Biologically, Puppeteers are highly intelligent herbivores; a herd animal, Puppeteers prefer the company (and smell) of their own kind. Their cycle of reproduction is unusual, but Earth equivalents exist in the form of digger wasps: the Puppeteers consider themselves to have three genders (two male, one female), except their two "male" genders are the equivalent of human female and male (one has an ovipositor, the other a penis, of sorts) and the "female" is the (non-sentient) parasitised host into which the ovum and spermatozoon are deposited.
Puppeteers are also extremely long-lived. The exact length of a Puppeteer's lifespan is unknown, but it is at least several centuries — Nessus is over 300 years old. This lengthy lifespan is also responsible for extreme crowding: the Puppeteer homeworld has a population of over a trillion, and four farming worlds are dedicated entirely to supplying the population with food. Technologically, the Puppeteers are very advanced, centuries or millennia ahead of most other species (including humans). For example, humans invented a method of cheap teleportation in the twenty-fifth century called a transfer booth, which requires an enclosed space at either end of the transmission. Puppeteers' use a much more elegant and sophisticated booth-less "open" version in the form of stepping disks, which require no enclosure. More impressively, Puppeteers transformed their home world, and several other astronomical bodies, into a Klemperer rosette, in order to flee a galactic catastrophe.
Socially, the three most notable traits of Puppeteers are their racial/cultural penchant for cowardice, their tendency to congregate in herds, and their steadfast honesty in honoring agreements. The cowardice is thought in Puppeteer society to originate with the Puppeteer instinct for turning one's back on danger. However, the trait is thought by many to actually originate from their herd instinct, as the instinct to turn one's back is linked to an instinct to kick the hind hoof at an attacker. In Ringworld, when Nessus and the expeditionaries are threatened, the Puppeteer defends himself quite effectively:
All in one motion, the puppeteer had spun on his forelegs and lashed out with his single hind leg. His heads were turned backwards and spread wide, Louis remembered, to triangulate on his target. Nessus had accurately kicked a man's heart out through his splintered spine. (Ringworld, Chapter 13, published 1970.)
Another noticeable behavioral trait is the coma state, broadly a cognate of the human fetal position — in the same way that ostriches are said to bury their heads in the sand, Puppeteers fold up into a ball, tucking their three legs and two heads underneath the padded cranial bulge. This is, in part, an explosion reflex, learned during childhood. Their cowardice is also reflected in their architecture and object design, as all the Puppeteer-designed rooms and vessels have no sharp edges, everything curves into everything else, giving a "half-melted" look and meaning that objects are less likely to damage someone inadvertently, through their own carelessness.
In Ringworld, Nessus, a Puppetteer, explains how his race's cowardice is partly a result of a science experiment (the details of which are not given) that proves the Puppeteers have nothing equivalent to an immortal soul, and therefore death is, for their species, absolute and eternal. As a result, the Puppeteer race is fanatically devoted to its own safety.
A courageous puppeteer is not merely regarded as insane (as Nessus mentions "the majority is always sane"), though, but is insane, showing symptoms we would associate with human mental illness, such as bipolar disorder, homicidal tendencies, clinical depression and so on. Incidentally, though, aside from the crew of the Long Shot in the novel Ringworld, no human has ever met a sane Puppeteer, as no sane Puppeteer would ever leave the safety of the Fleet of Worlds (see below), and even those who do would not venture out without a painless method of suicide, in case circumstances required it.
On occasion a puppeteer will express its amusement by facing its two heads towards each other, in effect, looking at itself. This is described by Niven to be the closest to laughter a puppeteer comes.
The leader of the Puppeteers is known as the Hindmost. Since Pierson's Puppeteers are foremost concerned with their own safety and the survival of their species, the most important Puppeteer is considered to be behind, or the protector of every other member of the species. It is a shortening from the more literal the one who leads from behind. A maddened, deposed Hindmost is responsible for Louis Wu's return to the Ringworld in the book The Ringworld Engineers.
Exposure to antimatter is the only known method for destroying a General Products hull until the recent novel Fleet of Worlds. In the story "Flatlander", a GP Hull is exposed to a constant stream of diffuse antimatter during a visit to a star system with some exotic qualities. Whereas a conventional hull made of metal, for example, would simply have ablated under these conditions, the General Products hull instead simply unraveled. This was due to the fact that a GP Hull essentially consisted of a single incredibly large, highly complex molecule. Once a sufficient number of the atoms which constituted the molecule were annihilated by the antimatter, the molecule could not remain stable, and thus degenerated into a selection of less complex compounds and elements, effectively causing the hull to vanish in an instant. Fortunately, the vessel's pilot was sufficiently paranoid to be wearing a vacuum suit at the time, and survived, as did the owner of the ship.
In "Fleet of Worlds", the characters tour a General Product factory and ask innocent-seeming questions of their tour guide, Baedeker. Baedeker reveals (apparently unintentionally) that the manufacturing process is extremely sensitive to gravity and impurities, that the hulls are constructed from a single supermolecule constructed using nanotech, and their strength is reinforced by an embedded power plant that reinforces the inter-atomic bonds. These facts provide the clues that allow them to later destroy a GP Hull from the inside and survive.
