Ratweasels are a widely-successful group of predatory rodents that first diversified during the Middle Muricene. Outcompeting all other mammalian land carnivores, their place at the top of the food chain has been secure for untold generations. Their size was limited in earlier ages; 1.5 million years ago, none were larger than a lynx. This opened the door for larger hunters to evolve, which would place the ratweasels in a mesopredatory niche. However, since every other group of meat-eating rats went extinct many years ago, this new apex predator could only emerge from within the ratweasel genus itself.
The Ratjackal (Tondendens aureops) is descended from the Middle Muricene sapbadger, a social species that regularly ganged up to attack prey larger than itself. These tendencies are even stronger among ratjackals, which form lifelong packs of about a dozen. With each member weighing in at about 40 kilograms, no herbivore is safe from their jaws. They are commonly seen on the rat-grassland, as well as both temperate and tropical pseudoforests. Their diet is primarily composed of rattalopes, but they don't shy away from various plant-eating rats, kiwis, giant castlebugs, and even smaller ratweasels.
By tackling such large game, the ratjackal can no longer employ the same method of dispatching its targets seen in other ratweasels. While its relatives use their shearing incisors to snip off extremities from their prey, ratjackals's front teeth are nearly conical in shape. This allows them to clamp down, digging deep into the flesh of an unlucky animal. Often, no more effort is needed, as blood loss and damage to internal organs are usually enough to secure a meal for the hungry rodent. When necessary, though, it can also use its molars to slice and tear through flesh. These have gradually morphed into a carnassial-like form, cutting through muscle and skin with little effort.
The pack structure of this species is surprisingly varied, often changing based on the pressures of the local environment. Grassland populations usually have larger groups, as more individuals are necessary to surround and corral prey in open areas. These large-scale hunts also promote strict hierarchies of social standing, in which a small number of dominant individuals command their subordinates in highly-organized attacks. "Alphas" (a somewhat oversimplified and inaccurate term, but used here for the sake of simplicity) may be either a mated pair or a single individual. In the former case, all low-ranking ratjackals will be prevented from breeding, being killed or driven away if caught. If the leader does not have a long-term mate, it will often meet with alphas from other packs, a union that sometimes results in the two groups permanently joining.
On the colder steppes, where other packs are few and far between, harems are more common, with the alpha mating freely with all packmates of the opposite sex. As a result, male-led packs are the norm in these regions (especially as the plains gradually turn to tundra), as opposed to in warmer climates, where females are just as likely to be the alpha. From a human perspective, the social life of northern ratjackals seems authoritarian, and there is indeed a great amount of pain inflicted upon subordinates if they act out of line. These violent methods are necessary to get by in such harsh conditions, and it's possible that many other species that currently live in fluid, egalitarian herds will soon adopt similar behaviors as the global climate approaches a tipping point.
In more hospitable biomes, packs are far less violent. Pseudoforests in particular offer a chance to form more easygoing groupings, as small prey is common and there is no need for any central authority. The boundary between leader and subject is often blurry in these environments, and the lives of the lowest-ranking members are usually no more difficult than those above them on the social ladder, with the only difference being one's access to potential mates. If there is any alpha at all, it is usually simply the oldest of the pack, though it's also not uncommon to see same- or opposite-sex siblings filling that role, or occasionally two unrelated same-sex individuals.
Taiga pseudoforests pose the greatest challenge of all, with temperatures nearly as frigid as the tundra but with far lower numbers of large prey. Like in the temperate woodlands, smaller animals are often the only food available, but in the desolate arctic winter, even these are hard to come by. As a result, ratjackals exist in very low population densities here, and different packs may go years without meeting one another. Packs operate similarly to their conspecifics in warmer wooded areas, though they lean even more in the direction of egalitarianism. The most notable difference between taiga-dwellers and other ratjackals is their body size; both the biggest and smallest members of the species coexist in this habitat.
As one might suspect, very large ratjackals are common in cold climates. This is a natural consequence of Bergmann's rule: similar organisms become larger in colder eras and areas, increasing their body mass to better conserve precious warmth. The diminutive subspecies that shares their range can be explained by the lack of large-bodied herbivores, a factor that promotes smaller, more agile predators. These contradictory forces result in two distinct niches in this environment, one for a huge, cold-hardy hunter and one for a lean, efficient killer. These populations are constantly in flux; in blizzards and extreme cold spells, the Timber Ratjackals (T. a. tempestus) are more likely to survive, while the fox-like Dwarf Ratjackal (T. a. invisus) fares better during prolonged periods of food scarcity. With time, it's possible that the two will diverge so greatly that they will cease to interbreed or compete with one another at all, forming two distinct species occupying the same home.