While zoos in the United States favor contraception as a means of keeping their resident populations under control, other zoos around the world take a different approach: euthanasia.

Dave Morgan, chairman of the Population Management Committee at the World Association of Zoos and Aquariums explained to the New York Times that international guidelines on the ethics of breeding zoo animals are sketchy. Apparently, since ethics and philosophies are so diverse among countries of the world, it's tough to make blanket regulations.

For instance, both the European Association of Zoos and Aquaria and the African Association of Zoos and Aquaria generally consider routine euthanasia a viable management and breeding strategy, while the Central Zoo Authority of India "has recommended that euthanasia of zoo animals may be carried out only in the specific circumstances when any animal is in such an agony or pain that it is cruel to keep him alive."

How Euthanasia Is Used for Population Control

Zoos that favor euthanasia over contraception generally allow animals to mate naturally and permit mothers to raise their young until an age at which the family groups would instinctively separate in the wild. At that point, zoo officials employ lethal injection to kill young animals that exceed the zoo's carrying capacity, don't fit into breeding plans, and are unwanted by other zoos.

In the spring of 2012, the Copenhagen Zoo euthanized a pair of leopard cubs who were approaching two years of age as part of their breeding management plan. Each year, the zoo puts approximately 25 healthy animals to death, including chimpanzees, whose similarities to humans make opponents of euthanasia particularly squeamish.

Arguments in Favor of Euthanasia

Contraception (pills, implants, injections) can pose health risks to animals.

Euthanasia allows animals the natural experience of bearing young and parenting.

Terry Maple, the former director of Zoo Atlanta and co-editor of Ethics on the Ark, knows of no definitive research that assesses the importance of raising young to animals' health, but he has said that observation indicates that most zoo animals are "motivated and protective parents that play frequently with offspring."

Euthanasia imitates animals' survival in the wild, where high percentages of young die early in life as a result of predation, starvation or injury.

According to longtime zookeeper and curator Peter Dickinson, "There is nothing wrong in killing an animal if it is done quickly and with forethought and kindness. When animals are euthanized for the correct reasons, then it is morally right and justified. The uninformed will often level accusations of being 'heartless' and 'not caring' when precisely the opposite is true. Good zoos with managed populations can see the bigger picture … it is the species which is being managed and not individuals. "

Arguments Against Euthanasia