The small mammal specialist group is responsible for three orders of small mammal – the rodents, tree shrews and eulipotyphlans (made up of the shrews, moles, hedgehogs and solenodons). These three orders contain more than 2800 species of which 456 are considered threatened with extinction by the International Union for the Conservation of Nature. The diversity of this group really sets them apart, with an astounding range of adaptations and life histories, from the fishing mice of South America and the river-dwelling desmans of the Pyrenees and southern Russia, to the venom-injecting solenodons of the Caribbean. Here we hope to give you a flavour of the breadth and diversity of small mammals. If you interest is piqued we encourage you to follow the links on the pages to additional sources of small mammal information…
Introduction to Rodents
Without question, rodents are the world’s most successful group of mammals, and represent an important ecological component of virtually every terrestrial ecosystem. There are over 2200 living species, comprising around 40 percent of all of the mammal species existing today, and they have an almost worldwide distribution. Rodents are characterized by a single pair of continuously growing, gnawing incisors in their upper and lower jaws, which enable them to feed successfully on a huge range of different food types.
Over half of all rodents are classified within just two families of relatively small-bodied species – the Muridae (the largest mammal family, comprising true rats and mice, gerbils and relatives) and the Cricetidae (including hamsters, voles, lemmings and rice rats). However, there are over 30 other living rodent families, containing species with a wide range of different ecologies and habits. From giant arboreal flying squirrels to blind subterranean naked mole rats, and from aquatic beavers to bipedal hopping kangaroo rats, giant jumping rats and springhares, rodents are truly diverse. New rodent species continue to be discovered by researchers, and many of these newly described species – such as the highly unusual Sulawesi shrew-rat Paucidentomys vermidax, a specialized worm-eating animal with a long thin snout and no molars – reveal more and more about the extreme evolutionary diversity of this remarkable mammal group.
Introduction to Insectivores
The eulipotyphlans, frequently referred to as “insectivores”, include over 400 species of shrews (Soricidae), moles and desmans (Talpidae), hedgehogs, moonrats and gymnures (Erinaceidae), and solenodons (Solenodontidae). These species represent some of the most ancient lineages of living mammals. The group as a whole differentiated very early on during placental mammal evolution, and it is currently estimated that the two species of solenodons – large shrew-like mammals that are found today on the Caribbean islands of Cuba and Hispaniola – diverged from the last common ancestor they shared with all other living mammals around 76 million years ago, during the time of the dinosaurs.
All of the eulipotyphlans have a fairly generalized diet which mainly consists of invertebrate prey, hence their common name of “insectivores”. However, different eulipotyphlan groups show a remarkable range of adaptations to different lifestyles and ecological habitats. Moles are highly specialized for subterranean burrowing, with powerful forelimbs, large paws, and reduced eyes and ears; whereas water shrews and desmans have become adapted for hunting in fast-flowing streams and rivers. Hedgehogs have instead evolved extremely specialized defensive adaptations, with an armoury of spines and the ability to roll up into a protective ball using highly developed back muscles. Many eulipotyphlans are also venomous – several species of shrews and moles are known to have venomous saliva for paralyzing their prey, and the two living solenodons species also have specialized grooves in their teeth for delivering venom directly from modified salivary glands.
Introduction to Tree Shrews
The tree-shrews are a small group of about 20 species of typically tree-dwelling small mammals in the families Tupaiidae and Ptilocercidae, found in south-east Asia from southern India and China to the islands of Indonesia and the Philippines. They are superficially squirrel-like in appearance, and the tree-shrew genus name Tupaia comes from “tupai”, the Malay word for squirrel. However, their evolutionary relationships to other mammals have been the subject of considerable debate for over a century, with different studies suggesting that they are either most closely related to primates or to the colugos, otherwise known as “flying lemurs”.
Despite their relatively low species diversity, tree-shrews are among the most remarkable and interesting small mammals. They have the highest brain to body mass ratio of any mammals, and display several other intriguing biological characteristics; for example, pen-tailed tree shrews (Ptilocercus lowii) have been used to investigate alcohol tolerance in humans, as they consume large amounts of fermented nectar yet do not exhibit signs of intoxication. Several tree-shrews have also developed symbiotic relationships with pitcher plants, feeding on nectar from the plants and in return defecating into them to supply them with much or all of their required nitrogen. It is highly likely that many more surprises await researchers investigating these enigmatic and understudied mammals.
Small Mammal FAQs
As the name of this IUCN Specialist Group suggests, the great majority of species in the three groups of “small mammals” are small-bodied in comparison to most other mammals. The mean body size of all rodent, insectivore and tree-shrew species is less than 1 kg, and some species are truly tiny – the world’s smallest mammal, the Etruscan pygmy shrew (Suncus etruscus), weighs on average only 1.8 grams. However, some so-called “small mammals” are actually relatively large.
