An animal rights group is questioning the ability of Wellington Zoo to house its animals following the deaths of four baboons.
The baboons were euthanised after violent fighting, resulting from a breakdown in their social structure.
Debra Ashton, chief executive of animal rights group SAFE, says the zoo needs to take care of its animals for life.
"They need to be responsible for those animals, whatever challenges they come up with. We don't think that killing them should be an option."
Zoo chief executive Karen Fifield said the decision was not taken lightly.
"When it comes to a decision like this we need to make a decision before the animals begin to suffer, in this case it is a matter of urgency."
But Ms Ashton wants to know why the baboons could not have just been separated.
"When the social structures break down and the animals begin to fight, vulnerable animals are not really in a position to be able to escape… like they would normally do in the wild."
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While Dr Ngaio Beausoleil of Massey University's School of Veterinary Science attributes the behaviour to baboons' life in captivity, the zoo made the right call.
"If there was another way they could have done this that would have had a better welfare outcome for the animals, they absolutely would have taken that option."
Dr Beausoleil says it must have been very difficult for zoo staff to come to the decision to euthanise the baboons.