German researchers who have studied Rico for several years say he shows a skill, normal in young children, to form a rough meaning of a new word.

As they enthused at a news conference Thursday about him and the significance of his mental skills, he lay uninterested and nearly motionless under a table - until a toy crocodile appeared.

Then he barked, awaited instructions and shook the relevant toy to photographers and reporters lying obediently at his feet.

"He just loves the cameras. He only has to hear a shutter opening to react," said Witold Krzeslowski, husband of Rico's owner Susanne Baus.

The couple from Dortmund, western Germany, say their 10-year-old dog shows little sign of tiring of the game, but also relishes hunting for well-hidden toys.

Rico developed his skills while laid low for nearly a year after a shoulder operation, with Baus trying to engage his mind and let his body rest.

"I discovered this talent and told my husband, who thought I was mad. At the start it was three to four objects, but it's risen to 200 or 250," she said. "I don't know what the limit might be, but we've now run out of space."

"We wonder what prevents animals from speaking," said researcher Julia Fischer of the Max-Planck Institute for Evolutionary Anthropology in Leipzig, and senior author of the study of Rico published Thursday in the journal Science.

"The limitations are not their capacity to hear or understand."

Rico won the German "Wetten Dass" (Wanna Bet?) TV program for weird talents five years ago and went on to be voted the best performer in the show's first 20 years.

But would-be collie owners be warned, Krzeslowski said.

"It can be hard to come home and get on with work. You first have to play 'fetch' with 20 objects. The only thing he accepts is when you are in bed. But he also gets up early in the morning."