Teronura

color-sensitive & semi-aquatic

Rain washes over everything, including you. Thick mud tries to claim your shoes as you squelch your way through the forest, but the heaviest downpours are over for now. You can now hear the sounds of many wet bodies slapping against rocks and low-pitched laughter closer to the gurgling river. As you crest a hill and look down at the river bank, you catch small piles of abandoned clothes, but no one is in sight. Just before you go to turn back, you see six round heads and twelve round eyes poke up just above the water. Then, one waves you welcome.

Teronura are medium-sized six-limbed semi-aquatic mammiles. With otter-like long torsos and short legs, they prefer a quadrupedal gait, but they have very nimble hands on their uppermost limbs. They only use all six limbs to swim. Color vision is split between their two sets of eyes (one set adapted for water vision, one set adapted to terrestrial vision). Fur that covers most of their bodies, scales that cover their snouts and hands/feet, and casques on top of their heads all shine in UV. Civilizations started in the warm, wet tropics of Mutania's equator, but they've swam their way across the planet.

Limbs

Teronura have six limbs, using two for object manipulation, four for walking on land, and six for swimming. Their forelimbs are like human arms, with full shoulder rotation and four-fingered hands, and their hind limbs function as legs in both bipedal and quadrupedal gaits. They use their middle limbs for aquatic propulsion and steering. Instead of dexterous hands with two opposable thumbs like the forelimbs, the middle limbs have webbed fingers and no opposable thumbs. When teronura walk on land, they tuck their shorter middle limbs against their body so they don't get in the way.

Senses

Vision

Teronura outer eyes are adapted for vision underwater. The lens is spherical, like many fish eyes, to accommodate the way light refracts in water. They have a transparent nictitating membrane that covers and protects the eye while swimming. Out of the water, those eyes are extremely near-sighted.

Their inner pair handles distance and clarity in air. These eyes also have a nictitating membrane, but it's translucent rather than transparent. When underwater, the membrane only blurs the vision more in these eyes, so teronura tend to close them fully when submerged.

All four nictitating membranes can be consciously controlled, and teronura use them as an aspect of their facial expressions.

Color

Both eyes have tetrachromatic color vision with four types of cones. The inner eyes have similar cones to a human plus an extra one sensitive to UV light. Long-wavelength cones of the outer eyes are shifted to green instead of red, allowing them to better discern between colors in the blue-green range at the cost of red perception. Certain colors look identical to them underwater, but their inner eyes can distinguish between them in the air, and vice versa.

Chemosensory system

Like the other sophont species on Mutania and snakes, lizards, and some mammals on Earth, teronura have a vomeronasal organ (VNO), also known as a Jacobson's organ. It detects close-range chemical signals like pheromones. Teronura can use these signals to detect reproductive status, emotional state, or sickness in other teronura. Signals output from other sophonts can be detected, but what the signals mean has to be learned like a language.

Hearing

Teronura hearing includes adaptations for hearing low frequencies, like deep rumbles and ground vibration, and is slightly less sensitive to higher frequencies than a typical Earth mammal.

Whiskers

On their faces, they have whiskers around their snouts and above their central eyes. As bipedal animals, their whiskers are primarily used for social touch and communication. Facial expressions include whisker movements.

Speech

Teronura have bifurcated tongues that give them more surface area for tasting. Fricatives, especially the unvoiced "th" sound (like the "th" in "thin"), can be buzzy or hissy. The bifurcation gives them more surface area for tasting objects.

With complex, large larynxes relative to their body size, teronura have rich deep voices. They can vocalize between 80–8000 Hz, which is just below the 85 Hz floor of human voices.

Scales and fur

Teronura have both scales and fur.

Their faces (around the muzzle and eyes), hands, and feet are scaled, with slight differences between their limbs. The muzzle scales are full of sensory organs as sensitive as the tips of human fingers, useful for detecting water movements. The backs of their hands and feet are covered in raised, textured, crocodile-like scales. Smaller, finer scales on the palms of the hands of their forelimbs and the soles of their feet allow them to touch things without fur in the way. Their middle limbs have the same crocodile-like scales on both sides of their hands.

Teronura fur has two layers: an outer layer of long stiff hairs that protect an inner layer of fine, densely packed underfur. Together, they can protect teronura from cold water without them needing a layer of blubber.

To maintain their fur, they need regular access to water to bathe or shower in, at least once a day. Water keeps their fur from matting or becoming too oily. Even a short camping trip through the desert, where the only water was for drinking, would result in some truly grody teronura.

Teronura fur is also biofluorescent. To a human, they would appear black or dark brown, but to creatures who can detect UV light, they appear purple. Their casques also fluoresce, with variations between individuals in color and pattern. Members of the same family tend to have similar casques to each other.

Respiratory system

Teronura have unidirectional multichambered lungs. Like birds and alligators, old and fresh air never mix, but they don't have air sacs like birds. Asymmetrical branching airways inside the lungs create aerodynamic valves where the geometry of the passages directs the rush of inhaled air through certain passages while exhaled air uses pressure differentials to move through different ones, all in a loop. When diving, teronura lungs can maintain the flow of oxygen via the pumping of their heartbeat to move air.

Digestive system

Fish and invertebrates make up the primary ancestral diet of the teronura. For gripping slippery fish, they've got many sharp incisors in both their lower and upper jaws, with the outermost incisors especially elongated and fang-like. Toward the back of the mouth are several powerful molars for cracking open invertebrate shells. They can open their mouths very wide to fit hefty fish and have more bite force when they need to crush and crunch.

Reproductive system and life cycle

There are two sexes of teronura. One sex has oviducts and uteri behind the cloacal opening and retains eggs inside the body after fertilization before eventually laying them. The other has a penis with a forked tip stored inside the cloaca when not in use. Both sexes have specialized skin patches on their abdomens for taking care of the eggs after they're laid—a brood patch for incubation and patches with milk-secreting glands for nursing hatchlings. Both sexes are fertile year-round.

Each clutch contains 1 to 3 eggs. Once fertilized, the eggs are retained in the parent's body for about 3 weeks so the young are able to receive nutrients directly from their parent and develop more quickly. After the eggs are laid, they only require a 4 week incubation period, then the babies use temporary egg teeth to break out of their leathery eggshells.

When the babies hatch, they are only about 3–4" (7.6–10 cm) long and relatively helpless, but their uppermost pair of arms are well-developed, enabling them to crawl around the parent's abdomen in search of milk. The babies are born furred and scaled, but their casques are undeveloped, visible only as furless patches on the top of their head. Distinguishing marks like fur pattern are visible at birth, but some scale patterns can develop greater complexity later in life.

Babies are weaned after around 4 months. The casque begins developing at around 5 years of age, but isn't fully finished until a teronura is around 15-20 years old, which is also approximately when they reach sexual maturity and full size.