat, but favours aquatic insect larvae and crayfish.
A number of predators are known or suspected to feed on tenrecs. These range from birds of prey and viverrid carnivores to snakes; some small shrew tenrecs (Microgale spp.) may even be attacked by larger species of their own genus.
The uniqueness and diversity of tenrecs is well demonstrated in their reproduction. All species have remarkably long gestation lengths. Whereas shrews give birth after 3 to 4 weeks, a similar sized shrew tenrec will take around 8 weeks! Regardless of their size, pregnancy in all tenrecs takes between 50 and 70 days. However, their litter size is hugely varied. Furred tenrecs of the Sub-family Oryzorictinae, the greater hedgehog tenrec (Setifer setosus) and the highland streaked tenrec (Hemicentetes nigriceps) generally have between 1 and 5 young per litter, yet the lesser hedgehog tenrec (Echinops telfairi) has up to 10, and the lowland streaked tenrec (Hemicentetes semispinosus) up to 11. The most productive species is the tailless tenrec (Tenrec ecaudatus) which can have up to 32 young, this is the second largest recorded litter size within a mammal (naked mole rats win the contest with 33!).
Most tenrec young develop relatively slowly. However, there are exceptions. The lowland streaked tenrec (H. semispinosus) is the most precocious; their offspring mature more rapidly than any other species in the family. The young open their eyes 7-12 days after birth and are weaned within 3 weeks. They reach sexual maturity very quickly too, at just 35-40 days; other spiny tenrecs of the Sub-family Tenrecinae do not mate until they are at least 6 months old. Lowland streaked tenrecs live in multi- generation groups with a very complex social system. Due to the speed of maturation and relatively large litter size, a family group may comprise more than 20 individuals from three generations. They forage together, maintaining contact by using their stridulating organs.
The large-eared tenrec (Geogale aurita) is unique among tenrecs in that it exhibits post-partum oestrus; this means females are able to suckle a first litter whilst a second litter is developing in their uterus. Although this is common in small shrews it is unknown in any other tenrec. It may represent an adaptation to an unpredictable environment, optimising reproductive output while favourable weather conditions prevail. This reproductive strategy also allows the female to save energy. The large-eared tenrec has even been witnessed entering torpor during pregnancy, and it is thought that this then delays the development of the embryo - a phenomenon only seen before in some bats.
Although tenrecs have inhabited Madagascar for some 60 million years, it is possible that some may not survive more than a few decades more. Habitat loss is the greatest threat to most species. Much of the forest in Madagascar has been cut down and burnt for slash and burn agriculture, and the island continues to lose about 200,000 hectares of forest each year. Rain forest that was once a solid block along the east coast is now, in many places, very fragmented. Some of the tenrec species that have been seen rarely (e.g. the pygmy shrew tenrec, Microgale parvula, and the four-toed mole tenrec, Oryzorictes tetradactylus) or those with very limited distributions (e.g. the montane shrew tenrec, Microgale monticola, and Nasolo's shrew tenrec, M. nasoloi) face a real threat of extinction if their habitat is not conserved. Uncontrolled fires and the introduction of alien species of shrew, rodent and carnivore may also threaten some species. There is also evidence that habitat disruption caused by too many visitors in certain parks may reduce tenrec species diversity.
The aquatic tenrec (Limnogale mergulus) is the greatest cause for concern among conservationists. It is known from only 10 sites in Madagascar and appears to be restricted to clear streams with abundant prey. Siltation caused by widespread deforestation is expected to cause problems as it will reduce prey species. Animals are also drowned in eel and crayfish traps.
Box 2 - Threatened tenrec species
Six Malagasy tenrecs appear in the IUCN Red List of 2006. Endangered species are considered
more threatened than vulnerable species. Data deficient species require more information before an
assessment can be completed.
Aquatic tenrec, Limnogale mergulus - Vulnerable
Tree shrew tenrec, Microgale dryas - Vulnerable
Jenkin's shrew tenrec, Microgale jenkinsae - Endangered
Montane shrew tenrec, Microgale monticola - Vulnerable
Nasolo's shrew tenrec, Microgale nasoloi - Endangered
Four-toed mole tenrec, Oryzorictes tetradactylus - Data Deficient
If representative samples of the eastern rain forest block - from north to south, from low to high altitude - are protected, there is a strong chance that many tenrec species will not be threatened with extinction. Various actions are underway to protect tenrec habitat. In 2003, Madagascar’s President Marc Ravalomanana committed to triple the island’s protected area coverage, adding a further 5 million hectares, and placing more than two-thirds of the country’s remaining forest under formal protection. A whole suite of conservation agencies, such as WWF, WCS and Conservation International (CI), are helping make that happen. Particularly important is the mobilization of funding resources and, to support this, the government of Madagascar, WWF and CI are creating the Madagascar Foundation for Protected Areas and Biodiversity. This aims to raise US$50 million in the next five years and has already made a good start.
Tenrecs with very narrow distributions or specific threats may need extra help. More research is required to confirm the distribution and abundance of poorly known shrew tenrec species (e.g. dryad, montane and Nasolo's shrew tenrecs). If they genuinely occur in only a handful of sites, conservation efforts will be needed to target their habitat.
Special attention needs to be paid urgently to aquatic tenrecs. Research should be conducted into their habitat needs and factors affecting their distribution. Land-use and fishing practices may need to be changed in areas where they occur. Although small mammals are often neglected in large conservation programmes, aquatic tenrecs would make ideal flagship species for integrated forest and freshwater conservation programmes in eastern Madagascar. Work to conserve forest habitats and maintain clear, unsilted streams will benefit a range of other plant and animal life as well as aquatic tenrecs.
Tenrecs are a unique and diverse family of mammals, from another world and another time. They make up a significant component of Madagascar’s faunal diversity and no doubt hold the answer to many scientific questions on the evolution and adaptation of mammals. Let's hope that current conservation efforts ensure that their time is not up yet!