Known affectionately as "Squossums", ratatoskrs are a small aboreal marsupial native to the Mossfell Islands which inhabit a niche similar to that of squirrels on the mainland. The oddest features of ratatoskrs are their flared lower canines. While somewhat unwieldy, ratatoskrs use these "tusks" to compete against rivals for mates in the case of the common ratatoskr, or to burrow into snow and soil in the case of the apline ratatoskr.

The common ratatoskr (also known as the red ratatoskr) truly has embraced an arboreal lifestyle. On the dense forests of St. James, the common ratatoskr uses its light frame and agile body to jump from tree to tree in search of food. The common ratatoskr weighs 0.4-1.2 kg (0.9-2.6 lbs.), with a usual body length of 38 cm (15 in). 

Unlike mainland opossums, the common ratatoskr is diurnal. It spends most of its time searching for food to compensate for a higher metabolic rate. As a result, ratatoskrs are very territorial and will defend valuable resources such as Idunn trees. The fruit of the Idunn tree has a fibrous outside, but contains a soft and nutritious center making it a valuable resource for these creatures.

Ratatoskrs have developed many adaptations for their aggressive arboreal lifestyle. They have developed better claws for climbing tree bark, as well as flexible wrists to aid in grappling. They have a heightened metabolism than mainland opossums, which helps them in scurrying about tree trunks. The naked and semi-prehensile opossum tail has been refitted into a bushy appendage. This tail is helpful for both maintaining balance while running on thin branches, as well as creating drag during leaps across gaps in the trees. During autumn common ratatoskrs create caches of nuts and fruits in hollowed out trees.

They are omnivorous and scavenge, but favor consistent sources of food such as Idunn trees and garbage cans leading to tense territorial disputes. During these disputes, individuals will begin to scurry up and down the location they are defending while chittering and flicking their tail. If that fails, the defender will attempt to use its tusks to nudge the intruder away from its territory and onto the forest floor. 

As a result of its territorial nature, common ratatoskrs are solitary animals, and usually only interact to mate. Females will attract males by making a high-pitched chattering noise. The average litter size is 5-7 infants, with a mortality rate of 80%. As marsupials, ratatoskrs give birth to young no larger than a dime which then spend a month and a half in their mother's pouch. Much like mainland opossums, ratatoskrs will carry their young on their backs for a short period of time before they leave their mother after three months. Mothers can have up to four litters per year if conditions are ideal. Common ratatoskrs generally have a short lifespan of 2 years in the wild. Despite being able to use their tusks as a rudimentary defense, they are still a common prey animal and prefer to flee. 

Despite this, common ratatoskrs are likely the most successful of the indigenous fauna on St. James by far. They were initially named by Norse explorers after the mythological squirrel Ratatoskr - a creature depicted with tusks and a penchant for scurrying up and down the world tree. Common ratatoskrs are found in plentiful numbers in any temperate forest on the southern island chain from Nishsystir to Musphelheim, with smaller introduced populations on the colder islands Gasmensyi and Tyrkimani.

Due to their generalist behavior, common ratatoskrs have been engorging themselves on human trash since initial Norse settlement in the 13th century. This has led to small speciation event creating the R. s. vulgaris and R. s. silvestris subspecies. R. s. vulgaris is most commonly found on St. Anne and the more developed parts of St. James and is notably larger and less arboreal. The larger subspecies is often found in neighborhoods and on the two college campuses on St. James. Individuals often will show reduced if not absent fear of humans. R. s. silvestris is found in denser woodland on St. James. It is much smaller and is more fearful of humans, often chittering and brandishing teeth if gets too close. 

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The apline ratatoskr (Ratatoskr  montanus) is a smaller, and rarer relative of the common ratatoskr. Found only in the highest reaches of the archipelago, the apline ratatoskr can only be found near the treeline on the islands of Neffannafjall and Musphelheim. Unlike the common ratatoskr, the apline ratatoskr prefers a terrestrial life. Instead of using its tusks to fight against intruders, it uses its fixed teeth to create burrows under soil and snow. 

The alpine ratatoskr is well adapted to life high above. A shaggy coat protects it from cold weather, and its dirty grey appearance blends well into snow, slush, and rocks. Like the common ratatoskr, it is a solitary animal that only interacts with others of its kind to mate. Additionally, it also favors an omnivorous diet, eating insects, small reptiles, and evergreen berries throughout the year. Unlike common ratatoskr, alpine ratatoskrs are most active at dusk and dawn, away from the peering eyes and ears of most predators on the mountaintops.

Very little is known about the alpine ratatoskr's lifecycle due to their secretive lifestyle. Unfortunately it may remain that way permanently as it is thought that the species is currently endangered due to increased habitat loss from climate change. Within the last 20 years, most of the permafrost found on the peaks of Neffannafjall has disappeared. The tree line has shifted upwards considerably on the islands, expanding the range of the common ratatoskr at the expense of its alpine cousin.

#Ratatoskrmoment

The grad student wipes sweat off her brow as she glances toward the thick canopy. It's a muggy mid-august day and certainly feels that way even in the shade. Only a few yards from the side of a road, she ventures further into the thicket of invasive willow trees. From a short distance she can hear an odd chatter, a clicking-hiss followed by the sounds of conflict. Going forth, the grad student comes face to face with a thick and broad Idunn tree, saturated with lumpy and unappetizing fruits. Finally she sees the cause of the ruckus. Two red blurs whisk about the tree trunk, hissing and snapping at one another. One lunges forward, and uses a sharp white tooth to swipe at the other. The two little creatures snap at each other again ferociously. One eventually relents and scurries off of the tree, leaving the victor to savor the Idunn tree's bounty. The creatures little feet grasp a branch of the tree as it curiously peers back at the human.

The grad student smiles at the toothy marsupial, its lithe red body relaxes and lays calmly on the tree branch. She pulls out a cracked iPhone from her pocket and snaps a glamorous shot of the Ratatoskr's blissful repose as it lets out a yawn. The grad student then turns and emerges back onto the road. A glance at the time and she realizes she's late for a meeting with her faculty advisor. Goddammit not again, she thinks to herself as she walks down the quad to the Roosevelt Biological Sciences Building. Well at least she can post a cute picture to @RatatoskrsofCanterburyU afterwards.