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Stottmice have undergone a remarkable radiation in the past 10 million years, reaching astonishing levels of biodiversity in both size and ecology. Modern "light" forms can be divided into two main clades: the open-dwelling fast stottmice and the mostly forest-dwelling pompomice. Both groups evolved strong hooves on all four legs, an adaptation that allows them to burst into speed when pursued by predators. Interestingly enough, hooves evolved convergently in the two clades, arising from partial unguligrade condition, but with different configurations. This difference in hoof number is one of the key features used to distinguish them, along with other traits:
Fast stottmice have four large hooves per feet on the forelimbs, of which only two touch the ground, while the hind limbs possess three hooves per feet with a single enlarged one that sustains the body. This likely represents an adaptation to open terrain. This configuration result in forelimbs that are less powerful but more maneuverable, helping with rapid changes of direction, while the hind limbs, which provide the main thrust during running, are built for strength rather than flexibility.
Their legs are proportionally long, suited for running across landscapes with few hiding spots. The skull is narrow and elongated, aiding in grazing grasses and herbs, while the long muzzle helps store food in the cheeks. Most species are grazers, though mixed feeders also exist. This clade includes 10 genera and around 15 species.Pompomice are generally smaller than fast stottmice and possess four hooves on every limb, with two touching the ground. Their bodies are shorter and their limbs proportionally smaller, reducing speed but enhancing maneuverability, which is crucial in closed or semi-closed environments. They are easily recognized by their fluffy, rounded tails (which resemble a pompon), used for individual recognition and sexual display.
Their skulls are shorter and more robust, with less cheek storage, because their diet is mainly composed of soft plant materials like leaves and fruits. In winter, their stronger bite force allows them to gnaw on bark and lichens. This clade includes 4 genera and about 29 species.
Among the over 40 species of "light" stottmice now present, some are more interesting and representative than others. The valley stottmouse (Postucavia pedimontis) is surely one of the largest fast stottmice, reaching the size of an elk.
It inhabits the Great Depresseaon, particularly the gallery forests along rivers. Although uncommon in the driest areas, it thrives in foothills where trapped humidity increase the presence of shrubby vegetation. Adapted to aquatic life, its broad hooves enable efficient swimming, helping it escape predators much like a holoceninc waterbuck.
If threatened, vallet stottmice can also kick with their hind legs, though they usually relies on flight rather than fight. Both sexes are similar in size and possess large skin flaps on the neck, used in display and possibly for thermoregulation. Valley stottmice live in small related social groups of fewer than 10 individuals. They form seasonal breeding pairs in late autumn, with both sexes helping in raising young, usually twins or, more rarely, triplets.
They are mixed feeders, grazing herbs and grasses but shifting to foliage in the colder, drier season. Their large size allows them to rear up on their hind legs and browse leaves nearly three meters above ground.
In the driest regions of the Great Depresseaon, where shrub cover is sparse or even absent, another (even) fast(er) stottmouse dominates: the clashing stottmouse (Pachycavia cursoria). This ungulate rodent is built for high speed, with long legs capable of reaching 75 km/h on short distances, an crucial survival skill in order to live in open savannas and grasslands.
More gracile than the valley stottmouse, its distinctive trait is the large dome of the head. The dome is rarely used against predators, but it's instead used in intraspecific combat. Unlike muskoxen, which clash head-on, clashing stottmice fight like cattle and bison, pushing against one another with their flat-domed heads. Victory goes to the rival that can push the other back, an ability enhanced by their long, rear-placed hind limbs. It's difficult to say if clashing stottmice evolved first long hind legs for running and only later for helping in fight or vice versa, but it is surely efficient for both things now.
Their environment is dominated by tall and short grasses with scattered trees, much like a savanna, and their diet consists almost entirely of grasses and forbs. Their dentition includes three pairs of lophodont molars in each jaw, which increase grinding efficiency. Few predators can ordinarily prey on this swift stottmice, and maybe because of this they are the most abundant stottmouse species, numbering between 1 and 3 million individuals. Though most numerous in the Great Depresseaon, they also occur in lower densities in the wetter forest-savannas of the western Transantarctic Mountains.
In contrast to fast stottmice, pompomice are on average less suited to open ground but thrive in the tangled forests and rocky slopes of the continent. The genus Folicavia contains several small species rarely exceeding 30 kg, a useful trait for move through the dense undergrowth of their habitat. The browsing pompomouse (Folicavia transantarctica) is the most widespread and adaptable species, occupying both cool and warm lowland forests, including the Strip Plateau and dense gallery forests of the Great Depresseaon. Its coat is orange with creamy-yellow spots, a camouflage pattern similar to that of young cervids and tapirs. They live in small groups of fewer than 20 individuals and feed mostly on leaves, buds, and fruits. Their molars are less complex than their ancestors’, reflecting a softer diet. Despite their short legs, browsing pompomice can discretely climb to feed on fruits and leaves or to escape predators, similarly to a goat. Their flexible feet, capable of rotating more than those of fast stottmice, enhance climbing ability.
On Neopolarica’s slopes, pompomice face little competition from fast stottmice and have grown larger. The genus Rupicavia, or slope pompomice, contains species adapted to cliffs and rocky highlands, like the Lorentia slope pompomouse (Rupicavia lorentia), the largest, with males exceeding 100 kg. Geographic isolation in mountainous regions has driven a great species diversification, with at least 15 known species, sometimes partially living in the same area but in different habitats. Adaptations for life at high altitude include semi-webbed hooves and flexible toes in order to better adhere while moving on almost vertical rocks. Their habitats vary from steep mountain forests to alpine grasslands, but all of them must have at least some slopes, used as a refuge from predators. In alpine zones, some species retain high-crowned teeth for grazing on abrasive plants. In southern latitudes, where food is scarce in winter, slope pompomice rely on bark and mosses, made possible by their powerful bite that can debark tree trunks. Unlike their lowland relatives, slope pompomice have an extremely variable coat pattern, but usually no spots. Some species retain only faint leg stripes, which fade with age.
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