Norbert Zajac got his first pet, a golden hamster, when he was 4 years old. He took good care of her and bought a second hamster one year later. By the time he was 8, Zajac had bred more than 100 golden hamsters in the basement of his family's little home. His parents, a highway cop and a housewife in Gladbeck, Germany, said he could keep as many pets as he wanted, as long as he paid for them himself. Zajac began selling hamsters to local pet shops. He diversified, adding guinea pigs, salamanders, tortoises, and a crocodile. He took over the family garden and started raising birds. "When I found out about an animal, I wanted to hold it, and when I held an animal, I wanted to breed it," Zajac says. When he was in fifth grade, schools began taking field trips to his house. He became Germany's youngest licensed parrot breeder in 1967, when he was 13, and quickly cornered the local market on parakeets by training them to breed at Christmastime. At 14, Zajac asked a career counselor what he should do with his life. He was told to become a steelworker.

It was easy advice to give in the 1970s to a young man from the Ruhr Valley, the heart of the West German steel industry and the most populated part of the country. Zajac, who never graduated from high school, worked early shifts at the mill so he could be home to tend his animals before dusk. At 18, he sold most of his pets after he was conscripted into the military. Two years later he was working again at the steel mill when he saw an advertisement in the local paper. A pregnant woman in the city of Duisburg, near the Dutch border, was trying to sell her pet shop before she gave birth. Zajac borrowed money from his father and took over the small store on a quiet residential street in 1975.

Zoo Zajac has become for the Ruhr Valley what Zajac's childhood home was for his classmates. "It's almost used as a gratis zoo by the people," says Ulrich Schürer, the former director of the nearby Zoo Wuppertal. As many as 12,000 visitors will arrive on a Saturday, and many of them won't buy anything at all, except for perhaps a currywurst or coffee at the cafeteria. Even the proper customers rarely obtain animals; the majority purchase only the tens of thousands of pet-care products that line the shelves between Zoo Zajac's tanks and cages: everything from aquarium filters to dog leashes to eyedrops for turtles. Live animals are expensive to maintain in a pet shop, and demand is relatively small. In the U.S., they make up only 6 percent of the retail pet market. "A pet store mostly subsists off of accessories and merchandise associated with the animal," Zajac says. "If I just sold only animals, I would lose €250,000 a year."

This has given rise to what he calls, sneeringly, "Die Tierhandlung ohne Tiere," or "the pet shop without pets." Many of the most successful pet businesses have stopped selling animals or scaled back to just a few low-maintenance species, even though their customers are crazier than ever about their little friends. In Western countries, where family sizes are shrinking, pet owners no longer treat their animals as property but as children, pampering them with products and services that would have once seemed ridiculous: bottled water, gluten-free kibble, doggy diapers, designer beds. The "humanization" trend has benefited more than just animals. The U.S. pet industry has more than tripled, to $60 billion, over the past 20 years, and pet care was one of the few retail industries to grow during the Great Recession.

Much of that growth, though, has accrued to big-box retailers rather than small pet shops. Chains slashed operating costs by eliminating live animals and exploited economies of scale to sell an ever-increasing variety of products as cheaply as possible. In the U.S., PetSmart and Petco Animal Supplies capture more than half of industry revenue. In Germany, the pet-care chain Fressnapf operates more than 800 locations and claims nearly a third of all sales, according to the research firm Euromonitor International. Germany still has more than 5,000 independent pet shops, but their market share has dwindled to less than 15 percent.

"See the Bible, David and Goliath," says Zajac. "I am the little one with the slingshot." It's not the identification you would expect from the owner of the biggest pet shop in the world, but bigness was, for Zajac, an adaptation. While many independent shops disappeared or turned to high-end services like grooming or day care, Zajac took the opposite tack. He survived not through specialization, but through spectacle—by building a pet shop so large that it has gravitational pull. "Zoo Zajac's consumer experience is its advantage," says Antje Schreiber, a spokeswoman for the German Pet Trade & Industry Association. "If you don't look like a zoo today, your physical retail will have problems in the future." 

Germany isn't the country where most people would expect to find the world's biggest pet shop. Most German businesses are modest enterprises, and consumers there aren't impressed by immensity in and of itself. But German pet ownership, like many German enthusiasms, gets a little weird. In the U.S. the humanization trend has ridden largely on the backs of dogs and cats, whose owners spend the most money on their pets but are the least likely to buy their animals in pet shops. Germany, however, has some of the lowest rates of dog and cat ownership in all of Europe. Instead, there's an abundance of so-called exotic pets. Germans keep more small mammals—everything from chinchillas to ferrets to rabbits—per capita than all other Europeans, save the Dutch, and the country also has large populations of reptiles and fish.

