Last weekend, a healthy juvenile male reticulated giraffe at the Copenhagen Zoo was killed. His name was Marius. The reason given was that his genes were already sufficiently represented in the giraffe population across the zoos of the European Association of Zoos and Aquariums (EAZA) – his brother lives in a zoo in England, for example – making him a so-called "surplus animal." Despite the international outcry against it, the giraffe was euthanized, a necropsy was performed by scientists while educators explained the dissection to the gathered crowd, and hunks of meat were fed to the zoo's lions, polar bears, and other carnivores.

The event reveals an ethical dilemma that rests at the core of zoo management: what tradeoffs are acceptable when it comes to animal welfare? (There are those who would rather zoos not exist at all; for the sake of this ethical exercise, we'll leave that conversation for another time.)

The reality is that zoos have an obligation to manage their populations as sustainably as possible, and that includes avoiding overpopulation and preventing inbreeding. Zoos operate at what is called "carrying capacity," which is the upper limit on the number of individual animals for any particular species that a given amount of space can sustain. In the wild, the carrying capacity of a geographic area is defined by the territory needs of individuals, and the amount of food and water available to sustain them. In a zoo, in addition to space, other limiting factors include the time available to care for animals and to maximize their welfare by keepers, curators, and veterinarians, and cost. And welfare is about more than just avoiding illness, injury, or boredom. As Terry Maple and David Bocian wrote in the journal Zoo Biology in 2013, promoting welfare is about "the effort to reach a higher plane of satisfaction, essentially techniques to improve the quality of life from good to great." Preventing overpopulation is, in part, how zoos can maximize the physical and psychological health of each animal; preventing inbreeding is how zoos can maximize the genetic health of each population.

In the earliest days of the modern zoological park, zoos did not have this problem. The problem, instead, was just keeping animals alive. When animals died, new animals had to be caught from the wild to replace them. In the intervening decades our ethics have evolved (in the vast majority of cases, wild animals are no longer imported to zoos), our scientific understanding of reproductive physiology and veterinary medicine have grown by leaps and bounds, and best practices for husbandry and management have improved. As a result, in many cases, we now face the challenge of limiting population growth.

The Association of Zoos and Aquariums (AZA), which oversees the Species Survival Plans for over 500 species in North American zoos, and other organizations like it (such as the EAZA, of which the Copenhagen Zoo is a member) achieve this through developing population management plans.

These plans consider the available space and resources that each participating zoo has in order to determine how many individuals of each sex and age class can receive adequate care. Combined with information about the genetic lineage of each individual animal, they can develop breeding recommendations. With luck, those pairs who have been matched will take to eachother and produce offspring.

How can zoos prevent individuals who have not been given a breeding recommendation from producing offspring? There are several approaches: they can be housed in a single-sex enclosure if appropriate for the species, they can receive contraception, they can be permanently sterilized, or if there is no space for the animals, they can be transferred to other institutions, or they can be euthanized. Some zoos instead will allow for breeding between non-recommended pairs, but that leaves them with the problem of what to do when the offspring themselves grow to reproductive age.

Each of those approaches comes with positive outcomes as well as tradeoffs with respect to animal welfare.

Starting with the simplest, the issue with permanent sterilization is that, in most cases, it's permanent. While a given animal might not get a breeding recommendation this year, they might next year. Or, perhaps they successfully reproduce but the juvenile doesn't him- or herself survive to breeding age, making it preferable to reintroduce the parent into the gene pool. While sterilization addresses the inbreeding problem, it doesn't address the overpopulation problem.

The problem with transferring the individuals to other institutions – whether that's an animal who hasn't been recommended for breeding, or a "surplus" animal who is the result of a non-recommended mating, is also fairly straightforward. Having cared for an individual animal since birth, one might argue that zoos are collectively responsible for the animal's welfare for the duration of its life. To that end, animals should only be transferred to places that can provide adequately for their health and welfare, which includes appropriate socialization (for social species, like giraffes). In fact, it is a violation of AZA's Code of Professional Ethics to transfer animals to inadequate facilities.

What happens if there is no space to hold and care for an animal, and there are no suitable alternative facilities? Some would argue that it is better to euthanize the animal than to send it to a facility where the quality of its life would suffer. As Ingrid J. Porton points out in Wildlife Contraception, "there are records of transferred zoo-bred mammals from big cats, lemurs, and chimpanzees to a range of ungulate species that have eventually reached roadside animal attractions, circuses, for-profit animal breeding facilities, exotic animal auctions, and the pet trade." And that's to say nothing of any progeny that transferred animal might have in the future. Should length of life be privileged over quality of life? Should length of life be privileged over the uncertain quality of life of potential descendents? Is it better to provide a humane death for an animal when the only alternative would find the animal in a circus or in the backyard of a wealthy private citizen?

