One problem with trying to reintroduce wild species is making the animal comfortable with his old habitat. These New World monkeys had to learn to balance in branches of trees and walk along swinging vines, before one could even think about reintroduction. That was one of the ideas behind letting them run free in the bird aviary. They now have Golden Lion Tamarins living unfettered at the zoo in preparation for reintroduction. They are even more endangered.
