This sign is posted in many places at the Denver Zoo.
Today, zoos save most of their space to house endangered animals. Since animals are no longer routinely captured in the wild, animals in accredited zoos are either zoo-born, rescued because of illness or injury or confiscated from poachers. To keep certain animals from being over-represented in the captive gene pool, DNA records are kept on all endangered animals and genetically appropriate partners are "loaned" to other zoos for breeding purposes. Our zoo used to have a sign, posted below the one above, which read, "Your zoo or mine?" but some parents complained so it was removed. In some cases, such as elephants, artificial insemination may be used rather than try to transport an elephant cross country.
This is just one bit of interesting information I picked up while volunteering as a docent at the Denver Zoo. Zoos have come a long way from the private menageries of animals " collected for display" in the middle 1900s. Today's zoos are very involved in conservation and protection of the animals. All training is done either for intellectual stimulation or to teach behaviors and trust that would allow keepers to take blood, check temperatures, treat hooves or do general health maintenance.
I will write tomorrow about one example of "Enrichment Training" to keep animals from getting bored in an environment where they don't spend time, energy and resources in foraging for food.