Mourners left teddy bears and other items outside the zoo and responded to a condolence message on the zoo's Facebook page.
Nearly 1,000 people commented on the post, some passionately condemning the boy's mother and others urging compassion and understanding. More than a few parents acknowledged lifting their children onto the rail, too.
Baker said zoo officials "discourage" parents from setting their children on the wide, wooden railing, which slopes toward the viewing platform at a 45-degree angle so a child placed there would be more likely to fall backward into a parent's arms than forward into the exhibit.
Nate Legler takes his 4-year-old daughter to the Pittsburgh zoo two or three times a month and said it's safe and there's no way for a small child to climb over the wooden railing without assistance. He said he has lifted his daughter to give her a better view but never close enough to the railing to be dangerous.
"You can take your child to the Grand Canyon and hold them up to let them look down but not feel unsafe like you're going to drop them into the canyon," he said.
The director of animal care at the Columbus Zoo and Aquarium in neighboring Ohio, Doug Warmolts, said exhibits are designed with visitors' expectations in mind because they all want to have unobstructed views and "the up-close experience" but the first priority is always safety.
He said accidents like the boy's death in Pittsburgh are "one of those things that keep you up at night."
"I'm sure it's going to send ripples through our industry and everybody's going to double-check their measures and how to respond to things like that," he said.
Associated Press writers Michael Rubinkam in Allentown and Kevin Begos in Pittsburgh contributed to this report.