Epic wolf hunt caught on camera
The extraordinary endurance hunting of grey wolves has been recorded by BBC film-makers in Alberta, Canada.
Cameramen battled howling winds and sub-zero temperatures to film the dramatic chase as wolves closed in on their American bison prey.
Fortune finally favoured the wolves when a panicked bull knocked over a calf, leaving it defenceless against the hungry predators.
End Quote Chadden Hunter, director
This was the coldest overall shoot on all of Frozen Planet”
The sequence features in the landmark BBC One series Frozen Planet.
Director Chadden Hunter and his team scoured Wood Buffalo National Park, an area the size of Denmark, to find the predators and prey in action.
Taking on a herd of bison, with males weighing in at up to a tonne, is a high-risk strategy for the wolves.
But with temperatures reaching -50 degrees and with winds of up to 100mph, conditions drive extreme behaviour.
"This was the coldest overall shoot on all of Frozen Planet," Hunter told BBC Nature.
"The electronics on the cameras would freeze up, the lenses would grow sheets of hoar-frost and batteries would die in seconds.
"We tried heating the cameras but the plastic cables to the battery packs snapped like bread sticks and the plastic covers broke apart like poppadoms.
"We resorted to 'ancient' technology and strapped tin cases containing sticks of burning coal into the insulated blanket covering the camera."
- American bison are also known as American buffalo
- They are the continent's largest living land mammal
- Despite their size, they can run at speeds of over 30mph
Both bison and wolves are wary of humans so a combination of techniques was used to capture the hunt in full.
Aerial cameraman Michael Kelem, more accustomed to filming Hollywood car chases, had to acclimatise to the weather quickly.
Operating the camera on board the helicopter required fine motor control: impossible in down gloves but excruciating without them.
"The helicopter obviously became our secret weapon for filming the hunt but it was some serious old-fashioned bushcraft that got us the ground shots to complete the sequence," said Mr Hunter.
Native Canadian Jeff Turner was stationed on the ground where he was able to film the climax of a second hunt.
Turner describes watching a lone wolf battle with a bison for over an hour as "powerful".
"The struggles that we'd had with the cold... felt insignificant compared with these animals that were struggling for their very lives," he said.
Frozen Planet starts on 26 October at 21:00 BST on BBC One