Zoos are a constantly evolving workplace. Over the past 50 years, exhibits have gotten increasingly naturalistic, diets for certain species have become more standardized, and captive breeding programs have turned into nationwide campaigns. Yet if one thing’s remained constant, it’s the fact that keeping the animals in our zoos both happy and healthy requires a great deal of time, coordination, expense, and old-fashioned willpower. It’s not an easy job, but most zookeepers say they wouldn’t trade it for the world.

1. PANDAS ARE VERY, VERY EXPENSIVE.

Giant pandas are one of the biggest draws for zoos that manage to snag a pair. But the big mammals also come with an extremely high price tag. Famously finicky, they dine almost exclusively on bamboo. Since these plants don’t offer much in the way of nutritional value, pandas need to consume about 26 to 84 pounds of them every day. Maintaining a fresh supply is a costly endeavor, especially for zoos located in cooler areas where bamboo doesn’t grow as well. The Toronto Zoo, for example, spends $500,000 CDN per year (about $370,000 US) flying in bamboo from a Memphis-based supplier.

Food-related expenses are just the tip of the iceberg: China’s government effectively maintains a global panda monopoly. To put one of these rare, in-demand critters on display, a foreign zoo must lease it from the Chinese for a full decade. During this period, an annual payment has to be made—and the going rate is sky-high. For example, the Edinburgh Zoo is currently paying £600,000 (about $740,000) per year for its resident pair. Across the pond, the Smithsonian’s National Zoo in Washington, D.C. shells out $550,000 annually in order to keep two adult pandas. By the way, if one of those bamboo-eaters should die because of some human error, China will administer a roughly $400,000 fine.

2. KEEPERS WARN EACH OTHER ABOUT GUESTS WHO DON’T FOLLOW THE RULES.

Using clearly marked signs, zoos warn their guests not to do certain things that might harm the animals. Unfortunately, some people ignore these notices. Glass-tapping is a particularly common offense. While it might not seem like a big deal to human patrons, this can really stress out captive creatures. “Imagine if somebody’s knocking on your living room window all the time,” Bruce Beehler of the Milwaukee County Zoo says. “I think you would be annoyed.” He adds that tossing coins—or, indeed, anything else—into an animal’s enclosure is another big no-no. Not only can these bits of currency get swallowed, they’re also liable to contaminate an animal’s water supply.

When mental_floss interviewed Bob, Terry, and Nancy*—three keepers who work at a zoo in the southern U.S.—and asked them to name their biggest job-related pet peeve, all three cited rule-breaking visitors. “Read signs and listen to keepers,” Bob implores. “If I ask you not to tap the glass, don’t tell me it’s just for fun and you can tap the glass all you like. If a keeper asks you not to stand your child on the railing of an animal’s enclosure, do not put them down and then wait ‘till we walk away. When we see anyone doing something that endangers our animals, we do follow you.”

Security guards are on hand to remove those who ignore repeat warnings. Additionally, zoo staffers will often use their radios to tip each other off about problematic visitors. “Depending on where they are, we might alert the next area down the line,” Nancy explains. “We’ll say ‘Hey, I saw these people disturbing the animals in this area and they’re heading towards your area. Keep your eyes open.’ Each area will then make the call about how serious the situation is and whether they should call security.”

Nancy also told us that she’s personally had to discourage patrons from, among other things, throwing food at gorillas and dropping various objects (money, juice boxes, etc.) into the alligator pool. It should go without saying, but the posted rules are there for a reason. Respect the animals’ homes and you’ll have a more enjoyable visit.

3. LOTS OF ZOO ANIMALS AREN’T ON PUBLIC DISPLAY.

Purchase a standard zoo ticket and you’ll get to see most of the critters in their collection. But you can bet that at least a handful of specimens will be kept from view, stowed away in backroom terrariums or birdcages. “Animals live behind the scenes for a number of reasons,” Terry says. Some of these so-called “off-exhibit” creatures are used for educational purposes, including occasional public shows and private birthday parties. By utilizing animals that most visitors never see, staffers can put together a live creature presentation without emptying any displays in the process.

Nancy adds that the newborn offspring of breeding animals are also sometimes withheld from the public. “If your zoo is breeding a given species,” she says, “then it’s likely that the species is already well-represented in your displays. So you wouldn’t need to put all of the babies in the public viewing areas. Visitors might like to see one or two burrowing frogs, but there’d be no point in having an entire wall full of them.” A good percentage of these unseen infants will probably end up getting shipped off to other zoos.

