And yet, so much of this diversity – even at the species level – remains undiscovered.
One of the best ways of understanding and visualizing the richness of the known small mammals, and how species relate to one another is using OneZoom. The OneZoom tree of life explorer is an interactive map of the evolutionary links between all living things. Discover your favourites, see which species are under threat, and be amazed by the diversity of life on earth.
Use OneZoom (click on the image below) to explore how the rodents, treeshrews, shrews, hedgehogs, moles and solenodons feature on the mammal tree of life.
Higher-order relationships between the major mammal groups continue to be revised on the basis of new molecular, morphological and fossil data. Rodents and treeshrews fall within the major placental mammal clade Euarchontoglires; within this clade, the treeshrews probably represent the sister group to the colugos (Dermaptera) and fall within the Euarchonta together with primates, whereas rodents and lagomorphs (rabbits, hares and pikas) make up the Glires. Lipotyphlans instead represent the most basal divergence within the Laurasiatheria, another major placental mammal clade, which also contains the pangolins, carnivores, bats, ungulates and cetaceans. These major placental clades all emerged close to the extinction of the dinosaurs, although different studies debate the exact timing of the main mammalian evolutionary radiations.
Although they are referred to collectively as “small mammals”, these three mammal groups have extremely different evolutionary histories, and do not represent each others’ closest relatives.
Within these major clades, different small mammal lineages have shown extremely different patterns of divergence and diversification. Several lineages – such as the Caribbean solenodons (Solenodontidae), the North American mountain beaver (Aplodontidae), and the enigmatic kha-nyou of Lao PDR (Diatomyidae) – are very ancient and species-poor, and may be considered “living fossils”. Other groups have undergone extremely species-rich recent evolutionary radiations. For example, the tuco-tucos comprise a single genus (Ctenomys) of at least 38 species which all diverged rapidly around three million years ago. Even more strikingly, the Muridae – the single most diverse family of mammals, containing over 1300 recognized species – diverged less than 25 million years ago, and experienced a series of extremely large-scale radiations within the past few million years.
The different groups of small mammals show distinct patterns of geographical distribution across the globe, reflecting their independent evolutionary histories. Treeshrews have the most restricted global distributions, with both living and fossil species only known from tropical forest environments in southern and south-east Asia. Lipotyphlans and rodents both have much more cosmopolitan distributions, with rodents occurring on all of the world’s continents excluding Antarctica. However, different taxonomic groups also show important patterns of spatial variation in their geographical occurrence.
Within the lipotyphlans, solenodons and the recently extinct nesophontid island-shrews are completely restricted to the insular Caribbean, whereas other families have broader geographical distributions. Both talpids and shrews are found across Eurasia and North America, with shrews occurring in the Americas as far south as the Andes. Erinaceids do not occur in the Americas, with gymnures and moonrats only found in south-east Asia. Shrews and hedgehogs also occur in Africa, where they coexist with small-bodied afrotheres such as elephant shrews or sengis.
Many distantly related rodent families are restricted to different continents, but can display superficial similarities in locomotion and appearance as a result of convergent evolution to similar ecological roles. For example, North American kangaroo rats, African springhares and Eurasian jerboas have all independently evolved saltatory or jumping locomotion, whereas North American pocket gophers, South American tuco-tucos and African mole rats all display different levels of adaptation towards an obligate burrowing lifestyle. The great majority of South American and Caribbean rodent families – including hutias, capybaras, maras, chinchillas and agoutis – are hystricognaths, descended from overwater colonists from Africa that reached the then-isolated island continent over 30 million years ago. More recent colonizations of other landmasses between five and seven million years ago by other rodent lineages – colonization of New Guinea by anisomyines, Australia by conilurines, and South America by muroids – led to further rapid, massive adaptive radiations that together account for nearly 10% of all current-day mammalian species diversity.
Along with the other mammal Specialist groups, the SMSG conducts Red List assessments of all its species. These assessments involve gathering up-to-date taxonomic and ecological information for species, the threats they face and the conservation actions they require. Not too challenging for a handful of species – our problem of course is that there are well over 3200 small mammals!
If there isn’t enough ecological information to properly assess their extinction risk or if there is taxonomic uncertainty, then a species will be classified as Data Deficient. The group has around 450 poorly known species which are currently considered Data Deficient.
The SMSG works with the Global Mammal Assessment Team at Sapienza University, Rome, to deliver this huge project. Assessments are usually drafted by our team, often with the expertise of our members. The assessments are then reviewed by independent experts and then sent to the IUCN Red List Unit for final checks before being published on the IUCN Red List website. Along with all the other Red List assessments of mammals, amphibians, fungi, plants and so on, they will form a number of global biodiversity indicators which allow progress towards global environmental and development goals to be tracked.
In 2022, the SMSG took the decision to use the Mammal Diversity Database taxonomy for all future Red List assessments. We are in the process of working with the GMA to make changes to the database and following this we will begin creating new assessments where necessary and reassessing species that are affected.
If you would like to provide your knowledge of certain species to help draft species accounts then please get in contact with the group.
The Mammal Diversity Database of the American Society of Mammalogists (ASM) is your home base for tracking the latest taxonomic changes to living and recently extinct (i.e., since ~1500 CE) species and higher taxa of mammals. The ASM was established in 1919 for the purpose of promoting interest in the study of mammals, and is currently composed of around 2,500 members, many of whom are professional scientists.
The Mammal Diversity Database is curating the taxonomic implications of new research publications in real time — with the goal of promoting rigorous study of mammal biodiversity worldwide.
To download the database, search for specific species, or take a tour through the taxonomies, check out the Mammal Diversity website.
