They feel certain that this will make the animals more physically fit, mentally stimulated, and even happier.
And scientific research may even provide proof.
The goal is for animals to "have a much more varied experience in their lives and truly travel, truly explore, in a way that creates all sorts of behavioral opportunities," said Andrew Baker, the zoo's chief operating officer.
Officials hope to blur the distinctions between visitors and animal residents.
Other zoos and sanctuaries have tried smaller projects - notably, the National Zoo's Orangutan Transport System, dubbed the "O-line," that orangs swing along to travel to a "Think Tank" exhibit.
But none has been as ambitious as Philadelphia's. Officials here plan to have about 1.5 miles of trails with many species using them.
The final trail, planned for 2015, will be for larger animals such as hippos and rhinos. "You can imagine, every day at 1 o'clock, the gates go down like you're at a railroad crossing," Baker said, and visitors have to wait while the giraffes go to their afternoon watering hole.
At one point in their new 200-foot trail, the orangs can spy on people eating in a cafe, and vice versa. Another spot overlooks a lake. Eventually, their trail will link to gorillas and big cats.
In 2012, a monkey lookout will mirror a children's climbing structure. Baker said it would provide an "eye-to-eye, parallel play opportunity."
Other zoos are watching closely, said Steve Feldman, spokesman for the Association of Zoos and Aquariums.
Rotating animals among exhibits - not with one another, but in one another's spaces on a kind of time-share basis - is an emerging trend, and Philadelphia is "really taking it to the next level," he said.
The animals' behavior will convey clues about whether they're more content.
For proof, keepers are collecting fecal samples to be tested for cortisol, a steroid released in response to stress. High levels over time would be a bad sign.
Baker also hopes to initiate the wildlife version of psychological - or "cognitive bias" - testing among the animals that use the trail system.
"We always say that when we provide animals enrichment, it makes them happier. But we don't know," said Heidi Keen, a doctoral student at Washington State University whose recent work with grizzly bears intrigued zoo officials.
At its simplest, think of a dog. A bowl in one place always has food. A bowl in another is always empty. The dog learns this. But what happens when you put a bowl precisely in the middle?
A dog that approaches is deemed happy, even optimistic, while a dog that ignores the bowl is unhappy.
The zoo came up with the trail system idea after conversations with Jon Coe, who designed the Peco Primate Reserve when he was in Philadelphia with CLR Design.
Now living in Australia and founder of Jon Coe Design, he began to wonder, "Why can't we hook everything in the zoo up to everything else and basically let the animals have the run of the place?"
That thought led to a rotation system here in Big Cat Falls, where jaguars, say, remain indoors, and lions are let into their outdoor exhibit.
The lions stop, sniff, scratch up the grass, mark the new territory, and even just lie in the sun - all signs of healthy engagement.
"It created a lot of interest," Baker said. "They've expanded their area of mastery and awareness. They know what's on the other side of the tree. They know what's across the path."
What followed was last year's pilot trail for lemurs and small monkeys. In shifts, different species have the run of a treetop steel mesh tunnel that encircles the Impala Plaza, where tables and chairs allow visitors to linger around a fountain.
Not long ago, a small primate named Storm, a red-capped mangabey, sat in the tunnel watching children play. He'd call out, "aaah-oooh!" and the kids would answer, imitating him.
Coe, who collaborated on the trail designs, was delighted. "Can you imagine? Before he had this trail, he would sit in his enclosure and hope something interesting happened. Now he can go around and find interesting things."
Although the trails are narrow, officials do not consider them confining. Map the territory of an animal in the wild, they say, and you'd see that it traverses a series of trails to resources - food, water, its den - just as humans often drive the same route to work and the grocery store.
"These trails are much more than just a way to get from here to there," Coe said. "The walk itself is the pleasure."
So, much of the time, the animals may hang out in the trail watching the nearby action. Even lounge or nap.
"Giving animals choices is the next frontier," Coe said.
As for the visitors, the idea is that they'll never know for sure when, or where, an animal might show up.
Many animals have tiny ID chips implanted under their skin. With GPS capability added, people could some day track animal locations on their smartphones.
Coe said the Philadelphia Zoo, which is hemmed in by the Schuylkill Expressway and railroad tracks, is ideal for such a system. Unable to build out, it's building up.
With all the mixing, zookeepers will now become a little like air-traffic controllers, Baker said. And they'll have to make sure one species doesn't transmit a disease or pathogen to another.
As Tua and Batu got access to their trail for the first time Saturday, Chris Oberlin, a keeper, watched intently.
Orangutans are "insatiably curious," he said. But also cautious. He couldn't predict what would happen when the door opened.
Tua and Batu approached. Then backed away. They came closer. Batu touched the doorway and sniffed her hand.
More time passed. More hesitant approaches.
Finally, it was Batu, the active toddler, who started climbing and swinging through the tunnel.
She'd made her choice.
To view the new animal trails at the Philadelphia Zoo, go to www.philly.com/trails
Contact Sandy Bauers at 215-854-5147, firstname.lastname@example.org, or follow on Twitter @sbauers. Read her blog, "GreenSpace," at www.philly.com/greenspace