The Cartwheel

Up high in the treetops of the jungles of Serinarcta in the early Ultimocene, food is abundant and life, it seems, is easy. Fruiting plants, especially vines that cross between the tall ant-trees, bear energy-rich clusters of berries designed to attract animal seed dispersers including unusual avians such as boras which climb out on thin branches with their tentacle-like facial appendages, and molodonts which crush food in their grinding jaws. Life in the canopy is warm, wet, and most importantly, predictable. Food is always available somewhere, and so even as individual fruiting branches are picked clean animals don’t have far to travel to find more. What more could life ask for?

Most of the animals which live up here get along. There is enough food that there is rarely cause for squabble, and so an ever-shifting tapestry of different animals moves in and out of fruiting trees throughout the day and night. Different specializations of sight, scent, and hearing balance one another out put together, and so birds and tribbets of all kinds help one another, intentionally or not, as they find patches of fruit or alert one another to possible danger, these mixed foraging parties becoming greater than the sum of their parts.

Among these parties can be found simiagibs, arboreal tribbets not closely related to tribbetheres but which have also become more active and energetic animals with a high metabolism. Descended from handfish, their most glaring definitive trait is in their fractal fingers; two pairs of which on both forearms and their tail-leg have hypertrophied from the wrist and become effective limbs themselves, branching at the distal tips into four additional digits due to a replication mutation of their finger bones. In all tribbets the same genes have evolved to work to form the structure not only of the forearms but the tail-derived hind limb as well, with this mutation also effecting this limb despite its originally different origins.

Simiagibs are omnivorous, fast-moving, and intelligent by tribbet standards, though less so less than the molodonts. They live in small familial groups for safety, care for their young, and have keen senses to find food and avoid danger in both large eyes and big ears that are able to move independently to pinpoint the direction of sounds. These senses allow them to give an early warning for the other animals they forage with to take cover when a predator threatens, but after that it’s every creature for itself. Birds that can, fly off; boras brachiate away suspended under the forest branches to find shelter. Simiagibs, however, are capable of a unique method of brachiation that no other Serinan creature is - a rotational, rolling movement which makes use of their three highly specialized limbs, in which the animal rotates its entire body at high speed, grabbing and pushing forward with alternating appendages. The descriptively named cartwheel, a small frugivorous simiagib, is particularly specialized to move in this manner, which is quicker than alternating to swing with just two arms but at the cost of being more disorientating.

Cartwheels are one of the smallest simiagibs, still closely resembling their more primitive gibbet ancestors even though they are well-within the simiagib clade. They are social, live in female-controlled groups, and eat mostly fruit, with a smaller percentage of invertebrate prey. They are tiny, with only a small stomach adapted to quickly digest energy-rich food, and so cannot eat very much at once but instead must feed frequently through the day. To reduce the time they need to spend foraging, cartwheels have evolved large pouches in their cheeks in which they can store extra food to eat later at their leisure.

The novel locomotion employed by the cartwheel is evident even when they are relaxed and foraging for food. They swing under a branch first with the right arm, then swing up the tail and allow the body to fall down supported, before swinging the left limb back up and so alternating, with the entire animals’ body rotating like a wheel under the branch. The eyes are huge, with four-pronged, star-shaped pupils which are structured to observe two possible horizons at a time as the animal moves along, reducing the strain on its eyes to focus on a changing landscape as it oscillates upside-down and right-side up. At regular “walking” speed this spinning is slow and the eyes alone are able to adjust to always determine which direction is upward without becoming confused. But it is only when the cartwheel is frightened by a predator that its real skill becomes apparent. To rapidly escape danger this simiagib effectively rolls away from its attacker at higher speed than it can run, making use of gradually descending branches to pick up speed as it rapidly rolls along, grabbing and releasing the branch with each paired limb several times a second and then using the powerful hind leg to leap upwards to another branch at the last moment to repeat the cycle. At high speed the eyes alone cannot maintain balance to determine which direction is up, and now a different, specialized organ comes into play. A rod-like finger on the wrist, vestigial in other simiagibs, attaches to the muscle of the arm and functions like a haltere, and is able to keep track of the changes in body position that result from this locomotion through gyroscopic properties. While moving quickly these digits are twitched continuously, and so any rotation of the plane of oscillation causes a slight change of force upon them due to the Coriolis effect. These slight differences are then detected by the nerves which attach to the wrist and give the animal an instinctive sense of what is up and what is down even while effectively rotating and unable to determine gravitational pull.

Cartwheels are also capable of a rapid rolling movement over the ground, in which they function like a biological wheel. This is effectively an inverted version of their brachiating. However, as these are arboreal animals, dropping completely to the forest floor is performed only as a last-ditch attempt to escape danger.

The rotational ability of cartwheels is also put to use for other means. The brain of cartwheels is large and well-adapted to track the number of rotations they put their body through to stay orientated while feeding, and male cartwheels rely on this ability in females to impress them. During mating season the males gather in groups on the branches and display by leaping into the air in a clearing and performing as many rotations as they can before reaching the ground, which the females attentively observe. The most acrobatic males, who complete the most cycles, demonstrate the greatest control of their abilities and show a potential partner that he is fit and well capable of fleeing predators. Bright blue patches of skin on the males’ fingertips and the bridge of his nose make it easier for the female to count each rotation of his body in the air, as the blue digits stand out against the dark skin of the rest of his limbs.