The Kurii are prevented from launching any large-scale assault on Earth or Gor to take possession of these worlds due to the superior technology of the Priest-Kings. The Priest-Kings have previously repulsed six large scale invasion attempts, and continually repulse smaller raiding parties. All of the Kurii on Gor are there because their ships were shot down by Priest-Kings and crash-landed on the planet. The Priest Kings permit the stranded Kurii to exist on Gor as long as they obey the anti-technology edicts of the Hive.
The balance of the Kurii race exists in large spaceships in the outer Solar system, the ships being the size of small moons and containing billions of the creatures. There is no instinct against killing in their physiology, which would tend to prevent a society from even forming, except that no Kurii expects not to be killed by its fellows. On their large world-ships, they constantly kill and feed on each other, with no compunctions. However on the world ships there exists a Ship Discipline enforced by the strongest, which seems to be the only thing preventing them from self destructing.
Their home planet seems to be destroyed in such internecine warfare, and this is why they seek a new home world. Further evidence of the ships being refugee clusters is that each ship contains multiple Kurii nations and multiple Kurii languages, as do the crash-landed Kurii on Gor.
Kurii are large and gorilla-like in appearance, with six digits on each hand, and each finger with many joints. The fingers terminate in claws, which are retractable. Like the gorilla, their front limbs are longer than their back limbs, and they can run on all four for a short time. When only on their back limbs, they move much slower than a human. The pelts which cover their whole bodies are brown when temperatures are above freezing, but they grow a mottled grayish-white pelt if they spend too much time in the cold. The winter-pelt is shed once the Kurii spend time in warm areas.
Their eyes are very large for their head, and the sclera (white of the eye) is yellow, giving them a bug eyed appearance. In daylight their black pupils are as thin as needle points and the Kurii vision is most impaired, although still equivalent or superior to a human. In darkness the pupil spreads to cover much of the eye giving a Kur excellent night vision, for that they are reputed to see much better during the night than humans can during the day.
Their noses are flat on their face, the nostrils slitted so they can be closed against moisture, and their sense of smell is most sensitive to spilled blood which throws them into a feeding frenzy. Their tongues are black, and the insides of their mouth a very vivid red. They have two rows of teeth similar to a shark, but their upper and lower canines are more prominent more than the other teeth. Especially, their upper canines protrude below their jaw, similar to a sabre toothed tiger. The Kurii can hinge their jaw open enough to swallow an entire human head and part of the shoulders.
Their ears are large and pointed, much like desert foxes, allowing them to hear human whispers at a distance of a thousand gorean feet. When speaking to humans, a Kur might fold his ears back to appear more human. Folding the ears back also seems to be a sign of aggression, which may or may not be a coincidence.
Four sexes exist in the Kurii and they eventually birth live young, although part of the life cycle involves eggs. The first and primal sex is the highly intelligent Dominant which is driven to mate by instinct. The Non-Dominant sex is a subset of the Dominant, it can mate but it does not have a mating instinct, is less intelligent, and most often performs common labors. The Egg-Carrier sex of the Kurii is their "female" sex, it is impregnated by the Dominant sex, and carries a small egg to term. And finally there is a Blood-Nurser sex, which carries the egg during the final stages of development. The egg can be any of the four sexes, but most favored offspring are Dominants. The reason why the Kurii do not run out of Blood-Nursers is because each can incubate dozens of eggs, and many of the eggs may be blood nursers themselves. In the life cycle of the Kurii, eight live young are born each year, usually two Dominants, two Non-Dominants, two Egg-Carriers and two Blood-Nursers. The Dominant and Non-Dominant sexes are sometimes interchangeable, as some Dominants may for some reason have their libido suppressed, or a Non-Dominant may have its mating instincts and libido enhanced. Even the Kurii are not certain if they have three or four sexes, as humans understand them.
The mating process is thus:
- Dominants kill other Dominants for the right to mate.
- The strongest survivor Dominant mates with and impregnates the Egg-Carrier.
- The Egg-Carrier carries and incubates many eggs to term.
- The Egg-Carrier injects the developed eggs into a Blood-Nurser.
- Eggs incubates inside the Blood-Nurser and eventually hatch while still inside.
- The young within feed on the blood of the Blood-Nurser during final development.
- Once the young are ready to be born live, they erupts from the Blood-Nurser, killing it
- The Non-Dominant raises the offspring.All Kurii are fully carnivorous and consider humans to be a delicacy. A single kurii can eat dozens of humans in a single day, taking little nutrition from each before excreting the remains. However their metabolism dictates that a Kur feed once every ten days. A Kur hunts its quarry from the moment it smells blood, and much like the sleen rarely gives up until the prey is dead. Once the prey is dead, the Kur will stand over the prey and defend it to the death even faced with many armed opponents, it will not abandon the carcass and run to save its life. The Kurii also behave this way toward puzzles and problems, single-minded pursuit of the solution, suggesting that this may be how they developed technology. This may be the reason the Kurii have not abandoned their conquest of Earth and Gor, despite being driven back by Priest-Kings half a dozen times.
While the smell of blood throws a Kur into a feeding frenzy, the smell of blood, the hunt, and combat also makes the Kurii more intelligent and vital. This might be why despite Ship Discipline, Kurii on the world-ships are still permitted to slaughter each other. Without regular combat, hunting and feeding a Kur seems to lose much of its intelligence as though conserving energy, though that seldom occurs outside captivity. Their intelligence on average seems to be on par with humans, yet far below Priest-Kings. Aside from their world ships and their smaller spacecraft, their technological development is also not dissimilar to the humans of Earth.
In short the considerable physical capability of the Kurii yet does not make them invincible, nor is their advanced technology unbeatable. These aren't the main reason for the antagonist position of the Kurii in the series. The reason they are the main antagonist is the fact that the Kurii are psychologically an intractably aggressive species and will not stop in their pursuit of conquest for any reason.