New photos, taken by a heat-and-motion sensing camera set up in the forest by the photographer Marius Angvik, show a wide-bellied female wolf making her way through the forest.
"It is very likely that there is a father and daughter who are expecting puppies,” concluded Øystein Flagstad, a senior researcher at Rovdata, the wildlife monitoring wing of the Norwegian Institute for Nature Research (Nina), who confirmed that the female wolf looked pregnant. "It's not very likely that there are other females around."
Fenris and his partner Freya were named in a nationwide poll organised by the Norwegian Conversation Society after they were first spotted in the forest in January 2013.
The pair went on to have two puppies, one male and one female, that May. However, Freya disappeared that August, leading the researchers to presume she had died.
"It’s not common at all," Flagstad told the local of the father-daughter pairing. "We’ve studied the wolf population very closely for 35 years, and we’ve seen parent-offspring mating five times in all these years."
He said that Freya's disappearance may have led Fenris to replace her with their daughter.
"It’s a bit speculative, but we know that when the female disappears, often the daughter may mark the territory. Why the father chose to mate with her, I really don’t know. It’s probably because there are not other females around, and she was ready."
He said that it would be interesting scientifically to see what happens to the puppies if the daughter is indeed pregnant with Fenris.
"It’s quite interesting from a research perspective to see what happens. In two of the earlier matings there were one or two pups born but they then disappeared, but in the three other cases, at least one of the pups survived to reproduction, so it will be interesting to see if there will be viable pups."
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He said Rovdata still needed to collect DNA data and other evidence from Østmarka before it would know for absolutely certain that Fenris was the father.
He said that the Freya and Fenris's son now appeared to have left the family, after staying with them much longer than normal.
Many Oslo residents travel to Østmarka on an almost daily basis to ski, run or hike.