A global uproar has followed the deaths of two African lions and their two ten-month-old cubs. Their lives ended because the zoo wants to introduce a new male to the remaining females to bear more lions.

The same outcry was heard when a healthy young giraffe named Marius, who had the wrong genes for the facility's breeding program, was killed with a bolt to his head—so as not to contaminate his body with poisons. The giraffe was publicly dissected and then fed to the zoo's carnivores, including lions.

None of the deaths were euthanasia, which is a mercy killing when an animal is suffering or lingering near death and must be "put down," as zoos always refer to such situations.

Rather, it was "zoothanasia," or killing done by zoo workers because an animal is no longer needed for one reason or another and is deemed to be a disposable object rather than a sentient being. (Related: "Opinion: Killing of Marius the Giraffe Exposes Myths About Zoos.")

The "Marius Effect"

Many people around the world were outraged by Marius's death. I call this the "Marius Effect."

Many of them had never previously voiced their opinion about the common killings of what are disparagingly called "surplus animals" by zoos, or had spoken out about other animal issues. (See "National Zoo Deaths: 'Circle of Life' or Animal Care Concerns?")

While some workers at the zoo and elsewhere said the giraffe had to be killed because he didn't fit into the zoo's breeding program, and therefore couldn't be used as a breeding machine (like dogs at a puppy mill), countless others disagreed. An online petition asking the zoo to hold off on the killing until another home was found received tens of thousands of signatures.

Marius was killed despite the fact that another facility had offered him a home in which he could live out his life in peace and safety.

Many others and I figured that the negative attention that the late Marius brought to the Copenhagen Zoo would serve as a catalyst to change the breeding policies of zoos in Europe. We thought those responsible for killing him would reassess what they did and question their killing ways—even if such killings were required by existing regulations put forth by the European Association of Zoos and Aquaria (EAZA). (Read more about zoos and saving rare species in National Geographic magazine.)

We couldn't have been more wrong. Now, with the deaths of the four lions, the Copenhagen Zoo wants to become a lion mill, I would argue, and still seems to think killing healthy animals is perfectly okay.

All of the newborn lions whose kin died to make way for them will spend their lives in captivity, and some will undoubtedly be "zoothanized" in the future because they, too, will be classified as disposable "surplus" animals without the right genes to pass on to future captive lions.

The zoo also argued that the new male lion brought to the zoo would kill the youngsters and the captive group, and thus the group wouldn't resemble a wild pride of lions, as if it previously had.

Of course, there is nothing natural about the cage in which they are kept. While some might call it an enclosure or pretty it up by calling it "lion habitat," it is still a cage in which future lions will be mercilessly crammed, from cradle to grave.

"Perversely Justified"

I see heinous acts like killing Marius and the four lions as a perfect subject for study for researchers in the field of anthrozoology, the scientific study of human-animal relationships.

These easily avoidable deaths, perversely justified "in the name of conservation," are horrible lessons for youngsters. They run counter to global programs in humane education and compassionate conservation, in which the life of every individual animal is valued—and not just because they can serve us in any number of ways, such as by making more of themselves for future captive breeding. (See "Is Breeding Pandas in Captivity Worth It?")

Zoos need to change their ways and respect the caged animals for whom they are responsible as long as an individual is healthy.