The three-month-old African black-footed penguins, named Bamm-Bamm and Pebbles, are being hand-reared at Whipsnade Zoo.

BBC: Endangered penguin chicks hand-reared at Whipsnade Zoo

To test whether ravens could follow gaze, Dr Heinrich and Dr Bugnyar used six six-month-old hand-reared ravens, and one four-year-old.

ECONOMIST: Now, it seems, even the bird-brained have theories of mind

Knut was hand-reared by zoo keepers after being rejected by his mother, a former circus bear from the former East Germany.

BBC: Candles by Knut's enclosure

Knut the polar bear, who became world famous after he was hand-reared by keepers after his mother rejected him, has died at the age of four.

BBC: Star German polar bear Knut dies