Zoo safety design remains far from cut-and-dried

PHILADELPHIA — 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.

Polar bears, lions and tigers are another matter. They are larger, swifter, smarter and more agile, not to mention with 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 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.' "

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.

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, design may come down to what has worked at other zoos.

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 3 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.

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 yellowwood tree just inside the fence of the outdoor gorilla exhibit is trimmed regularly so the savvy apes can't use it as a ladder.

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 the 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. 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.

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 semiretired 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 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."

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."