Kurtback

The emerging ecosystem of the Vonnegut islands is a unique one indeed. As these islands emerged so far from any other land, only the smallest and most abundant terrestrial organisms managed to colonize the archipelago. Though most of the region’s native fauna are unable to attain a large size, one group stands out amongst the crowd: the teacup saucebacks. Being descended from much larger creatures, it was only a matter of time until one of these species began getting big again.

Hunting
With little competition to hinder its growth, the kurtback is over three times longer than its closest relatives on average. Due to this size advantage, mature kurtbacks are able (and, in fact, specialized) to feed on other shrewbacks. Kurtbacks hunt in a manner not dissimilar to an earthling cat. Their feet are covered in soft down, keeping them warm in the dead of winter while also allowing them to stalk their prey without making a sound. This, of course, is hugely important to a species whose main prey item relies chiefly on sound to perceive the world around it. Additionally, kurtbacks do not emit sonar calls while hunting, using their prey’s calls to echolocate instead.

Once a kurtback is close enough to its prey, it gives chase, holding its robust jaws wide open. While a teacup sauceback has a top speed faster than that of a kurtback, its small size and high metabolism mean that it loses momentum quickly enough for the kurtback to pounce on it after only a few seconds of chasing. A kurtback pounces head-first, locking its jaws on the first part of a prey item they touch, killing it instantly.

Life Cycle
Similarly to their ancestors, kurtbacks begin their lives as tiny eggs, though now they are buried in a shallow depression in the soil instead of laid in the open. While kurtback larvae are very similar in appearance to those of their ancestors’, they are distinguished by several important features, just like the adult. Kurtbacks are more K-selected than most shrewbacks, laying slightly fewer eggs but having the larvae hatch at nearly 5 millimeters long to compensate. This adaptation originally gave these larvae a developmental advantage over other shrewbacks, but as in the adult kurtback, this size difference also helps kurtback larvae catch and eat (the larvae of) teacup saucebacks. To avoid competition with adult kurtbacks and other shrewbacks, larval kurtbacks often supplement their carnivory with a variety of roots and tubers, making them mesocarnivores. It’s worth noting that while adult kurtbacks have a slightly lower metabolism than their relatives due to their larger size, larval kurtbacks have a slightly higher metabolism than other shrewback larvae, enabling them to dig through the ground a little faster than their prey.

As the soil of a subpolar volcanic island can be a harsh place for an ectotherm, kurtbacks metamorphose earlier in their life cycle than other teacup saucebacks. While teacup sauceback larvae usually reach the length of the adult form before metamorphosing, kurtback larvae only reach half of their maximum length (17 cm), with the remainder of the growth happening in the adult form. Kurtback larvae still grow hair and store a large amount of fat before metamorphosis.

Integument
Since kurtbacks spend longer out in the open than their relatives, they have developed a variety of integumentary adaptations to deal with the cold environment. Kurtbacks have a distinct summer and winter coat of feathers, with moulting between the two triggered by the year’s first thawing and first snowfall, respectively. A kurtback’s summer coat is composed of thinner, black feathers, which help it blend in with the volcanic rock and soil it calls home. The winter coat, meanwhile, is composed of a dense layer of white feathers, keeping the kurtback warm and helping it blend in with the snow. A kurtback’s feathers are replaced as they fall out, giving the kurtback a mottled grey (or “blue”) look during the brief moulting period.

The sclerites of a kurtback undergo a similar color change to the rest of the body, but by a different mechanism. When changing to the winter coat, the black pigment in the jaws, teeth, sauce, claws, and tail spike is dissolved, with the pigment being regenerated during the transition to the summer coat. While the changes in feather and sclerite color don’t hide the kurtback from the blind teacup saucebacks, it does hide it from their other prey items like kruggs.

Courtship and Behavior
Perhaps the most unusual thing about the adult kurtback is its tail spike, which now appears more like a strange fin than it does a tiny spike. This structure is for sexual display, helping kurtbacks identify each other quickly and making them more attractive to mates. Although kurtbacks are completely blind, they are able to “see” this tail spike especially well via sonar. It is much larger, flatter, and denser than in other saucebacks, perfect for reflecting sound. Larger tail spikes are also favored by both sexes as a display of fitness: the logic goes that individuals able to maintain a larger tail spike must be better at hunting and surviving. This is because the structure essentially serves as a handicap. It radiates a lot of heat due to its flat shape, making it harder to thermoregulate. It is also somewhat cumbersome and makes the kurtback more “visible” to its prey, making it trickier to hunt.

Outside of courtship, kurtbacks are largely solitary, viewing other kurtbacks of the same sex or of much smaller size as competition. Rival kurtbacks generally don’t fight each other, but they do use the size of their tail spike to try and assert dominance. While this may seem like a purely selfish trait, it actually benefits newly-metamorphosed kurtbacks, which could otherwise be seen as food by fully grown individuals. Kurtbacks are not particularly smart for a sauceback, typically ignoring or avoiding new stimuli that they don’t recognize as food, foe, or partner.