The Cambridges took their eight-month-old son to Sydney's Taronga zoo to meet a bilby - a rabbit-like marsupial - that has been named after him.
But they had to warn keeper Paul Davies about their son's iron-like grip when he threatened to grab one of the ears of the creature, affectionately known as Australia's Easter bunny.
The couple supported George as he stood up and held on to a low clear plastic fence that surrounded the marsupial's pen. However, the couple had to restrain him when he tried to climb into the enclosure to get at the bilby.
The Duchess said "he's trying to grab his ear," and William added: "If he gets it he'll never let go," kissing the top of his son's head.
Mr Davies told the couple it would be fine for George to stroke the animal and went to coax the bilby closer but Kate stepped in to stop him, saying: "He's got quite a strong grab actually."
Zoo officials had to apply to the national Stud Book Keeper to have George, the bilby's new name, officially recognised and his old moniker Boy dropped.
Mr Davies said of the creature: "If you think of kings of the past, he has got that worldly presence. He has very little fear of anything. He calls his own shots. He is a very confident little animal."
Greater bilbies, nocturnal marsupials who hide in burrows during the day, were once common in Australia's grassy woodlands but have been driven to the verge of extinction by predators such as foxes and cats and competition from rabbits, all introduced to the country by British settlers.
With only 10,000 left in the wild in northern Queensland and Western Australia, a conservation campaign begun in the late 1960s has gathered pace in the last decade, with chocolate Easter bilbies replacing chocolate bunnies in many Australian children's homes.
George, dressed in blue shirt, navy shorts, socks and shoes, was carried by Kate inside the Australian Nightlife House to view other nocturnal animals, including an echidna and spinifex hopping mice.
William and Kate later unveiled a plaque that read: "A national gift from the Commonwealth Government to commemorate the birth of His Royal Highness Prince George of Cambridge."
Australia's government under former prime minister Kevin Rudd gave Taronga Zoo's bilby preservation programme a £5500 donation to commemorate George's birth on July 22 last year.
The baby prince interrupted the zoo chief executive's speech with some enthusiastic squealing before gleefully taking a present of a stuffed toy bilby from him.