The Copenhagen Zoo put down a 16-year-old male lion, a 14-year-old lioness, and two young lions this week to make way for a new male lion from the Givskud Zoo, also in Denmark.

"The change in the lion pride had to happen now because Copenhagen Zoo currently has two young females from the 2012 litter and it is ideal to keep these as part of the new pride and then find a suitable male," the zoo wrote in a Wednesday statement on its website.

"If the Zoo had not made the change in the pride now then we would have risked that the old male would mate with these two females—his own offspring—and thereby give rise to inbreeding," the statement says.

What's more, the 14-year-old lioness was too old to give birth and raise another litter without complications, the zoo said. (Read more about zoos and saving rare species in National Geographic magazine.)

The news quickly reverberated across the globe, with 10,000 stories as of Wednesday and Twitter abuzz with people expressing shock and disgust.

The U.S. Association of Zoos and Aquariums did not respond to requests for comment about zoo euthanasias, but the organization said in a February statement that "incidents of that sort do not happen at AZA-accredited zoos and aquariums for several reasons."

That's because those facilities carefully manage breeding programs and exchange animals between institutions as needed, according to the AZA website.

But the European Association of Zoos and Aquaria (EAZA) noted in a statement that the Copenhagen Zoo "has not broken any of its codes of conduct."

EAZA said that the Copenhagen Zoo "has been consistent in its approach to animal population management, and high standards of animal welfare.

"As a result, while EAZA regrets the death of the animals in question, we recognize the right of Copenhagen Zoo to humanely cull them in line with their policies," the statement continued.

Giraffe Death Sparked Outrage

The Copenhagen Zoo's killing and public dismemberment last month of a healthy giraffe that the facility had named Marius also sparked outrage on social media.

The news even prompted death threats against zoo staff, and was seen by some animal lovers as a provocative response to a campaign to spare the giraffe's life in the days and hours leading up to his death. (See "Opinion: Killing of Marius the Giraffe Exposes Myths About Zoos.")

An online petition asking the Copenhagen Zoo to hold off on killing its unwanted giraffe until another home could be found had received tens of thousands of signatures from around the world. Wildlife parks in Britain, Sweden, and the Netherlands had offered to take Marius off the zoo's hands.

"When breeding success increases it is sometimes necessary to euthanize," Bengt Holst, Copenhagen Zoo's scientific director, said in a February 9 statement on their giraffe's death.

"We see this as a positive sign and as insurance that we in the future will have a healthy giraffe population in European zoos."