Escapes happen. There isn't any way around in, and any keeper who works at a collection of any size where nothing has ever escaped is probably not entirely informed. Or not considering certain escapes "escapes." Incidents like my near-miss with the cheetahs, or the lion escape up in Canada, are infrequent. Most escapes don't make the news, usually because a) the situation is quickly and easily corrected, b) the animal involved is not very big or scary, and c) we get really, really embarrassed when this stuff happens.
In the age of mass social media, when an escape does happen and word leaks outside the zoo, everyone has a field day. Think of Rusty, the red panda from the National Zoo who spent a day cheerfully exploring DC, or the Egyptian cobra from the Bronx Zoo who was given her own Twitter account to detail her fictitious escapades. Upon recapture, she was also given a new name - Mia...as in M.I.A.
So escapes happen. But how do they happen? That's the first thing I usually ask after an escape, mostly so I can verify to all interested parties (i.e., my bosses) that it wasn't my fault. As near as I can tell, there are three main methods of animal escape, and I've gotten to see them all. Illustrated with anecdotes!
1.) Poor Enclosure Design
When we put to two cobras together, no one thought anything of it. The male had lived in his enclosure peaceably for years, and it held him just fine. It held the female, on the other hand, for about two days. That's when we found her wandering around in a room completely on the other side of the building. You see, the cobra exhibit may have had a hole or two in it. The male was much larger than the female and couldn't fit out of them. She, on the other hand... whoops.
Exhibits, needless to say, shouldn't have holes. Climbing animals shouldn't be able to climb out. Burrowing animals shouldn't be able to dig out. Exhibits should be thoroughly checked before animals are introduced to them to make sure they are secure. Some zoos go to extremes - when the Woodland Park Zoo in Seattle prepared to welcome the gorillas to their new exhibit, it invited (human) rock climbers to try and climb out and find any potential escape routes.
It's also always a good idea to recheck an exhibit before introducing a new individual to an exhibit. That newcomer is likely to explore and test things that the current occupants won't... like our cobra did.
2.) Structural Damage
The big cat exhibits at my zoo aren't much to look at... they're old. They were built with one thing in mind - security. If one of the giant trees that surround them fall and land directly on top of them, they will still stand. That doesn't mean that after every storm the first I do isn't to race around the zoo checking fences and roofs and viewing windows for damage.
Sometimes, damage inflicted upon exhibits is human-induced. At one zoo where I worked, I spent an hour carefully coaxing back in an escaped marmoset. After securing it, I set to work trying to find out how it had escaped. It took all of three seconds. Right along the bottom of the mesh at the front of the exhibit was a gash running the entire length of the exhibit, the mesh flapping ideally in the breeze. Stuck to it, at various parts, was freshly cut grass.
Our director was an old school zoo man, especially on the horticultural front. No tangled growth or jungle scenes, he liked his flowers and trees planted in neat rows and he liked his lawns mowed nice and short. Unfortunately, our new (and soon to be ex) groundskeeper got a little too close while trimming around the exhibits with a weed-whacker.
3.) The "Whoops" Factor
What every keeper dreads most is the moment when an animal gets out for the simple reason that you were an idiot and let it out through the front door. That's what happened with the cheetahs I described yesterday. Sometimes you forget to close the door. Sometimes the animal is lying in wait for you and makes a dash for the door as soon as you open it. There are some exhibits which I have lived in fear of because the animals inside were dashers... especially the very small, very fast ones.
Sometimes, even the not so fast ones can throw you for a loop. At the same zoo as the marmoset incident, I watched an increasingly flustered keeper chase a binturong in circles around (the outside of) her exhibit. Binturongs don't go especially fast, but this one was doing just enough of a trot to keep out of our keeper's clutches (not sure what she was going to do if she caught it - they're generally pretty good-natured animals, but man, do they have some teeth on them still). Eventually, a few other staff arrived and we were able to herd the little beasty back in. It turns out that the keeper had opened the door, turned around for a second to set a water bucket down, and then looked back just in time to see a shaggy, black prehensile tail slithering around the corner.
To guard against this, many exhibits have what are called keeper areas, or shifts (I've heard others call them "air locks"), which act as a series of double-doors between the animal's exhibit and the outside. The theory is that if the animal makes it through the first door, it'll still be confined in a small area from which it can easily be pushed back into the enclosure. Note: this only works if you actually remember to close the first door before opening the second one.
There are other ways an escape can happen - an extreme amount of motivation giving an animal the ability to push through a normally insurmountable barrier, heavy rainfall allowing animals to swim across filled-in moats, I've even come across one incident of (I suspect) a keeper deliberately letting an animal escape (presumably so she could recapture it and be a hero). The point is, at some point, an animal will escape.
The question is, what are you going to do about it?