Suddenly, people walking along a main pathway at the Philadelphia Zoo jerked to a standstill.
Some gasped. "Tiger!"
Just overhead, 320 pounds of gold and black fur stalked into view. No cage bars. No thick glass wall. Just barely visible mesh netting.
The mesmerized visitors looked up.
He peered down. Those teeth! Somehow, when those big incisors are 14 feet above ... gulp.
The Philadelphia Zoo -- America's first zoo built 155 years ago -- this month opened Big Cat Crossing, a 330-foot addition to the zoo's innovative trail system.
The $2.3 million project gives the zoo's tigers, lions, pumas, jaguars, and snow leopards unprecedented access to more areas, more smells, and more views -- including gawking humans.
Philadelphia is the nation's first zoo to so fully embrace the concept, which is considered transformative for both animal and visitor.
The first trail -- 1,735 feet -- was a treetops runway for small primates. The next, at 200 feet, was big and sturdy enough, for on-the-move orangutans.
In 2015, hippos, giraffes, zebras, and the like will go where they have never gone before.
"It's a game-changer," said Terry Maple, an expert on zoo animal welfare and a former Zoo Atlanta president. "It's going to change the way we build zoos in the future."
Most zoos have gone from barred cages to naturalistic "enclosures" and now incorporate various enrichment equipment designed to keep the animals engaged and active.
But no matter how cleverly designed or enhanced, enclosed exhibits are still "pretty boring," said Jon Coe, who has worked extensively with the zoo. Founder of a Philadelphia design firm that specializes in zoos, Mr. Coe now is based in Australia.
"Zoo animals suffer all around the world from a lack of physical fitness and behavioral competence," Mr. Coe said.
The trail system is a way to give animals change, flexibility, and, above all, choice -- tough to achieve in captivity. It allows them to get exercise and see interesting new things.
With the new trail, the cats can travel out of the main exhibit area, over a major visitor path and down to the ground near the cafeteria entrance -- prepare for gridlock there, too -- and Bird Lake, where the initial cat travelers were "fascinated by the swan boats," said Andrew Baker, the zoo's chief operating officer.
Or, perhaps, the meaty humans paddling them.
(Nearby in their trail, the orangutans were not as pleased to see big cats in such close proximity.)
The zoo is incorporating the concept in other areas as well. Most mornings, pink flamingos cross a bridge to a new pond. In KidZooU, goats clamber up a stairway to an overlook.
Indeed, the zoo's new theme is "Zoo360" -- as you move around the zoo, the zoo moves around you.
The switcheroos have to be done with caution, however. For instance, big cats pass through bacteria from their raw-meat diet. Plant-eating primates aren't adapted to that. So the big cats might get the run of the orangutan trail in winter, when the warm-climate primates are inside, and the passage will get a cleansing come spring.
The whole notion stems from brainstorming by Mr. Coe and the zoo some years ago. By 2006, the zoo had begun animal rotations, moving, say, the panthers to the puma exhibit for a day.
Mr. Coe began to wonder, "Why can't we hook everything in the zoo to everything else and let the animals have the run of the place?"
Now, he describes the idea even more simplistically. At a zoo conference in New Zealand in March, Mr. Coe asked the group: "How many of you have a dog? How often do you take the dog for a walk?"
When it comes to zoos, "how often do these animals get to go for a good walk, get an outing, get some serious exercise, see something new?"
Mr. Maple is reminded of a zoo director in Jakarta, Indonesia, who used to drive up to the orangutan enclosure in a horse-drawn carriage. The orangs would pile into the back and the director would drive them around the zoo.
Simple: They liked it.
When Philly came up with its trails idea, "I said, 'Good Lord, this is exactly what we should be doing,' " Mr. Maple recalled.
The network of trails works for Philadelphia, in particular, because the zoo is small. With a railroad on one side and a major highway on the other, it has no place to expand but up.
And now, Mr. Maple said, "zoo architects will begin to use this idea, and we'll see all kinds of iterations."