Assuming that there is space to keep a non-breeding animal, contraception might be a better approach. Unlike sterilization, contraception is meant to be reversible. But the issue is that in most cases and for most species, contraception is still considered experimental. Long-term research on the safety and toxicity of hormonal contraception is still ongoing. And for some species, it isn't clear yet whether an animal can become pregnant and give birth to viable offspring after months or years of contraceptive administration. "Without conclusive data on the consequences of contraceptive use in all species, the ethical dilemma becomes whether the possibility of health risks associated with contraceptive use is outweighed by the benefit of not separating animals or of not producing surplus animals," Porton writes. Could the contraceptives result in side effects that could shorten the life of an animal, or in illness? Is that an acceptable risk to take when the alternative is producing offspring for which the zoos within a given network can't provide? When some risk is inevitable no matter what approach is taken, is a known risk to the parents preferable to an unspecified gamble on the welfare of future unplanned progeny?

Another downside to contraception, according to some, is that it deprives animals of expressing behaviors related to mating, reproduction, and parenting. They argue that if zoos endeavour to allow the animals in their care to express the widest range of species-typical behaviors as possible, that necessarily includes breeding. That mating and raising young is a "fundamental and enriching part of life," as Porton puts it. She continues, "this view holds that all social aspects of mating and rearing offspring are of overriding importance to the well-being of captive animals and to prevent this experience could be considered unethical." For institutions that have adopted this philosophy, animals are not contracepted and social species are housed in mixed-species enclosures. If there are offspring born from individuals whose genes are already well-represented in the population, then when the offspring is old enough that it would naturally disperse from its mother's social group, as was the case with the Copenhagen Zoo giraffe, then the zoo must decide what to do with that individual.

Here lies the critical tradeoff at the heart of the controversy surrounding the death of Marius at the Copenhagen Zoo.

On the one hand, those against the killing argue that the welfare of the animal in question (Marius) is of paramount importance. They might offer that contraception would have been a better alternative to prevent his birth in the first place, but that once he was born, the zoo has an obligation to continue caring for him, even if he is a so-called "surplus animal." They might argue that he could be contracepted or sterilized to prevent him from breeding and further propogating his genetic materials. Each of those arguments, of course, has its own downside as described above.

On the other hand, advocates for humane euthanasia argue that the welfare of the parents is of greater importance. They point out that the euthanasia would only occur once the juvenile has aged to the point where it would normally seek out a new social group anyway. From the parents' perspective, euthanizing their progeny (outside of their view) is not functionally different from relocating the animal to a different social group in a different institution. And if there's no space to care for the juvenile at a different zoo, or if that space would be better used by an individual with a different genetic endowment, then it may actually be preferable to euthanize the animal, while deriving as much educational and scientific benefit from the process as possible.

In addition, advocates for this position would point out a silver lining, which is that the carcass can then be broken down and used to feed the carnivores. Thus, the giraffe would enjoy maximal welfare while alive, an entirely humane death, and the lions, tigers, and other carnivores, for whom intact carcass meat itself serves as an enrichment item, would enjoy improved psychological welfare and dental health, as research repeatedly demonstrates. It's worth pointing out that, as obligate carnivores, big cats can only survive by eating meat. If the remains of Marius were not fed to them, then they would simply be fed with meat derived from the slaughter of other animals. And you can be sure that the life and death of the Marius the giraffe reflected greater welfare concerns than the factory-farmed animals who wind up in the chow served more routinely at the zoo.

By definition, zoos are an imperfect system insofar as they can not care for an infinitely increasing population and it would be irresponsible to allow it to do so. But that's not to say that nature, red in tooth and claw, is any less imperfect. A giraffe in the wild who is too weak, or too slow, or injured, will be taken down by a hungry lion, with no regard for the welfare interests of its meal, nor for the rest the giraffe's social group who bear witness to that predation. No natural ecosystem can support an infinitely increasing population any more than the artificial ecosystem created by a network of zoos, and without the selective pressure offered by predation, starvation, or drought, zoos must restrict population growth by other means.

For those who are horrified by the idea of Marius's euthanasia, I would ask what alternative they would choose, given the risks and downsides associated with each. And for those who supported the euthanasia, I would ask whether the parents' ability to experience mating and parenting behaviors was worth the uncomfortable outcome for the juvenile.

The answer to these questions is not something that science is equipped address. Science can only describe the various alternatives and what the consequences of those decisions would be. The hard part is weighing the variables and deciding what the most optimal outcome would be, from a set of imperfect choices.

Further reading:

Re-Writing the Death of a Giraffe

Why Did Copenhagen Zoo Kill Marius the Giraffe? Cultural Differences in Conservation

Header image and second image: Masai Giraffe, San Diego Zoo. Third image: Masai Giraffe, Los Angeles Zoo. All photos copyright Jason G. Goldman.