Although the find was made by studying Australian birds, it supports the idea that our big human brain evolved because it is a sexually attractive organ, not just a useful one.

Signs of intelligence - such as creating art, music, and humour - could have made the brainiest people luckiest in love, according to this theory, championed in the book The Mating Mind by an evolutionary psychologist, Geoffrey Miller, almost a decade ago.

Testing the idea in humans is very difficult, said Jason Keagy, of the University of Maryland in the US, which is why he chose to observe satin bowerbirds at Wallaby Creek in NSW instead.

Bowerbirds are intelligent, he said. ''But they're not as complex as humans.''

Unlike with men, Dr Keagy was also able to get an accurate record of the male birds' sexual success by videotaping their every movement. ''They can't really lie to us.''

Renowned for their fascination with blue objects, bowerbirds have a strong aversion to red. The first IQ test Dr Keagy and his colleagues devised was to place three red objects under a clear plastic container in their bower.

The smartest males could remove the cover and carry away the offending objects in 20 seconds.

''It looks pretty simple, but some weren't able to do it,'' said Dr Keagy, whose study is published in the journal Animal Behaviour.

In a second braintwister, he glued a red object down. Some bowerbirds kept on trying in vain to pull it out, while the brighter ones quickly twigged this was impossible and covered it with leaves.

The males who failed the plastic container test were spurned.

''No females were mating with them,'' Dr Keagy said.

The smartest birds, on the other hand, attracted up to 20 female partners a season.

''This is the first evidence [in any species] that individuals with better problem-solving abilities are more sexually attractive,'' he said.

Greater intelligence may allow male bowerbirds to woo more females because they can build more elaborate bowers, are better dancers or are more responsive to subtle cues from the females during courtship.

Alternative theories to the mating mind include that our large brain evolved because it was advantageous for hunting or living in social groups, and cultural creativity was simply a fortuitous byproduct of the struggle to survive.