Nothing at all, although I will point out that the museum in Great Plains Zoo is very well done but the similar museum at Rolling Hills Wildlife Adventure is truly great.
This is what the zoo's website said about the tusks:
"Are the elephant tusks real?
No. The tusks in the elephant are cast replicas- real tusks that big would be too heavy for the mount to support. The tusks in the case ARE real- but if you look closely you can see that the replica set is much bigger than the real set. Between the time the collection was sold and moved from West Sioux Hardware and into storage- a set of tusks disappeared. There is a copy of the reimbursement check to the Delbridge family for a set of missing elephant tusks in the amount of $60,000."
