It's not if you take a simple view of emotion. What's controversial is whether or not they feel complex emotions. Is that really depression or just sadness? Do adult elephants have periods of melancholy that are suddenly brought on by seeing elephant children playing in the watering hole which causes the adult elephants to remember their home town and childhoods and wish they never came to this distant country for better opportunities -- leading to regret, and then anxiety because they realized they did this all for naught are and feel trapped in their current situation? Who knows. I doubt it.