For the record, certain departments hide their critters more frequently than others do. “Reptile, aquarium, and maybe bird areas are most likely to have larger numbers of animals behind the scenes,” Terry says. “It’s easier to house and hold many small animals than large ones … not a lot of places [have] off-exhibit elephants!”

4. TRANSFERRING ANIMALS BETWEEN ZOOS INVOLVES A LOT OF PAPERWORK.

Bob says that when an animal goes from one zoo to another, a “ton of paperwork” usually travels with it. These documents are loaded with need-to-know details about the critter’s health issues, behavioral tendencies, and the amount of training it’s received.

Unhelpfully, new beasts that aren’t acquired from other zoos seldom come with comprehensive paperwork. “Sometimes their history is a mystery,” Bob admits. “Many zoos will get animals through confiscation from Fish and Wildlife services. I’ve even met a South American tamandua [a genus of anteater] who was found walking the streets of Houston!” Over the years, Bob’s also worked with a cougar that had previously been a school mascot, as well as two bobcats believed to have been escaped pets.

In any event, zoos subject all new acquisitions to a mandatory quarantine period. Usually, this lasts anywhere from 30 to 60 days and may take place in an isolated enclosure or at the zoo hospital. “This is to make sure they bring no ailments or parasites to the general zoo population,” Bob says. “If they do show signs it is treated. Once that passes, then the animal is taken to its appropriate new home within the zoo.”

5. FEEDING THE ANIMALS ISN’T EASY (OR CHEAP).

Zoos have high standards when it comes to the quality of their residents’ food. “We’re probably pickier than some restaurants. We have to be very careful because we’re dealing with endangered animals and animals we want to reproduce and live long lives,” Kerri Slifka, the Dallas Zoo’s curator of nutrition, told the Dallas Morning News last year. Nowadays, a growing number of zoos are hiring full-time animal nutritionists to make sure that their critters receive the healthiest possible diets.

Furthermore, in recent decades there’s been a big push to standardize the meal plans for certain species. (For example, the Association of Zoos and Aquariums advises member zoos to feed orangutans a balanced diet consisting of 86 percent produce and 14 percent “nutritionally complete primate biscuits.”) The standardization trend can be traced back to the rise of nationwide breeding programs in the latter half of the 20th century. Under these initiatives, specimens were transferred between different zoos with increasing regularity. As zoological nutritionist Barbara Toddes told the Smithsonian, “Animals need consistency in their diet when they move from place to place. It’s much better for them stress-wise and nutritionally.”

Big appetites are another complicating factor. Consider elephants, which devour 200 to 600 pounds of food every day when fully grown. The cost of feeding a single adult is usually around $15,000 per year. And some animals require specialized diets. In her interview with the Dallas Morning News, Slifka mentioned four Marabou stork chicks that had recently been hatched. In the wild, newborns of this species mostly subsist on the corpses of small animals. To supply its little birds with intact dead prey, the Dallas Zoo paid a pretty penny: By the time the young storks were 110 days old, their food-related expenses had totaled a whopping $10,000.

6. TO PREVENT THEIR CRITTERS FROM GETTING BORED, KEEPERS OFFER WHAT’S KNOWN AS “ENRICHMENT.”

Adequate food and space will keep captive animals alive, but stimulation—both the physical and psychological sort—is what helps them to thrive. “Enrichment” is a process whereby zookeepers prompt their critters into exercising their minds or displaying certain behaviors they’d normally exhibit in the wild. A quick scenery change can make for a good start. At zoos, caretakers occasionally add or remove certain things from their animals’ enclosures, forcing the residents to utilize their natural instincts as they mentally process the alteration. For example, Japanese macaques at the Minnesota Zoo wake up every so often to discover a brand-new leaf pile to dig through. Enrichment can also be aromatic: At Disney World’s Animal Kingdom in Orlando, the staff place various perfumes and spices around their tiger paddock. When confronted with odd new smells, the big cats might respond by rubbing, scratching, or marking their territories.

According to the Fort Worth Zoo, enrichment increases the “behavioral choices available to animals.” Simply put, by changing the status quo, enrichment provides animals with the opportunity to make decisions about how to react. Give an elephant a bright-pink volleyball (as the Columbus Zoo did recently), and he might bat at it with his trunk, kick it through a pond, or try to squish it with his feet.

