This post is mostly for Cassandra's enjoyment, because
she'll like the article, but I'll take a moment to answer the question the authors ask in the tagline:
"We now have solid evidence that elephants are some of the most intelligent, social and empathic animals around—so how can we justify keeping them in captivity?"
Well, we can justify it precisely because of their limited access to reason. In the
Politics, Aristotle suggests that some men are slaves by nature. Specifically, those who "are as different [from other men] as the soul from the body or man from beast—and they are in this state if their work is the use of the body, and if this is the best that can come from them—are slaves by nature. For them it is better to be ruled in accordance with this sort of rule, if such is the case for the other things mentioned."
What he means by "for them it is better" is that the slaves themselves will enjoy better results if their affairs are managed for them, i.e., if they are not left to their own devices. This should be an improvement that they themselves could recognize, rather than one that comes from outside of them (i.e., not "I think you would be better off if you lived as I want," but rather, "I realize that, though I'd prefer to do heroin every day, and would choose it if I were left free, it really would be better if I weren't free to make that choice").
Because they have enough reason to see the good, but not to choose it, there is a kind of objective justice to organizing their lives for them. This is true even if they don't
choose this state, because it's the ability to choose to do what they can see would be better that is at work. Thus, if a judge should involuntarily commit an addict, the addict may be angry about it, and certainly wouldn't have chosen commitment for himself. But he should be able to see the justice of it, to recognize that in an objective way he will be better off for it.
So it is possible to justify the captivity of elephants in the same way. Note, though, that the force of Aristotle's assertion that there is a kind of just and natural slavery is to bracket it as the only acceptable kind. It turns out to be a harsh criticism of every kind of actual slavery being practiced in his own day.
We might apply a similar critique to our favorite zoo.