In all the chatter about whether zoos "have a place" and whether zoos "really" do conservation, little is said about the traditional role zoos have had in assisting field biologists in developing the techniques they need to study elusive animals in the wild. A current example: Using DNA to investigate Giant panda diet – Botanics Stories What other similar projects are you aware of?
Research looking at stress hormones in lemurs springs to my mind, which first tested the applicability of the method on captive lemurs in Apenheul, Netherlands, before going to Madagascar. And there will be many more, even when you exclude all the research done in zoos, that is very difficult to replicate in the wild.
Soil (special microbes) can make people and other animals happy (not depressed), research found. Difference in zoo animals kept mainly on concrete (indoor) encosires vs. those kept on soil? Soil Microbes And Human Health ? Learn About The Natural Antidepressant In Soil
Cincinnati Zoo's work on embryo transfers in ocelots which translated into several births in Brazil http://cincinnatizoo.org/wp-content/uploads/2011/03/OneGiantLeap1.pdf