Wolves at a Scottish wildlife park have been culled and replaced with another sub-species of the animal.
Highland Wildlife Park's male Tor with cubs in June 2000
A pack of Mackenzie River wolves - a North American wolf - has been a feature at Highland Wildlife Park, Kincraig, near Aviemore, since 1972.
But experts said the six animals had to be euthanised because they were "not portraying their natural behaviour".
The park is now part of a breeding programme involving seven rare Scandinavian wolves.
Park owners, the Royal Zoological Society of Scotland, said the cull followed lengthy discussion and research and with the approval of the Animal Welfare and Ethics Committee.
The society said that in the past five years the six wolves stopped "portraying their natural behaviour", or pack dynamic, which it said was essential to the survival of the species.
Society chief executive David Windmill said: "Animal management is a complex, difficult but rewarding work.
"With any kind of management, at times difficult decisions need to be taken and this was one of those times.
"The welfare of the animal is always paramount in our minds and no decision is made until a full investigation has been carried out, taking into account all aspects of the species and situation."
He added: "In the wild, animals are competing in the deadly game of 'survival of the fittest'.
"Zoos have saved a number of these species from imminent extinction.
"The challenge will be to manage them in captivity to the best of our ability in the future, until perhaps one day 'the wild' is safe enough for their return."
Pack dynamic is the term used to describe how every wolf knows its place within its group.
An alpha pair leads the pack and the top male and female have a second-in-command called a beta male and beta female.
This hierarchy continues down through the pack to the omega wolf, which is usually picked on by the others.
Only the alpha pair is allowed to breed within the pack, but all the wolves take responsibility for caring for the cubs.
The wildlife park's Mackenzie River pack arrived in 1972 , but breeding of the animals ended in 2000.
The wolves were aged between six and eight years old and were put down in January.
Two new females from Scandinavia have been introduced to the park and will be joined by five males in about a month.
Ross Minett, director of campaign group Advocates for Animals, said the Mackenzie River wolf pack should have been allowed to "live out their lives in peace".
He said: "Zoos have a responsibility for these animals and not just treat them as a disposable commodity."
On the pack dynamic, Mr Minett said the wolves would have lost this behaviour because they had been kept in a controlled environment.