Humans’ gruesome tradition of imprisoning nonhumans in cages for display, goes 5,500 years, with kings across the world demonstrating their power and wealth.

Despite the modern justification of a concern for animals, the purpose of zoos was never changed. Zoos are still collections of "interesting items", demonstrating humans’ power and domination. Animals in zoos are treated like a stamp collection. The more species the better, especially if they are large animals from foreign places that the public would be willing to pay money to watch. The "specimens" are arranged in cages to make it easy to observe them at close range, at all times, despite how extremely stressful it is for the animals.

Justification

Zoos talk a lot about their essential scientific research, their total commitment to wildlife conservation, and their vital role as educators. Meanwhile, people do what they have always done - they go to the zoo to be entertained.
The expectation of the visitors is that the animal would please them. Humans demand to be taken notice of and they are insulted to find that usually the animals ignore them. They expect the animals to entertain each and every one of them. The mentality is of: "come here and say hello! Do something cute". The ugliest examples of humans demand for attention include teasing, banging on cages and throwing things at them.
Zoos foster the assumption that humans are the center of the universe.

Life in Confinement

Animals in zoos are deprived of their normal and natural behavior.
Tigers can’t run, birds can’t soar the sky, monkeys can’t swing through the trees, and elephants can’t roam over large distances.
Animals which would naturally roam for tens of miles a day, tread the same few paces in a small cage.

Zoos enclosures deprive the "prisoners" of their most basic behaviors including exercise, social interaction and bathing.
Animals that naturally live in large herds or family groups are often kept alone, or at most, in pairs. Foraging and mating behaviors are virtually eliminated by regulated feeding and breeding regimens. The animals are closely confined so they lack privacy. Solitary and shy animals live in cages with viewing from all sides.

Zoochosis

Complex behaviors and deep instincts that have evolved over thousands of years can't find any outlet. The result is boredom and stress. The animals show signs of mental disturbance through abnormal behaviors. Sometimes they become apathetic and just sit in one place, but the most common abnormal behavior is repeated movement: rocking from side to side, pacing up and down or round and round, waving or circling the head, over and over again.
Such obsessive and repetitive behaviors, including self-mutilation, are very common among zoo animals and are a result of no mental stimulation or physical exercise and a chronic frustration and boredom. This stereotypic, self-destructive behavior is called Zoochosis.

Lunatic Asylum

This is what boredom, frustration, habitat deprivation, stress and loneliness cause:
Great apes and elephants rock, sway or shift repeatedly from side to side.
Giraffes are licking the walls and chewing the bars of their pens.
Jaguars chew the end of their tail, which becomes completely bald.
Giraffes, llamas and some monkey species, are twisting and rolling their neck unnaturally, often flicking the head around or bending the neck back.
Big cats pace the same path again and again.
Primates often over groom themselves or each other and this can lead to self-mutilation. Disturbed maternal behavior may also involve over grooming but also the rejection or killing of young.
Reptiles climb or scratch their glass tanks because they don't understand why they can't get out.
Other reptiles may become completely sedentary, spending all their time behind a rock.
Bears in zoos tend to bite, rub the mouth along the bars of the cage and even suck it.
This repetitive behavior can result in damage to the teeth and mouth, particularly if the bars are rusty.
Gorillas smear faeces on walls and eat it.
Gorillas and chimpanzees vomit and then eat their vomit.

Animals in zoos tend to over groom themselves, grooming to an excessive extent, pulling out hair or feathers, often leaving bald patches, irritated and torn skin.
Self-inflicted physical harm, such as biting or chewing tail or leg, or hitting a head against a wall are all, also very common. Zoos are lunatic asylums.

"Surplus" Animals

Zoo babies are great crowd-pleasers, but what happens when the babies grow up? Zoos often sell or murder animals that no longer attract visitors.
Some are murdered for their meat and skin. Other "surplus" animals may be sold to smaller, poorer zoos or to laboratories to be experimented on.

Deer, tigers, lions and other animals that breed often, are sometimes sold to "game” farms where humans pay for the "privilege" of "hunting" them.

Zoo supporters claim that zoos offer people the opportunity to see something that they would never see otherwise. The zoo people are right. They give humans the opportunity to see what imprisonment does to animals. In zoos humans can see primates hide their faces, birds who don’t fly, mothers eat their young, elephants rock and swing for hours, bears and tigers pace all day long, monkeys eat faeces and gorillas eat their own vomit.
But still humans fail to see the boredom, the frustration, the madness and the suffering of the caged animals.

Some humans believe that caging animals in a zoo, is somehow for their own good.
They argue that zoos protect the animals from harms.
They see themselves as animal lovers and the zoo as a place that enables people to get to know and love animals. But the confinement in the zoo harms animals more than anything they might face during their lives. Zoos cannot protect animals. In fact animals need protection from zoos.

Zoos also claim to be educational. But what do they teach us?
Animals, which have become crazy and show unnatural stereotypic behavior are only 'educational' in showing how humans can drive animals mad by keeping them imprisoned.
The confinement educates people for relationships based on domination and control.
It teaches hierarchy and speciesism. It teaches how to objectify sentient beings.
Not that humans need these lessons - humans are… natural born exploiters.