Bellwether and Doug used the flowers to concoct a serum to turn predator mammals aggressive, as the central part of a master-plan to take over Zootopia by turning the prey (the majority of the city's population) against the predators. There were multiple victims, including Mr. Manchas the black jaguar and Mr. Otterton the otter.
The night howlers are small-medium flowers that have six violet petals and several yellow pistils and stamens. Night howlers are grown from root bulbs, which can easily be mistaken for rotten onion bulbs.
Night howlers contain toxins that are capable of inflicting highly potent psychological effects on mammals. When consumed or injected, the flowers' toxin causes a chemical reaction in the brain that decreases activity in most parts except the cerebellum, which becomes hyperstimulated, causing the victim to lose higher reasoning functions and lash out at anything that moves. The effect of this is that essentially, any mammal reverts to an aggressive, wild, violent state that is described as "savage".
The night howlers can even affect animals that do not have a naturally violent or ferocious disposition. Bonnie and Stu Hopps mention that Bonnie's brother, Terry, once ate one whole and went berserk, attacking Bonnie and "biting the dickens out of her". This shows that night howlers have the ability to turn prey animals, such as rabbits, aggressive.
As natural flowers, night howlers can outright kill small animals such as insects, which makes them an effective pesticide.
Stu grows the flowers around his crops to scare off bugs but has his children and other fellow rabbits stay away from them to prevent exposure.
Bellwether orchestrated a plot to use night howler chemicals to turn predators in Zootopia feral, giving the illusion that they were biologically reverting back to their "primitive savage ways". One of her henchmen, a ram scientist named Doug, uses his chemist expertise to modify the flowers' mind-altering toxin into a concentrated pellet, which can be loaded into a dart gun for long-range injections. Doug uses his sniper aim to dart every predator he can, especially ones that could risk exposing the plot. Duke Weaselton was paid to steal bulb roots of the plant for Doug to create night howler darts.
After Bellwether was exposed and arrested, doctors created an antidote for the afflicted predator mammals, including Mr. Otterton, who was reunited with his wife.
Role in the film
The night howlers were mentioned early on as the last thing Mr. Otterton talked of before going savage. At the time, Judy and Nick assumed that the name referred to the wolves involved in the disappearances of the other animals around Zootopia. Judy also thwarted one of Duke Weaselton's thefts of night howler bulbs, although the owner of the store and Chief Bogo believed them to be onion bulbs at the time.
The flowers themselves first appeared at Bunnyburrow where Stu uses them to ward off insects away from his crops. When she hears about the severe psychotropic effect they have on mammals, Judy realizes that she made an error—"night howlers" referred to the plant, not the wolves—and rushes back to Zootopia.
After reconciling with Nick, the duo gets information from Duke, who was doing it for money. He states that he did it for Doug in an old subway car. Judy and Nick go to the subway car, which is filled with night howlers being used in experiments, and they watch Doug liquefying them into a serum. Doug then gets a phone call from his employer, who informs him of his next target, making Judy realize that Doug is responsible for making Mr. Otterton and Mr. Manchas go savage by shooting them with the night howler toxin.
At the natural history museum, Judy and Nick discover that Bellwether is the mastermind behind the night howler operation. After her henchrams knock them into a pit, Bellwether fires a dart at Nick, intending to make him go savage and kill Judy to conceal her role in the plot. The seemingly savage Nick appears to bite Judy, but it is all an act; the duo had replaced the night howler serum with blueberries from Judy's farm before Bellwether reclaimed the dart gun. Bellwether threatens to frame the two like she did with Mayor Lionheart, but Judy reveals that she has recorded the former's confession with the latter's carrot pen, just as Chief Bogo and his Zootopia Police Department forces arrive and arrest her.
At the end, an antidote for the night howlers was created, curing all afflicted predators, including Mr. Otterton.
- According to Judy, the night howlers are related to the crocus variety and are a class C botanical.
- The word Midnicampum is a portmanteau of "midnight" and Latin "campum", field, whereas holicithias of "howl" and Greek "Colchis", the origin of autumn crocus.
- Victims of the night howlers cannot spread the savage disease onto others. This was shown when a bite from a savaged rabbit did not affect Bonnie when she was younger, nor did Manchas become savage by an afflicted Mr. Otterton, despite sustaining severe cuts and slashes from the encounter.
- In one pre-production script of Zootopia, night howlers caused affected individuals to become feral, but that did not necessarily translate into mindless physical violence. In this draft, Nick did get afflicted by the night howler serum for real, but the established friendship between him and Judy led his instincts to see the rabbit as something to protect, thus preventing him from attacking her.
- When under the influence of night howlers, the eyes of afflicted individuals resemble those of their real-life counterparts. The most notable example of this is Mr. Manchas; in his natural state he has dull green eyes with round pupils, but after being darted with night howler serum, his eyes turn a much brighter green and gain slit pupils, much like a real-life feline. Nick's eyes do not change after getting hit by Bellwether, and this serves as a visual cue that he and Judy are only pretending.