Rukh

Rukhs, known to themselves as ruus (both singular and plural), are a sentient species of gigantic, vaguely bird-like predators native to Saturn, specifically the lower layers of the atmosphere where the temperature and pressure are sufficient to support liquid water. The species is notable for having two independent but cooperating brains, one male and one female. They appear in Robert Forward's novel Saturn Rukh.

Biochemistry
Despite their alien environment, rukhs have a very similar biochemistry to Earth life. They live at a similar temperature, and use liquid water as a biochemical solvent. They are carbon-based and use DNA, and are sufficiently similar to Earth life that they may be descended from terrestrial microbes transplanted to Saturn by meteors. However, there is no oxygen in their environment, and they are presumably anaerobic hydrogen breathers. There is also sufficient biochemical difference between Saturn and Earth biochemistry that one type of life cannot eat the other.

Anatomy & Physiology
Rukhs are tremendously large animals, with a wingspan of up to 4 kilometers (2.5 miles; 13,123 ft). Their bodies contain large volumes of air, decreasing their weight so that they can fly, but they likely mass hundreds of thousands of tons regardless. Their bodies are vaguely bird-shaped, with a keel in place of a head and hundred-meter black "feathers" on their backs. Rukhs are too large to fly by flapping their wings, so they fly using biological jet engines. They feed using two 200-meter-wide mouths, one on each side of the keel. The mouths are full of sharpened feathers that shred prey.

The rukhs' most remarkable trait is their bodies' division into male and female halves. Each brain is housed in a caterpillar-like neck tipped with a single, ten-meter-wide eye. The female brain's neck lies on top of the keel, and the males' beneath. The necks are lined with stubby claws that they use like hands; these appendages are extremely weak for such a large animal, barely stronger than human arms. The vagina lies near the back of the top half, and the penis sits in a similar location on the bottom. The brains can coordinate and communicate, but usually one is in control of the body while the other sleeps. The male brain is in charge of hunting, while the female brain handles the flight upward in preparation for the next hunting dive.

Diet
Rukhs sit comfortably at the top of Saturn's food chain. They can kill and eat virtually any animal they choose, and do so in large numbers. They are ram feeders, eating by plowing into schools of smaller animals, sucking them into their maws, and tearing them apart on their feather "teeth."

Reproduction & Life Cycle
Reproduction is initiated by the rukh's female half. This rukh mates with the male half of a different rukh, initiating sex by stimulating the male's penis with her neck. The female then guides the penis to her vagina, and the two dock until copulation is complete. Usually the female will try to mate with a male from a different flock, but if this is not possible then they will mate within their own flock.

Due to the rukhs' huge size, their life cycle takes place over a very long period of time. Gestation takes 13-15 Earth years, at the end of which the pregnant rukh's female half gives birth to a single offspring about 100 meters (330 feet) in wingspan. The baby must learn to fly almost immediately lest it be blown off the mother's back and die, in which case its parents will not rescue it. However, baby rukhs spend most of their time on their mother's backs, feeding off bycatch until they are large enough to hunt on their own. Rukhs live for about eighty Saturnian years, or close to 2,400 Earth years. The brains do not necessarily die at the same time; usually one will die of old age before the other, dooming the survivor to fall to its death once it can no longer stay awake.