My grandfather is not far off 100 yrs old. He spent his whole life living near Edinburgh Zoo.
As a child he remembers going into the big cat house, where there were rows and rows of cages on top of each other, with lions, panthers, tigers and all sorts of other big cats stuck next to each other with not enough space to turn around properly. He was disturbed by this as a child and never had pets.
Fortunately things have moved on a great deal since then and Edinburgh Zoo in particular has an extensive 'enrichment' programme for all the animals.
The Polar bear enclosure is, I think, the last to be substatially improved since the 80s.
We have learnt a lot more about animals, people didn't used to think they were capable of any thought processes, that they were in effect machines, with no feelings, for people to use how they liked.
Some people still hold that view.