Of course, we have also caged and killed each other!
The question we take on is whether or not animals experience emotion, and therefore also emotional pain. If they do, that would require of all who believe themselves to be humane people of conscience to pay closer attention to how we treat them.
Where does emotion come from?
From a biological standpoint, emotion is our instinct’s alert system, letting our survival instinct know when we are in the presence of threat or not. The degree of a negative emotional reaction will be in proportion to the perceived threat to survival or security. (Positive emotions mean we feel safe.)
What will cause an animal (or human) to feel an increased level of threat?
The further they are from their natural habitat or surroundings, the more threatened they will feel.
The more unfamiliar creatures, sights or sounds they see, the more threatened they will feel.
The smaller the area to which they are confined, compared to what is normal for them, the more threatened they will feel.
Just as in the case of humans, the greater an animal’s confinement, the more extreme is their emotional distress. Confine a man to a country and he probably will not notice. Confine him to a city and you begin to cramp his style. Confine him to a prison cell and you can drive him out of his mind. Confine a lion to Botswana and he will not feel particularly put out. Confine him to a single game reserve and you may have incidents around the borders. Confine him to a zoo and he will experience prison.
The key difference between imprisoning a human and caging an animal lies in the third layer of the brain, the cerebral cortex, which is more well-developed in humans, giving us expanded mental capacities. While both humans and animals will experience the ongoing emotional pain of confinement, the human has the advantage of meaning and perspective. A human can rationalize his imprisonment in order to cope. He may not agree with the reason for his imprisonment, but at least he knows why he is there and that there is always the hope that in the future he might get out. This understanding can work to mitigate the symptoms of his confinement and make it somewhat more bearable. If animals lack reason, they do not have this advantage, which would cause their emotions and the associated suffering to be ongoing, if not heightened.
It is noteworthy that the Bible contains many commandments against causing pain to animals. It is told of the great Leonardo Da Vinci that he was walking in the market one day and saw a bird seller with several birds in a cage. He bought all of the birds and the cage, only to open the cage, free the birds, and break the cage in front of the bird seller. Da Vinci was passionate about the concept of flight. He studied birds and their wings and designed flying machines. It must have seemed clear to him that caging a bird, normally capable of flying free in three-dimensional space, was that much more cruel than caging a creature normally confined to our two-dimensional surface. And it was too much for him to bear.
The greater the degree of confinement the cage represents for an animal, the more forlorn or dejected it will be. This can usually quite easily be read in its body language.
The way we treat animals will always be one measure of our own degree of evolution — of our degree of civilization — as a species.