Marius the giraffe is pictured in Copenhagen Zoo February 7, 2014. [Photo/Agencies]

Stenbaek Bro said a significant part of EAZA membership is that the zoos don't own the animals themselves, but govern them, and therefore can't sell them to anyone outside the organization that doesn't follow the same set of rules.

He also said it is important for the breeding programs to work.

Bengt Holst, Copenhagen Zoo's scientific director, said it turned down an offer from Yorkshire Wildlife Park in Britain, which is a member of EAZA, because Marius' older brother lives there and the park's space could be better used by a "genetically more valuable giraffe."

Yorkshire Wildlife Park said it called the zoo on Saturday with a last-minute offer to house Marius in a new giraffe house with room for an extra male. It said it was saddened by the killing of Marius, but "without knowing the full details it would be inappropriate to comment further."

Copenhagen Zoo also turned down an offer from a zoo in northern Sweden, because it was not an EAZA member and didn't want to comply with the same high standards, Holst said.

"I know the giraffe is a nice looking animal, but I don't think there would have been such an outrage if it had been an antelope, and I don't think anyone would have lifted an eyebrow if it was a pig," said Holst.

Copenhagen Zoo doesn't give giraffes contraceptives or castrate them because that could have unwanted side effects on their internal organs, and the zoo regards parental care as important, said Holst.

EAZA said it supported the zoo's decision to "humanely put the animal down and believes strongly in the need for genetic and demographic management within animals in human care."

However, the organization Animal Rights Sweden said the case highlights what it believes zoos do to animals regularly.

"It is no secret that animals are killed when there is no longer space, or if the animals don't have genes that are interesting enough," it said in a statement. "The only way to stop this is to not visit zoos."

"When the cute animal babies that attract visitors grow up, they are not as interesting anymore," said the organization.

Elisa Allen, spokeswoman for People for the Ethical Treatment of Animals in the U.K., said Marius' case should serve as a wake-up call for anyone who "still harbors the illusion that zoos serve any purpose beyond incarcerating intelligent animals for profit."

She said in a statement, "Giraffes rarely die of old age in captivity, and had Marius not been euthanized today, he would have lived out his short life as a living exhibit, stranded in a cold climate, thousands of miles away from his true home."