At first the fox kept its distance, but gradually it became more and more confident, coming out to join Berge and Tinni regularly on their walks. Berge's extraordinary series of photographs are available here.
Throughout the winter, spring and summer Berge would call for Snusen, which roughly translates as 'Sniffer', whenever he took Tinni out for a walk near his house in Norway's Ski municipality.
"I tend to whistle to tell Snusen that we are here, and then he comes running," Bergen told Norway's state broadcaster NRK. "Sometimes we have seen him for several days in a row, and sometimes several days go by in between meetings."
Berge has now teamed up with Berit Helberg, a local author, to write a book about his dog's unusual friendship, with the proceeds going to groups campaigning against fur-farming.
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"The photos are not staged," Helberg stressed to The Local. "He just takes a walk in the forest with Tinni, his dog, and when he whistles the fox Snusen comes to play. They do whatever they please and they have never fought. Snusen is a real wild fox, coming and going as he pleases."