Zoos try to refine ways they keep animals in

To be sure, nobody worries overmuch about Galapagos tortoises.

If one should decide to break for points beyond its enclosure, zoo keepers would have ample time to thwart the mission.

Gorillas, orangutans, polar bears, lions and tigers are another matter. They are larger, swifter, smarter and more agile, not to mention their sharp teeth.

So from the time of the Romans, who kept animals in pits, people have taken special care to ensure that when they put a large and potentially dangerous creature in an enclosure, it will stay there.

But as San Franciscans found out two weeks ago, the science of zoo design is complicated.

"How high can a tiger jump? That's hard to answer," says Andy Baker, vice president for animal programs at the Philadelphia Zoo, which opened its big-cat exhibit in 2006. "You can't really say to a tiger, 'Jump as high as you can, so we can measure it.' "

(Not that some haven't tried: A German zoo pioneer is said to have hung pigeons from various heights in the late 1800s to see whether a tiger could reach them.)

So, finding the best way to keep animals captive often comes down to experience. "Most of it is what we've learned over many, many years," Baker says.

Centuries, really.

At one point, massive bars were the norm. Naturalistic exhibits that offer visitors an "immersion" experience now dominate American zoos, and new materials - from electrified wires to sturdy stainless steel mesh - have come into wide use over the last few decades.

Visitors to a viewing area at the Philadelphia Zoo can come within an inch-and-a-half of lions and tigers weighing hundreds of pounds, thanks to a "sandwich" of three layers of glass. If one breaks, the others might not.

These materials are highly engineered and extensively tested in laboratories. But uniform building standards do not exist, and in the absence of real-world data such as San Francisco's - luckily, Siberian tigers rarely cross dry moats, get over walls, and kill people - design may come down to what has worked at other zoos. Architects plan for what seems like the worst-case scenario.

Leopards, pumas and jaguars are champion climbers. So their Philadelphia enclosure has a roof made of the same woven stainless steel mesh as the sides.

Lions and tigers don't climb. So they have 16-foot mesh walls with an overhang slanting three feet inward, in addition to the viewing area glass. The mesh is sized so an adult tiger can't stick a paw through, and its effectiveness comes from both the tensile strength of the steel and the flexibility of the netting, which is intended to absorb the shock of a charging animal.

This, like the glass, provides transparency that gives visitors a feeling of closeness with the animals - in other words, the Wow! factor in the design challenge.

"When you're looking at a tiger and it's five inches from your face, that's a completely different experience from seeing an animal on a rock 40 feet away," explains Marc L'Italien, the architect of Philadelphia's big-cat exhibit.

Lemurs don't swim. So the small primates' outdoor area is surrounded by a 12-foot-wide water-filled moat.

Keepers must track changing conditions. If ponds freeze solid, polar bears are kept inside, because they might be able to stand on the ice and reach the top of a nearby wall. The yellowood tree just inside the fence of the outdoor gorilla exhibit is trimmed regularly so the savvy apes can't use it as a ladder.

Alas, there are no guarantees when it comes to orangutans, notorious for scaling walls and undoing bolts - now routinely tack-welded.

The Philadelphia Zoo's most dramatic escape in recent years came when an orangutan named Mango got onto the primate exhibit's roof not long after it opened in 1999.

Using the pressure of his fingertips, Mango had worked his way up a vertical ledge, which now has been beveled. A keeper saw it all, which led both to Mango's immediate capture (with a tranquilizer dart) and an explanation of how he got out, which no one would have believed otherwise.

Investigators in San Francisco have yet to figure out how and why a 350-pound tiger named Tatiana made it across a dry moat and over a wall before attacking three men, killing one of them. Police shot and killed the tiger.

The Association of Zoos and Aquariums said that Christmas Day incident marked the first time an escaped animal killed a visitor at any of its accredited 216 zoos and aquariums.

But there have been close calls. Among them:

Three years ago in Dallas, a gorilla attacked a toddler, his mother and another visitor before being killed. He apparently vaulted over a 14-foot fence.

In 2003, a Boston gorilla evidently scaled an electrified wall and crossed a 12-foot moat, then attacked a 2-year-old girl before he was tranquilized.

In 1982, a Siberian tiger at a California safari park, now closed, escaped from its cage, grabbed a toddler by the head, and dragged him 50 feet into bushes. An employee saved the boy by spraying the tiger with a fire extinguisher.

After last month's killing in San Francisco, zoo officials across the country checked their exhibits with tape measures to confirm that the enclosures were as secure as everyone thought.

"It's not like there are any single experts you can go to," says L'Italien, the San Francisco-based architect.

Plus, he says, "there's no real policing agency in the zoo world, like there is in architecture, where we follow code books for structural design."

The U.S. Department of Agriculture sets minimum square footage for animal holding areas, he says, but nothing else.

Guidelines have emerged over the years, however.

Species-specific committees within the Association of Zoos and Aquariums - the accrediting organization for North American zoos - have developed dozens of documents on animal care, including how to best keep them in.

"They represent the best available knowledge and experience for that animal," says Jon Coe, who designed the Philadelphia Zoo's nine-year-old primate exhibit. He semi-retired and moved to Australia four years ago, but still works worldwide and consults for the Philadelphia-based zoo architecture firm CLR Design.

When Coe is working on a design, he talks to zoo officials about their institutional policies: Is the minimum sufficient? Do they want to be conservative and add a 20 percent margin?

Where standards don't exist, he says he looks for a "logical precedent." For a railing, say, Coe might adopt the local specifications for a railing on a convention center balcony.

The team of designer and zoo also considers the specific animal.

"Is this bear an old couch potato or an Olympic-class bear?" Coe says. "Obviously, the bottom line is you don't ever want it to get out. But at some point, you can design an exhibit to such an extreme that it's ugly, impractical and expensive."

All this assumes the animals want to get out. But most were raised in zoos, not captured from the wild, and are acclimated, says Baker, the local zoo executive. "They're home."

(It also assumes that people do not want to get in. This, too, has proven false on a number of occasions, "usually with very poor outcomes," Baker notes.)

Reflecting on recent escapes from exhibits that had not been breached for decades, Coe wonders if better zoos, with improved nutrition and "behavior enrichment" programs, aren't creating stronger, fitter animals.

He is quick to note that's good thing. But there's a caveat: "It could also result that barrier heights that were sufficient in the past are no longer adequate."

Video, slide shows and news updates on big cats at the Philadelphia and San Francisco Zoos: http://go.philly.com/scienceEndText