And if a zoo is treating the bear well - as he believes Berlin Zoo was with Knut - then keeping some in captivity is a price worth paying.

BBC: Knut: A polar bear story

Many animal welfare organisations argue that the capturing and keeping of dolphins in captivity is cruel.

BBC: Dolphins helped my child

Blackfish, about the consequences of keeping killer whales in captivity, is among the documentaries that will be screened at the O2's Cineworld cinema multiplex between 25 and 28 April.

BBC: Sundance London film festival announces 2013 line-up