Serious fighting between the baboons at Wellington Zoo has seen four of the animals euthanised.
Habib, 14, Osiris, 7, Les, 17, and Rafiki, 15 were all put to death today due to a "breakdown in social structure".
Animal welfare advocate SAFE said the incident "again" highlighted the inherent cruelty of animal captivity for entertainment.
SAFE chief executive Debra Ashton said killing the animals was irresponsible and she questions the ability of Wellington Zoo to house their animals.
"Clearly the behavioural and habitat needs of the baboons were not being met at the zoo, which goes to show the flaws of keeping animals in captivity," Ashton said.
"Social structures suffer in enclosed environments and could be attributed to fighting and anxiety for animals. When these social systems break down and there is fighting, vulnerable animals are not in a position to be able to escape as they would in the wild,"
Wellington Zoo Chief executive Karen Fifield said social structure was critical for Hamadryas baboons, and the breakdown had led to serious fighting which meant the baboons were being injured and suffering from anxiety.
Fifield said all other options, including re-homing, were considered before deciding to euthanise the group.
"This is not a decision that we have made lightly and it is very tough on all of us but particularly those who dedicate themselves to caring for our animals," she said.
However the zoo was certain death was the kindest and most humane action it could take.
She said it was an "incredibly sad" day for the zoo.
The four creatures were the zoo's only baboons. Hamadryas Baboons are a species from Africa, marked by the silver-white cape of the males.