Cold river life

Despite the increase in aridity, which has severely affected organisms adapted to humid climates, the parallel decrease in average temperatures in Antarctica has led to a reduction in evapotranspiration. As a result, especially in areas closer to the pole, the flow of rivers has not undergone significant variations, which has maintained a trophic chain similar to that of the cambiocenic era.

Once again, rodents play a major role in river ecology, with numerous species adapted to diverse microhabitats. In addition to the ever-present herbivorous aquatic species, the presence of a purely carnivorous rodent stands out: the lakevole (Longiarvicola aquatica). This medium-sized cricetid can be found in all major river systems of the Coldvanna and Tramplesteppe, but exhibits higher densities in the western plains of the Antarctic Peninsula.
Although externally resembling a typical rodent, the most significant evolutionary change in this species lies in its dentition: the large molars of its ancestors have undergone a trend towards so-called "carnassialization," which make teeth more suited for cutting meat than grinding vegetation.
The structure of the lakevole's vertebral column has developed strong lateral elasticity, enabling rapid movements that aid in capturing fish, its main prey. In addition to fish, the lakevole's diet includes insects, mollusks, and rarely even other aquatic vertebrates. The species exhibits a surprisingly high intelligence quotient and is among the most gregarious rodent species. During summer basking moments, groups of several hundred lakevoles can gather and engage in conspecific grooming. The species has been observed to frequently hunt in groups, especially targeting larger prey such as other mammals. Young individuals are used as decoys to flush out aquatic rodents from their burrows in the riverbed, prompting them to flee towards the water. At that point, two or more adult individuals will emerge and encircle the unfortunate prey.

Despite the lakevole's notable intelligence and agility, it and many other vertebrate species often fall prey to another large aquatic predator, namely the newnewotter (Ancestrilutra simiaregressus). As the name suggests, this mammal is not a rodent but a descendant of the geotters, the group of otters that colonized Antarctica tens of millions of years ago. It represents a sort of return to origins, an event that has also occurred with the bellydruggers, albeit with a slightly different ecology than its ancestors. 

Although the newnewotter possesses adequate aquatic adaptations, this carnivore rarely feeds on fish; it instead exclusively targets aquatic and terrestrial mammals, from the size of a rat to a stottmouse. Partially durophagous molars, robust conical canines, and a proportionately enormous, short head are all adaptations that allow this species to secure a tight grip on its prey, which is inevitably crushed like in a vice.
Unlike lakevoles, newnewotters hunt by ambush. Typically, the otter lurks near the water surface, waiting for rodents to come to drink or feed on aquatic plants. Only then the otter make a powerful leap using its muscular flat tail and hind limbs, pouncing on the prey.
The species prefers lowland rivers and turbid lacustrine areas rich in clay sediment, where its camouflage abilities reach their peak. They are typically solitary and territorial carnivores, exhibiting extremely aggressive behavior towards potential intruders in their territory: interspecific fights can be devastating, often resulting in a severely injured loser. No predators, not even the largest on the continent, dares to challenge this formidable animal.