When Marius, a young male giraffe at the Copenhagen Zoo, was shot to death by his keepers a few days ago, the world caught its breath. How could the zoo and Marius's keepers do such a thing—particularly when people around the world were clamoring for the zoo to spare him?

Zoos, most of us think, are meant as safe havens for animals, places where they are loved and protected. Zoos tell us that they are educational places, too, where we can watch and learn about creatures we might otherwise never have a chance to see.

Zoos also bill themselves as the only places where certain highly endangered species, such as the Hawaiian crow, survive.

Indeed, zoos say this is why we need zoos. They're the guardians of some of Earth's rarest species, caring for them in captivity, and breeding them with the hopes of one day restoring them to the wild. Zoos, we're told, are one of the best ways—and in some cases, the only way—to preserve a species and its genetic variability. (Read "Building the Ark" in National Geographic magazine.)

So why, then, would a zoo kill a healthy, young giraffe?

Bengt Holst, the Copenhagen Zoo's scientific director, offered answers, but these only caused more alarm. It turns out that Marius's species, the reticulated giraffe (Giraffa camelopardalis reticulata), is not endangered in the wild. Plus, the zoo has a "surplus" of giraffes, especially males with genes similar to those of Marius. He did not fit into the zoo's captive breeding program, or that of the European Association of Zoos and Aquaria. And at 18 months old, past the cute stuffed-toy stage of a baby giraffe, Marius would soon be keen to mate.

According to the zoo's calculations, Marius was of more use to it dead than alive.

So, Marius's keeper lured him with a piece of rye bread, his favorite food, into a yard away from the other giraffes, and as he bent down his long neck to take the treat from his keeper's hand, a veterinarian dispatched him with a shot from a bolt gun to his head.

Zoo patrons, including children, were then permitted to watch and learn about giraffe anatomy as the vet butchered Marius. "It helps increase the knowledge about animals but also the knowledge about life and death," said Holst about the anatomy lesson. The giraffe's remains were subsequently tossed into the lions' den—which, some have said, would likely have been his fate anyway if Marius had lived in his natural environment, the African savannah.

What Is the Purpose of Zoos?

Yes ... but Marius was not living in the African savannah. He was living in a zoo, one that asserts that its mission is to be "known and respected for its high standards and quality regarding the keeping of animals," and for its ethics.

All of which raises some questions: Why is the zoo breeding reticulated giraffes, when they are not endangered in the wild? And why did they let Marius's parents mate?

For answers, you need look no further than the Copenhagen Zoo's Facebook page, where it celebrates the birth of a baby giraffe (possibly Marius) in 2012. Humans, science has shown, are drawn to babies of all kinds; we love the big eyes, the floppy limbs, the fluff and fuzz of infants. Baby leopards, baby pandas, baby elephants ... baby giraffes. They all draw huge, paying crowds to zoos.

And while the Copenhagen Zoo, and other European Union zoos, may celebrate themselves as conservation sanctuaries protecting animals on the threshold of extinction, a 2011 report from the Born Free Foundation tells a different story: "An average of only 13% of species kept in European zoos were classified as Globally Threatened" and on the International Union for Conservation of Nature's Red List of Threatened Species.

So, if these zoos aren't really engaged in conserving species as they claim, what is their purpose?

View Images A zoo veterinarian holds up a hoof as he performs an autopsy on the euthanized 18-month-old giraffe Marius. PHOTOGRAPH BY KASPER PALSNOV, AFP/GETTY IMAGES

Very likely the keepers and veterinarians and director at the Copenhagen Zoo all consider themselves animal lovers. They do the hard, often heartbreaking, work of caring for creatures destined to a life in captivity. And they serve another master: profitability.

Inevitably, the two motives will clash; one must betray the other, because caring for zoo animals, especially those that are long-lived, is an expensive undertaking. Zoos have limited space, and if a zoo attempts to give animals the experience of parenting, as the Copenhagen Zoo says it does, it's going to run out of room. It will soon have too many adult animals and not enough of those popular, crowd-drawing, profit-turning babies.

Genetics and the carefully planned breeding program aside, it's hard not to suspect that the real reason Marius had to die was simply that he was past his human-appealing prime.

Broken Trust

I have no doubt that the animal keepers and veterinarians at the Copenhagen Zoo work hard to find ways for the animals in their care to learn to trust them. I've watched the gorilla and chimpanzee keepers at the Lincoln Park Zoo in Chicago care for their charges, and have some idea of the long hours they must devote to this task.

I've also seen how willing and ready young animals are to trust us.

Once, on Equatorial Guinea's Bioko Island, a baby tree hyrax (which looks something like a large guinea pig) came bursting out of the bushes—not to get away from a group of us humans, but to seek our care. (Later, we found his dead mother.) If we'd been another kind of primate, we would have eaten the hyrax. Instead, we scooped him up, nursed him as best we could, and took him to a small animal shelter.

Of course, humans don't always respond this way to needy, young animals. But we think that is what the people running zoos do.

And so our hearts were broken when we saw the keepers at the Copenhagen Zoo break their trust with Marius. He should never have died so young and at the hands of his caretakers, the very ones who should have done all they could to protect him.

Zoos may feel that it is necessary to bill themselves as big players on the conservation stage. But what most of us want to see from zoos and their keepers is compassion for their charges, all of whom live such narrow, corralled lives.

If zoos cannot offer this to the Mariuses in their care, they will lose the public's goodwill, and will deservedly find themselves heading toward extinction.