Tnuctipun were small, arboreal pack predators, averaging about 3–4 feet long. Their heads were long and lean, and their eyes opened laterally. They were highly intelligent (IQ estimated around 130–140, according to the short story "In the Hall of the Mountain King") and social. As befits their carnivorous nature, they were also ruthless, aggressive, and cunning. Their word for alien most closely translates to "food that talks". Two billion years before humans evolved, the galaxy was ruled by the Thrintun, who telepathically enslaved other species, including the Tnuctipun.
The Tnuctipun invented most of the technologies from that era, including stage trees (trees containing solid rocket fuel in their trunks, originally used as cheap rocket boosters, which in the present era of the Known Space universe had evolved to seed themselves across star systems), sunflowers (flowers with integral parabolic mirrors that can focus sunlight to deadly effect), and stasis fields (a time dilation device). They were also known to have direct conversion of mass to energy and a telepathy shield (these two technologies are lost by the time most Niven stories take place). In order for the Tnuctipun to think creatively, the Thrintun allowed them some limited freedom. The Tnuctipun used that freedom to stage a rebellion against their masters, the culmination of a carefully thought out, centuries-long plan.
In the novel World of Ptavvs, the protagonist Larry Greenberg, a telepath who reads the mind of a Thrint, theorizes that some of their inventions were traps: Bandersnatchi, thought to be non-sentient livestock, were in fact intelligent, created as spies immune to telepathy. Sunflowers turned against their masters and burned Thrint homes to the ground. Other Tnuctip inventions were designed to shape Thrint society to weaken it. Sunflowers encouraged a trend for the slavers to live in isolated manors, surrounded by slaves. Mutated racing viprin (fast-running creatures raced for entertainment and gambling) ruined the existing viprin herding business, which along with other similar inventions led to an economic depression prior to the Tnuctip revolt.
The war escalated until the Thrintun, rather than accept defeat, employed a device that amplified the sphere of influence of a Thrint's mind control to encompass the entire galaxy. And they gave a simple command: Die. And everything in the galaxy that had evolved a backbone perished, including any Thrintun not protected by a stasis field. The Bandersnatchi were one of the only sentient races that survived this on a large scale, because they were already immune to telepathic commands. This course of events is alluded to in the novel World of Ptavvs and a still functioning suicide amplifier itself is discovered in the short story "Peter Robinson" by Hal Colebach, at which point it is destroyed.
Several other Tnuctip inventions are inadvertently discovered in the various known space novels, including a prototype hyperspace shunt, discovered during the first Man-Kzin War (in the novelette Inconstant Star by Poul Anderson). The Kzinti lose the war before they can bring news of it home, and the device itself is lost.
A recent Man-Kzin Wars short story - "Teacher's Pet" by Matthew Joseph Harrington, in Man-Kzin Wars XI - claimed that the Tnuctipun are responsible for creating the Pak Protectors. As with most Man-Kzin Wars material, its canonicity has not been confirmed by Niven.
In 1968, Niven worked with Norman Spinrad to draft a story outline entitled Down In Flames, in which much of the history of Known Space is revealed to be a hoax, and in which it is revealed that the Kzin are the Tnuctipun. The outline was published in Tom Reamy's fanzine Trumpet, and released on the internet, but was never intended to be completed or published, and was superseded by the Ringworld series of novels.
- "Future Histories", The Bulletin of the Science Fiction Writers of America, Summer 1989, Vol. 23 #2, issue 104.