Tribbles: Born Pregnant, Hated by Klingons and the Furry Alien That Nearly Got Star Trek Sued

"The Trouble with Tribbles" aired on December 29, 1967, in the second season of Star Trek, written by David Gerrold, then 23, from a story he had pitched as a comedy about an animal that breeds. The Enterprise is called to Deep Space Station K-7 to guard a shipment of quadrotriticale, a grain, against Klingon interference; a trader named Cyrano Jones gives Lieutenant Uhura a tribble, a purring ball of fur the size of a hand, and within three days the ship and the station are full of them, breeding in the corridors, the food slots and the storage bins. The tribbles in the grain turn out to be dead, poisoned, which exposes the grain's sabotage and, when one shrieks at a Federation official, the Klingon agent who did it; Scotty solves the infestation by beaming the lot onto the departing Klingon ship, where, he reports, they will be no tribble at all.

Gerrold's model was Robert Heinlein's flat cats, from the 1952 juvenile novel The Rolling Stones, and the resemblance was close enough that the studio sought Heinlein's permission; he gave it, and pointed out that he had taken the idea from Ellis Parker Butler's 1905 comic story "Pigs Is Pigs," about guinea pigs in a railway freight office. The episode was voted among the series' best by its own fans and has been the franchise's standard comedy since.

A tribble is a reproductive organ with fur. Dr. McCoy's examination in the episode establishes the essentials: tribbles are born pregnant, bisexual and asexual by turns, breed roughly every twelve hours, produce litters of ten, and put half of their metabolism into reproduction, the other half into eating; from one tribble in three days the ship's computer projects 1,771,561, which Spock announces to the bridge. They have no face, no limbs and no visible senses, come in browns, greys, whites and, occasionally, more alarming colours, and their purr, a soothing trill, has an effect on humans that McCoy calls tranquilising and Kirk calls a nuisance. They eat grain, and anything else, and starve to death in silence. Their only defence, and their only aggression, is the shriek they give in the presence of Klingons.

The franchise has explained them twice. The original series left their home world unnamed; later reference works settled on Iota Geminorum IV, and the 2019 short "The Trouble with Edward" told, as comedy, the story of a Starfleet scientist who took a slow-breeding, harmless tribble and edited it into the reproductive engine the episode encountered. The Klingons, in the same short and in the 1996 Deep Space Nine episode, are revealed to have hunted the species to near extinction as a pest; the 2009 film's Scotty keeps one in a bunker on Delta Vega, and Into Darkness (2013) revives a dead one with Khan's blood, which is how the crew learns that the blood can revive Kirk.

Gerrold wrote a sequel for the animated series, "More Tribbles, More Troubles" (1973), in which the Klingons have bred a tribble predator and the tribbles, genetically altered not to breed, instead grow enormous. The franchise's best tribute came on November 4, 1996, for the thirtieth anniversary: "Trials and Tribble-ations," a Deep Space Nine episode in which the station's crew are sent back in time into the 1967 episode itself, composited into the original footage, walking the corridors of the Enterprise behind Kirk and Scotty and discovering that Arne Darvin, the Klingon spy, has come back to plant a bomb in a tribble. Worf, asked why the tribbles hate Klingons, answers that Klingons regard them as mortal enemies, and, asked why the 1967 Klingons look human, declines to discuss it. The episode is widely regarded as the franchise's finest hour of fan service and Gerrold appears in it as an extra.

Tribbles have been in every era since, in Enterprise, in the 2009 film series, in Discovery, where one lives on a captain's desk, and in the animated Lower Decks, and they have sold, as plush, more units than any other Star Trek creature; the toy's purr is a mechanism the original prop, a bag of fur with a motor, never had. They are the franchise's answer to the question of what an alien is for, when it is not for fighting: a joke about biology, sixty years old and still breeding.

The tribble is a story about an invasive species, and the fear it plays for laughs is one that space agencies take seriously. When the Apollo 11 crew returned in July 1969 they were sealed in a quarantine trailer for three weeks, and their samples handled in a Lunar Receiving Laboratory, on the chance that the Moon carried organisms that might do to Earth what rabbits did to Australia; the rules were dropped after Apollo 14 when the Moon proved sterile, but planetary protection remains a discipline, with sterilisation standards for probes sent to Mars and Europa and a whole architecture of containment designed for the samples the Perseverance rover has been caching since 2021. The life on Mars article covers what those samples might hold, and the Europa article covers the ocean that missions are forbidden to contaminate; a tribble in the grain is what the rules exist to prevent, in either direction.

Within the franchise, the tribble is the Klingons' inverse, a creature with no honour and no fight that beats them anyway, and the episode's use of it as a spy detector is the oldest trick in the lore: the animal that knows what the humans cannot see, as dogs are said to in every haunted-house and saucer-landing story. The Vulcans, whose Spock calculates the tribbles' growth to the individual, are the franchise's other biology lesson; the tribble is the one it delivers with a purr.