Officials said they decided to put down Marius because his genes were too common within the zoo's giraffe population. They shot him with a bolt gun because euthanasia by intravenous injection would have contaminated his meat.
”Copenhagen Zoo’s giraffes are part of an international breeding programme which aims at ensuring a healthy giraffe population in European zoos," the zoo said in a statement. "This is done by constantly ensure that only unrelated giraffes breed so that inbreeding is avoided. If an animal’s genes are well represented in a population further breeding with that particular animal is unwanted."
"We see this as a positive sign and as insurance that we in the future will have a healthy giraffe population in European zoos," the zoo added. "This is something that Copenhagen Zoo believes strongly in."
The zoo's reasoning didn't mollify animal-rights advocates, who started an online petition to save Marius' life.
The zoo also rejected an individual's offer to buy the giraffe for $681,000. Zoos in the U.K. and Sweden offered to take Marius, but one of them already had a full supply of his genes, while the other couldn't guarantee that the giraffe wouldn't be sold again.
The zoo also doesn't consider contraceptives an option, because the hormones can harm the animal's internal organs.
"It just shows that the zoo is in fact not the ethical institution that it wants to portray itself as being, because here you have a waste product -- that being Marius," Stine Jensen, of Denmark's Organization Against the Suffering of Animals, told BBC News. "Here we have a zoo which thinks that putting this giraffe down instead of thinking of alternatives is the best option."
Copenhagen Zoo's scientific director, Bengt Holst, said he and his colleagues stood by their decision.
"We have been very steadfast because we know we've made this decision on a factual and proper basis. We can't all of a sudden change to something we know is worse because of some emotional events happening around us," he told The Guardian. "It's important that we try to explain why we do it and then hope people understand it. If we are serious about our breeding activities, including participation in breeding programmes, then we have to follow what we know is right. And this is right."
He also defended Marius' public dissection, in which a group of visitors, including children, watched zoo workers cut the giraffe's body into chunks.
"People are fascinated by it, both adults and children, and they would like to hear stories they normally don't have access to. I think that's good. It helps increase the knowledge about animals but also the knowledge about life and death," he said.