7. ZOO VETS USUALLY MAKE LESS MONEY THAN REGULAR VETS.

You might think that the opposite would be true, but according to data provided by the Bureau of Labor Statistics and the American Veterinary Medical Association, vets who work at zoos have a lower median salary than general veterinarians. Why? To begin with, many AZA-accredited zoos are nonprofit establishments. Therefore, vets who work there don’t always make the sort of income that a private practice might yield. Also, since there are only so many zoos in the world, job opportunities are rather limited.

Still, to hear most zoo vets tell it, you’d be hard-pressed to find a more rewarding career. “[There] is an exciting moment every single day,” says Dr. Suzan Murray of the Smithsonian’s National Zoo. As chief veterinarian, she’s expected to tackle a wide array of fascinating challenges. “Each one is a little bit different, whether it’s coming up with a treatment for coral, diagnosing a problem in a Burmese python, or visiting an elephant we’re hoping is pregnant,” Murray explains. “Every day offers a bounty of surprises.”

8. ANIMALS IN NOCTURNAL EXHIBITS DON’T ADJUST RIGHT AWAY.

Certain zoos have designated nocturnal houses, thick-walled buildings that allow guests to check out bats, bearcats, civets, and other creatures of the night during normal business hours. By day, they’re usually lit with dim red, blue, green, and yellow lights. But late at night, bright white fluorescent bulbs are turned on. This has the effect of reversing the resident animals’ normal sleep cycles so that they’re more active when zoo visitors are around and sleep when the humans do.

For the critters involved, the transition can take time. “When we get animals from a non-nocturnal building, there is an adjustment period,” Bob says. “Most seem to adapt in about a week’s time. We had one [kinkajou, also known as a honey bear], though, that took over a month to adjust.”

9. CAPTIVE BREEDING TAKES CROSS-COUNTRY COORDINATION.

What do Przewalski’s horse, the Arabian oryx, and golden lion tamarin have in common? Without captive breeding efforts—mating orchestrated in controlled environments like zoos and wildlife preserves—they might be critically endangered, or worse.

One of the ways zoos contribute to conservation efforts is by participating in Species Survival Plans (SSPs). Organized by the AZA, these are rigorously regulated breeding programs for rare, threatened, or endangered animals. The goal is to form a genetically diverse captive population, with member animals usually dispersed among several zoos and/or aquariums. In total, there are almost 500 individual SSPs, each headed by a coordinator.

Craig Saffoe, a curator at the National Zoo, leads several different breeding programs for big carnivores, all done in accordance with the appropriate SSP committee. “The first step is that we have to find two animals that actually get along together and are compatible breeding partners,” he says. “For that, we don’t just look at the current collection at the National Zoo. We look at the whole zoo population within the United States.”

Choosing the right pair is a process that involves working closely with the relevant SSP. “When the Species Survival Plan group gets together, they decide what the best route is to keep the entire North American population genetically healthy,” Saffoe notes. “Once my team and I have worked successfully with the SSP to match two animals on paper … it’s our job then to find out if the animals are actually physically compatible.” More often than not, at least one animal will have to be transferred between zoos before any first dates can take place.

10. THE WORD “DEDICATION” WAS INVENTED FOR ZOOKEEPERS.

Make no mistake, this isn’t an easy line of work to break into. Just ask the San Diego Zoo’s HR department, whose employees report that it’s “not unusual” for them to receive literally hundreds of applications when a single animal care job opens up. If you beat the odds and get hired, note that the average American zookeeper takes home a salary of just $29,000 per year.

Despite all this, keepers can rank among the most passionate and devoted people you’ll ever meet. “Just recently when Hurricane Matthew hit, tons of keepers [in affected areas] slept in their zoos, hunkered down in case the animals needed emergency help,” Bob says. In his eyes, such dedication is the rule, rather than the exception. “We go in at two A.M. to check on new moms … We are constantly researching ways to improve welfare and our own personal knowledge.”

What’s more, zookeepers enjoy a tight-knit community. According to Bob, “Everyone knows someone who works at another zoo and on Facebook, everyone is so supportive. There are closed groups of keepers where new ideas are constantly exchanged and people help support strangers when they lose an old, beloved animal. What we do is so hard and stressful and you always have to fight caregiver stress syndrome, but we power through and I wouldn’t trade this life for anything!”

*Some names have been changed.

All photos via iStock.