The orangutan banged windows with rocks presumably to get someone's attention for a reward.
Associated Press - A rock-loving orangutan named Rubih went ape on the observation windows of her St. Louis zoo enclosure, forcing nearly $200,000 in repairs and the temporary closure of the exhibit.

Zookeepers say the 12-year-old female orangutan repeatedly tapped and banged rocks against four 7-foot-tall windows over several months, causing considerable damage. The windows were replaced in mid-November and the exhibit is expected to open later this month, the St. Louis Post-Dispatch reports.
Susan Gallagher, a spokeswoman for the zoo, said replacing the triple layer of glass with 3-inch thick acrylic required re-engineering the frames that hold them, plus caulking requiring three weeks to cure.

Zoo officials say Rubih even dug up cement from the base of a tree for her destructive endeavors.
The zoo's ape care team taught the orangutan to bring them rocks in exchange for treats. But the ape started banging on the windows with rocks when zookeepers weren't around, presumably to get someone's attention for a reward.
Zookeepers now hope to train Rubih to drop rocks in a tube — regardless of whether staffers are around to reward her — and give her a treat if they later find rocks when they check the tube.

With insurance coverage, the zoo ended up paying $71,000 out of pocket for the new windows.