I found this study interesting. Definitely suggests that in some cases, maybe anti-zoo activists have a point about stress in captive animals — but in my opinion, that doesn’t take away from the importance of zoos for conservation efforts.
Frontiers | Stress in wildlife: comparison of the stress response among domestic, captive, and free-ranging animals
Frontiers | Stress in wildlife: comparison of the stress response among domestic, captive, and free-ranging animals