Honi and Kira spared no time exploring their new gorilla highway through the trees above the Philadelphia Zoo. Motuba, the zoo's male silverback, was the shy one: He climbed the stairs and then retreated to a more familiar field in the gorilla enclosure.
The overhead path that opened Wednesday is the fourth in a series of trails that have been lauded as not only innovative but transformative for zoos, taking the experience - for the animals that live there as much as the Homo sapiens who visit - into new realms.
The tigers, for whatever reason, have become entranced by golf carts. In the mesh-tunnel system that lets them wander far beyond the normal barriers, they can prowl above a main visitor pathway and check out the goings-on below. Infant strollers, for example. (Like the golf carts, things with wheels.)
It's been a long time since zoo animals simply sat in a tiled "cage," looking bored. Now, they spend their days in "enclosures" designed to replicate as closely as possible conditions in the wild.
With the zoo's new trail system, the big cats and many of the primates can venture far afield, traveling more freely than had been thought possible in a zoo setting, experiencing new sounds, smells, and sights. More important, officials say, is the fact that the trails give the animals far more opportunity for choice in their daily lives.
"What you're seeing in Philadelphia is an evolution of the modern zoological exhibit," said Rob Vernon, a spokesman for the Association of Zoos and Aquariums.
The $2.5 million Gorilla Treeway is a mesh-enclosed path 12 feet above a visitor path. It runs 300 feet from the Peco Primate Reserve, looping around some trees and connecting to the Great Ape Trail, which opened to the zoos' inquisitive orangutans and gibbons in 2012. At some points, the ground portion of the big cats' trail, which opened last year and now is being extended 170 feet, will be directly underneath as well. Also close by is the small primates' Treetop Trail, which opened in 2010.
This is the year where everything starts to get connected, and people can see these animals traveling long distances, said Andy Baker, the zoo's chief operating officer. Different animals may use different trails on different days. The idea is that it offers exploration opportunities not only for them, but also for the visitors, who will never know for sure just where a big-toothed carnivore or massive, hairy primate may show up.
Other zoos are watching, Vernon said. Following Philadelphia's example, the Jacksonville, Fla., zoo has built a trail for its big cats. Indianapolis constructed a habitat of connected towers for its orangutans. Mexico's Zoo Guadalajara recently opened 2,000 feet of Senderos Aereos, and Taronga Zoo in Sydney, Australia, is developing an overhead tiger crossing.
The concept had its beginnings in the design of Philadelphia's Big Cat Falls, which incorporated the ability to switch out the exhibits for the animals. Imagine the new smells, officials thought, if the cougars were let into the tiger area. (Once the tigers had left.)
Then Jon Coe, who founded a Philadelphia zoo design firm but worked on the current trail designs from his new base in Australia, got a bright idea. Why stop with rotating exhibits? Why not just connect everything in the zoo to everything else and let the animals have the run of the place?
"Providing environmental enrichment for zoo animals is an ever-expanding game," Coe said.
The zoo calls the concept Zoo360 - as visitors move around the zoo, the zoo moves around them.
The trail systems give the animals opportunity for mental engagement and physical exercise in settings that all too often result in obesity and boredom.
"I have seen zoo-house crocodiles with pot bellies, something we never see in the wild," said British animal behavior and animal welfare expert Robert John Young, who recently wrote about Philadelphia's concept in Scientific American, calling it the "Next Generation Zoo." With trails like this, he said, the map of a zoo would start to look like the London Underground.
For the Philadelphia Zoo, overhead trails also solve a logistical problem. Hemmed in by a highway, a river and a railroad, there was no place to go but up.
The trails also replicate more closely how animals use space in the wild, in which they generally follow paths between "nodes" where they sleep or eat.
The big cats seem to be "more responsive, literally more engaged," said Kay Buffamonte, their lead keeper.
Now, when the big cats are at the top of the mesh trail, looking out at the humans, they are clearly alert but calm, said Buffamonte. Ears are forward; nothing resembles the distress position - crouched, ears back.
"They're people-watching," Buffamonte said.
The trails also offer what Baker, the zoo executive, calls "social dispersion," allowing the animals time away from their enclosure-mates.
He said zoo officials are hiring a postdoctoral student to produce data in hope of confirming what they think they are seeing.
Are the animals really more active? Do they show more diversity of behaviors? How far are they ranging in a day? In short, do the trails change where the animals spend their time and what they spend it doing - sleeping, eating, watching baby strollers?
In essence, Baker said, do they value the trails? "We're creating a new experience for them and then, as best we can, asking them, what do they think?"
Inquirer staff photographer Ed Hille contributed to this article.