Partly this is because of a "dog tax" that Germany imposes on owners of man's best friend, but there are other reasons for the popularity of rodents and reptiles. These animals suit Germans' famously fastidious lifestyles. "Small animals make less mess," Zajac says, "and they do not bother the neighbors." (A majority of Germans live in apartment buildings, while most western Europeans live in single-family homes.) Germans are also, by reputation, more analytical than sentimental, which could help explain their interest in animals that are observed in tanks or cages rather than commingled in the family home. In 2011, for instance, a group of British and Australian researchers counted 98 reptile shows in the European Union, 41 of which were in Germany. No other country had more than 15. Similarly, 7 of the 10 most visited European zoos in 2013 were in Germany, and the remaining three were in German-speaking Austria and Switzerland. (Zoo Zajac, which has about 1 million annual visitors, would rank in the top 30 of European zoos if it were eligible.)

Zajac always understood his countrymen's attraction to unusual animals, and he never thought of his pet shop as a simple site of exchange. He learned shortly after buying his first shop in 1975 that people would come to see strange creatures even if they didn't want to buy them. "There was always something new to look at," he says, "and since customers were already there, they would just buy anything else they needed." He took this revelation as a license to indulge his wildest pet fantasies. As a teenager, he'd tried to import animals from Singapore; as a shop owner, he hopped on an airplane and picked them up himself. He asked his customers which animals they dreamed of keeping and did his best to add them to the store.

By the early 2000s, Zoo Zajac had claimed all the space it possibly could in its original location. "The building authority said if I built one more thing, they'd tear it all down," Zajac says. He started looking at industrial warehouses so he could continue growing. Banks were wary of lending money for such a mammoth upgrade, but the city of Duisburg stepped in to help. The steel industry had declined over the decades, and Duisburg was losing population every year. Zoo Zajac's 140 employees were important to the local economy, so the city offered to lease a warehouse to Zajac until he could afford to buy it. He wrote a fundraising notice in his catalog, asking customers for loans to renovate the warehouse, which he would repay at 7 percent interest—a better rate than he could get from any bank. He raised €2 million in three months.

Zoo Zajac opened in its current location on Nov. 17, 2004. A few months later, Guinness World Records paid a visit. Zoo Zajac was, at the time, two-thirds of its present size, still large enough to earn it the distinction of the biggest pet shop in the world. Zajac lives on the second floor in a modest apartment behind his office with his wife, Jutta, and a mother-daughter pair of dachshunds. "I had to help her give birth here on the living room table," he says. When Zajac is watching TV or relaxing in his hot tub, he's never more than a flight of stairs away from his animal collection, just like when he bred hamsters in his basement as a little boy.

In 2012, Zajac added a controversial mammal to his inventory. Animal-rights activists picketed the store and called him greedy and irresponsible. More than 25,000 people sent a protest letter from PETA that included a cartoon of Zajac strangling the creature with a price tag around its neck. Zajac says he received multiple bomb and death threats. One pet food manufacturer withdrew its products from the store's shelves. Even the German Pet Trade & Industry Association didn't support him. "He offers some animals that we, the association of this industry, aren't very happy with," Schreiber says. The addition wasn't vicious or endangered, but the most conventional pet of all. Zoo Zajac started selling puppies.

Zajac always sold dog food and supplies, but like all German pet shop owners, he'd stopped selling dogs themselves in the 1970s. He quit cats in 1985. People increasingly disapproved of the sale of the most affectionate species, and, despite Zoo Zajac's continual growth, it was hard to find sufficient space for cats and dogs in the store. Germany never legally prohibited their sale, but no other German pet shop was selling dogs when Zoo Zajac resumed in 2012. (The shop had started selling purebred cats again in 2007.) Zajac spent €800,000 on large kennels with heated floors and outdoor sections.

Zajac was responding to a changing market. Germany's dog and cat populations are growing, and the country's small-mammal population shrank more than 20 percent from 2008 to 2013, according to Euromonitor. Taste is becoming more conventional, shedding the quirks that made Zoo Zajac possible in the first place. But Germans may be no more willing than Americans to buy dogs and cats at pet stores. Zoo Zajac's puppy section is usually crowded with cooing spectators, but the shop has sold fewer dogs than anticipated—just 300 last year. And other pet shop owners have proved reluctant to follow his lead. Zoo Zajac is the only pet store in Germany where you can buy a dog.