The diversity of small mammals is jaw-dropping

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.

Use OneZoom to explore how the rodents, tree-shrews, shrews, hedgehogs, moles and solenodons feature on mammal tree of life.

https://zsltemp.wpengine.com/assets/mammals.html

Evolutionary History

Higher-order relationships between the major mammal groups continue to be revised on the basis of new molecular, morphological and fossil data. Rodents and tree-shrews fall within the major placental mammal clade Euarchontoglires; within this clade, the tree-shrews 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.

Biogeography

The different groups of small mammals show distinct patterns of geographical distribution across the globe, reflecting their independent evolutionary histories. Tree-shrews 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.

IUCN Red List

We have a challenge.  A rather large one.

Along with the other mammal Specialist groups, by 2015 the SMSG has pledged to conduct Red List assessments of all it’s 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 around 2800 small mammals.

Considered the most authoritative and objective system for assessing the status of and threats to the world’s species, the IUCN Red List of Threatened Species facilitates the expert-led assessment of species extinction risk based on the size and trends in species’ geographic range and population size. Based on these criteria, species are then placed into one of the following categories of extinction risk:

 

  • Least Concern
  • Near Threatened
  • Vulnerable
  • Endangered
  • Critically Endangered
  • Extinction in the Wild
  • Extinct

If there isn’t enough ecological information to properly assess their extinction risk, then a species will be classified as Data Deficient.  Our other big problem is that as a group the small mammals are poorly known, and we have around 450 species that are currently considered Data Deficient.

The SMSG is working with the Global Mammal Assessment Team at Sapienza University, Rome to deliver this huge project.  Assessments will be drafted by our members and programme officer, and then submitted to the Global Mammal Forum for a consultation period which will be coordinated by SMSG’s ‘Red List Focal Point’, Dr Giovanni Amori.  Following comments from relevant experts through the Forum, the assessments are will be then be reviewed by independent experts and ultimately 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.

For more information please visit the Global Mammal Assessment website.