The footage was recorded in a studio where they had constructed a tank with glass sides. They filmed Rudi underwater so they could see exactly how otters swim by slowing down the film. The accedemically ground-breaking sequence towards the end of the documentary showed how otters smell underwater by making an air-bubble and then re-imbibing it. Fascinating.
Rudi appears in the opening sequence and in later scenes that can be watched in the UK on BBC iplayer by clicking here:
The first time Charlie Hamilton-James saw an otter was when we took our very tame Asian short-clawed otter Bee to his primary school about 33 years ago. He said he has been 'an otter fanatic' ever since.