I agree it definitely doesn't look original, but it's been like that for a very long time. I'm guessing it was done to allow access to the old polar bear dens, possibly during the time the zoo had a large group of bears in the right hand pit who didn't all get along.
I know it's not recent but this is the first time I've remembered to take a photo! I really can't remember when it first appeared, but you could be right about the multiple bear theory.
I can't remember exactly how long it's been like that either; it must have been done before the Tectons were all listed of course (I wish they had knocked it all down or linked all three pits properly while they still had the chance). At least they have a way of linking those two enclosures via the dens in future if they want to.
This was the door to the indoor area for the original Tiger den as the tigers were in here first with polar bears in the middle den and lions in the end den hope this helps
