Jack Hanna

All this time I'd been assuming that a human had chewed the gum.  

However, Jack mentioned during his guest segment that the capybara, a large South American rodent, is the only animal other than man which chews gum and then discards it on window ledges.

Capybara sightings are rare in midtown Manhattan.  In all the years I've worked for the show, I can't recall seeing more than four or five of them.  But I have to allow for the possibility that we have the world's largest rodent to thank for CelebriGum.