The world’s largest rodent, the capybara (Hydrochoerus hydrochoeris), can weigh more than 50 kg, and several other rodents (such as porcupines, pacas and pacaranas) can also reach over 10 kg. Conversely, many other mammal species – such as bats, lagomorphs, tenrecs, small marsupials and even many carnivores – can also weigh far less than 1 kg.
Despite their generally small body size, small mammals – rodents, eulipotyphlan insectivores and tree-shrews – display an otherwise incredible diversity of successful adaptations and modes of life. From the fishing mice of South America and the river-dwelling desmans of the Pyrenees and southern Russia, to the venom-injecting solenodons of the Caribbean and the eusocial naked mole rats of Africa, these species showcase the many different ways that mammals can make a living in almost any terrestrial ecosystem.
The diverse range of small mammals contains many poorly-known species with unusual, memorable, or simply endearing names. For example, have you ever heard of the moon-toothed degu, the fire-footed rope squirrel, the crafty vesper mouse, the beautiful squirrel, the monster rice rat, Dickie’s deer mouse, the pleasant gerbil, or the tuco-tuco of the dunes?
And many other species have truly remarkable adaptations to match. Possibly the greatest small mammal “superhero” is the aptly-named hero shrew (Scutisorex somereni) of tropical Africa. This innocuous-looking species has evolved an incredibly strong backbone supported by elaborate interlocking spines and projections, which accounts for 4% of the shrew’s total body weight. This unique anatomy allows the shrew to withstand being trodden on by a fully-grown human – but the evolutionary reason for developing such a strengthened backbone remains unknown.
Some small mammal species – notably black, brown and Pacific rats, house mice, and musk shrews – have become adapted to living alongside humans and exploiting the opportunities that we provide for food and shelter. However, the great majority of small mammals are less ecologically flexible, and are instead dependent upon the existence of intact natural environments for their survival.
As human-caused habitat destruction continues to escalate at a global scale, the habitats that small mammal species depend upon have become increasingly threatened. In addition, many small mammal species are also threatened by invasive species – such as foxes and cats in Australia, and American mink in the UK – as well as by the effects of climate change and overexploitation for human use.
Unfortunately several small mammal species have already died out in recent centuries as a direct result of human impacts. The Caribbean Islands experienced a major small mammal extinction event following the arrival of Europeans and their associated invasive mammals (rats, cats, dogs and mongoose), which led to the disappearance of several species of solenodons and hutias (large-bodied rodents), and wiped out the entire family of endemic nesophontid island-shrews.
More recently, the introduction of cats, foxes and other invasive species to Australia led to widespread extinctions in the endemic rodent fauna. Today further extinctions of small mammal species, such as the Brasilia burrowing mouse (Juscelinomys candango), are also being documented from larger continental regions as a result of habitat loss.
The Top 20s
Top 20 “Lost” Small Mammals
As a group, small mammals are very poorly known. And many species have been ‘lost’ to science. In other words, these are species that haven’t been observed by scientists for many years – and in some remote areas of the world, some species haven’t been recorded for over 100 years. Have they gone extinct or have we not looked hard enough? The red-crested tree rat Santamartamys rufodorsalis provides a clue to answering this question. Originally described in 1898 when two specimens were collected from the Santa Marta Mountains in Colombia, the species wasn’t seen again until 2011 when one was seen shuffling along a walkway handrail in the El Dorado Nature Reserve – that’s 113 years of being ‘lost’.
Here is our list of the Top 20 ‘Lost’ small mammals – surveys are urgently needed for these species to establish whether they are still alive today and, if so, to determine their status and conservation needs. For more information, click on each species to take you to the relevant account on the IUCN Red List of Threatened Species.
1. Namdapha flying squirrel (Biswamoyopterus biswasi)
2. Zuniga’s dark rice rat (Melanomys zunigae)
3. Wimmer’s shrew (Crocidura wimmeri)
4. Vernay’s climbing mouse (Dendromus vernayi)
5. Shortridge’s rat (Thallomys shortridgei)
6. Puebla deer mouse (Peromyscus mekisturus)
7. Manusela mosaic-tailed rat (Melomys fraterculus)
Top 20 Threatened Small Mammals
Think of endangered mammals and most people will think of giant pandas, tigers and organg-utans. But there are also a jaw-dropping 437 species of small mammal that are currently considered to be globally threatened with extinction. Among these are those that are on the very brink of extinction – Critically Endangered species. For some of these species, populations have collapsed down to fewer than 100 individuals and often the entire global range may be just a few hectares on a tiny island or mountaintop.
Here is our list of the ‘Top 20’ threatened small mammals. All of them require urgent surveys, action-plans and conservation measures. For more information, click on each species to take you to the relevant account on the IUCN Red List of Threatened Species.