The monkeys have access to the islands all year round and 24hr a day (except possibly when the moats are icy). I guess they just haven't got to these leaves yet!
I think you'll find its because they don't go outside that much and there's a lot of foliage on the islands nowadays- so they don't make inroads on it.
The islands are hot-wired around the outer perimeter of the canal, and there some hot-wires between the islands too, although they may not be large enough to cope with a male mandrill.