In Ringworld, it is revealed that the Puppeteer government meddled in human and Kzinti gene pools. They started a series of wars (the Man–Kzin Wars) between the warlike Kzinti and humans, and guaranteed that the Kzinti lose each time, not least by using a starseed lure to guide an Outsider ship into human space, introducing FTL travel to humanity. This was a mechanism to cause rapid Kzinti evolution, since the most aggressive Kzinti would die in battle, leaving the more docile individuals to breed, eventually suppressing their racial instinct for aggression.
Another Puppeteer breeding experiment was the Lucky Human Project. The puppeteer government concluded that humans' most notable quality was luck, and decided to improve this trait. Manipulating politics on Earth through bribery and blackmail, the Puppeteers caused 'Birth Lotteries' on Earth around 2650, biasing human genetic selection (controlled, in the Known Space universe, by the "Fertility Board of the United Nations") towards encouraging luck. The character Teela Brown, who journeys to the Ringworld, is an outcome of this Lucky Human Project, though not quite the outcome the Puppeteers would have liked. Her luck was highly selective, bending probability so that the outcome most beneficial to her or her descendants would come to pass, without regard to its effects on those around her — which was contrary to the interest of the rest of the Ringworld expedition on more than one occasion.
Puppeteers were willing to pay large sums of hush money in order to suppress even trivial details about their homeworld. In 2641 AD, it was discovered that the Puppeteers' homeworld had no moon, information deduced as a result of the single failing of General Products hulls — GP hulls are not impervious to tidal forces because the Puppeteers have no experience of tides. (As told in Niven's short story, 'Neutron Star')
The Puppeteers had to make some drastic alterations to their home system, during their history, as global warming and overindustrialisation was rapidly making their planet uninhabitable. They moved their planet further from their sun, to lessen the effects of global warming, but overindustrialisation forced them to move four of the other planets in their system closer to their world and terraform them into "farming worlds", arranging the five planets into a Klemperer rosette (though Niven misspelled "Klemperer" as "Kemplerer"). Nessus explains this to Louis Wu and the crew of the Long Shot thus:
"I had explained," said Nessus, "that our civilisation was dying in its own waste heat. Total conversion of energy had rid us of all waste products of civilisation, save that one. We had no choice but to move our world outward from its primary."
"Was that not dangerous?"
"Very. There was much madness that year. For that reason it is famous in our history. But we had purchased a reactionless, inertialess drive from the Outsiders. You may have guessed their price. We are still paying in installments. We had moved two agricultural worlds; we had experimented with other, useless worlds of our system using the Outsider drive. In any case, we did it. We moved our world.
"In short, we found that a sun was a liability rather than an asset. We moved our world to a tenth of a light year's distance, keeping the primary only as an anchor. We needed the farming worlds and it would have been dangerous to let our world wander randomly through space. Otherwise we would not have needed a sun at all.
"We had brought suitable worlds from nearby systems, increasing our agricultural worlds to four, and setting them in a Kemplerer Rosette."
—(From Ringworld, Chapter 5, published 1970.)
Eventually, their sun converted from a yellow dwarf to a red giant, so the Puppeteers moved the "Fleet of Worlds", the five planets, to their system's Oort cloud. This is one of the reasons the Puppeteers were so successful at keeping the location of their homeworld a secret — explorers would be looking for a yellow dwarf (as one could surmise that Puppeteers had evolved around a yellow dwarf from their biology and that they were comfortable on Earth-like planets without pressure suits) when their planet(s) were actually near a red giant.
In the short story "At the Core", Beowulf Shaeffer, who made the discovery about tidal forces five years previously, in "Neutron Star", discovers that the Galactic Core is exploding. This news prompts the Puppeteer Exodus, where the Fleet of Worlds flee the galaxy at just under light speed for the Magellanic Clouds, in the hope that by the time the explosion reaches the Fleet of Worlds, the Puppeteers will have found a way to protect their civilisation. This exodus prompts a major stock market crash in human society; in 2864, the Fleet of Worlds leaves Known Space.
It should be noted, however, that the speed at which the Fleet of Worlds is moving (0.8c) would cause nearly as much damage as the Core explosion itself. This means that the Puppeteers may, in fact, have a means to deal with radiation affecting entire worlds. In "Crashlander" it is speculated that the Puppeteers are planning on moving to the uninhabited Core, isolated from potentially dangerous species (one would think that Louis Wu would have noticed this when flying there on the Long Shot in Ringworld). "Crashlander" also reveals that the Puppeteers may have feigned their ignorance of tidal forces.
One of the Green Lanterns shown on the cover of the graphic novel Green Lantern: Ganthet's Tale written by Larry Niven, appears to be a Pierson's Puppeteer. It also appears in one panel of the graphic novel.