Well, generally I don't agree with keeping animals in the circus.
But I guess in some rare cases, provided that the animals are really well treated (which excludes the bigger species, since they can't possibly not be stressed by the small amount of space circus offers), and provided that for whatever reason they can't live in the wild, (their environment is disappearing or they're hurt and would be rejected in a group), I guess it would be acceptable to keep them in a circus.
It would probably work best for the species that more readily socialize with humans: dogs, some monkeys. Again, this would only be acceptable if animal welfare was always in the first place, and if for examples the animals could refuse performing on a given day.
In those circumstances, a circus could provide a predator- free environment for them, which would be beneficial for the animals (but again, I am very skeptical this is ever the case).
