Over the years, Star Trek has been known for many things. It's revered as a science fiction fantasy show that highlights amazing technology, complex alien races and civilizations, and known for being a cornerstone for almost all space faring science fiction TV shows made ever since. At its core, when looking past all the sci-fi narratives, it is a show that focuses on allegorical storytelling to tackle very real and topical issues of the time it is made.
While this is a great guiding light, the show's creators have also never been afraid to inject humor into their franchise, taking a step back from themselves and laughing at the absurdity of it all. In the Original Series, potentially the pinnacle of this is the Tribbles, a cute yet surprisingly devastating race of procreation-obsessed furballs. They are also, hilariously, the sworn enemy of the mighty Klingon empire.
This unexpected paring of races was largely made for comedic reasons. The idea that some of the biggest and scariest aliens within the franchise were terrified of such adorable little creatures must have caused the writers a serious amount of laughs. What made it better was that, at the time the Tribbles were introduced, the Federation was still at war with the Klingons. The tough, scary alien warriors were made to be the main villains of The Original Series.
The little puffball aliens were introduced in the episode “The Trouble With Tribbles.” The episode's plot centers around the creatures being sold by space hustler Cyrano Jones (a fantastic name for a devious salesperson). He was selling the Tribbles as pets, making great companions due to their fairly sleepy nature, as well as being ridiculously cute. It’s suggested as well that the noises they make, an almost cooing purr, was also supposed to give off a calming aura, but whether this is true or just a sales pitch is not explicitly stated.
The episode features a lot of fantastically camp acting, and some comedy gold moments. But the biggest reveal is that while the Tribbles were docile towards everyone, they would grow hostile towards Klingons. This mutual effect would result in, for all intents and purposes, a growl off between the two races. Their hatred of each other at this time was fairly tame. The Tribbles hated the Klingons for some unknown reason, but the Klingons simply found them irritable. The Tribbles acted like a Klingon radar, and served a great purpose during this episode for speaking out Klingon spies that have infiltrated the ship. Their aggression could be used to find the hidden conspirator that was masquerading as a human (which was easier back then before their re-design). Ultimately, these cute creatures helped Kirk and his crew save the day. While neatly tidying everything up at the end of the episode, things started taking a turn for the worse off-screen.
These deeper, somewhat horrific consequences were explored many years later in the Deep Space 9 episode “Trials and Tribble-ations.” Starfleet officer Worf explains that the Tribbles almost wiped out his race after the encounter in "The Trouble With Tribbles," as they infested the Klingon empire. This might not seem like a bad thing at first, the cute little fluffs being a welcome addition to any planet. However, the Tribbles have one massive draw-back: they breed faster than rabbits.
Each Tribble is born pregnant, and their reproduction rate is astounding. It has been calculated that an average Tribble could produce 10 offspring every 12 hours, resulting in 1,771,561 Tribbles from just one creature in 36 hours. They caused havoc aboard the Enterprise, and as Worf explains, they continued to do the same on his home world afterwards. This created an ecological disaster that nearly wiped the Klingon empire off the map. They successfully managed to stop the creatures, killing them all, but only after significant damage had been done. It’s ironic that the biggest cause of destruction to the mighty Klingons during their war with the Federation was not the Federation itself, but these creatures.
As payback, the Klingons went on the warpath, and hundreds of them were sent on missions to kill every last Tribble they could find in the galaxy to ensure this never happened again. They went so far as to destroy the Tribble home world. The Klingons' fear of these creatures turned out to be justified. While they were mocked for their fear and genocide of these creatures, audiences know of the damage they can cause and the destruction that lies in their wake. While non-aggressive, they almost single-handedly wiped out not only the USS Enterprise in a matter of hours, but the entire Klingon home world. They may appear to be cute balls of happiness, but the Klingons and anyone who has encountered them previously know the consequence of finding a few aboard their ship or planet.
So why were the Klingons afraid of the Tribbles? Because they should be, like everyone else in the galaxy.