It seems clear that husbandry practices are more successful for some species than for others. Zoos are very good at keeping some species alive and breeding them, while other species have never fared well in captivity for a variety of reasons. While many hard-to-keep species may be of little concern, others are endangered and could stand to benefit from ex situ management if the husbandry was successful; unfortunately, this is often not the case. Furthermore, establishing captive populations when the species is already in a precarious state is risky (case in point: Sumatran rhino). With that being said, it is also true that husbandry develops over time. It was once very hard for zoos to keep gorillas alive, let alone breed them, but eventually zoos got the hang of it and now the husbandry for gorillas is very well-developed. That expertise and husbandry knowledge required quite a bit of trial-and-error in order to succeed; had it not been done, however, gorillas would not be the mainstay of major zoos around the world like they are today. So my question is this: are there species for which we should accept that captive management is too problematic to pursue? Perhaps we only need more husbandry knowledge and expertise to make it possible to keep them; on the other hand, perhaps there is a challenge with keeping that species for which there is really no good solution. Obviously, this might vary in a case-by-case basis (and I would love to hear any number of those), but the root of what I'm getting at is whether developing good husbandry technique is simply not possible for some species.