Brainy male birds are more sexually attractive to female birds, scientists have discovered.
Researchers gave male bowerbirds a set of cognitive tests to evaluate their problem solving ability.
Bowerbirds that performed well in the tests also mated with the most females, when compared with their more stupid rivals.
This is the first study to show that males who are better problem solvers also mate with more females.
Scientists studied satin bowerbirds (Ptilonorhynchus violaceus) living in the forest just south of Brisbane in Australia.
Bowerbirds are famous for their complex courtship behaviour and construction of elaborate bowers considered by some to be a true natural wonder.
Bowers consist of intricate structures built out of sticks that take the birds hours to create.
Bowerbirds are the kind of birds that make you realise that bird brain should be a compliment
Males will decorate these structures with colourful objects including flowers and even bottle tops. Female bowerbirds visit the bowers and then select a mate.
"Bowerbirds are the kind of birds that make you realise that bird brain should be a compliment," says Jason Keagy of the University of Maryland, US, who led the research, which is published in the journal Animal Behaviour.
The research team wanted to find out if male cognitive performance played an important role in sexual attractiveness.
Due to the complex nature of the birds' mating ritual, scientists suspected there may be an advantage to birds that are more intelligent.
To find out how "brainy" the birds were the team developed a set of problem solving tests.
"Satin bowerbirds males do not like red objects placed on their bowers and immediately attempt to remove them," Keagy explains.
"We created situations where males had an obstacle to removing red objects. The first test had a clear container placed over three objects and the males had to figure out how to tip the container over so they could get rid of the offensive red things."
"The better problem solvers removed the container faster," he says.
The team also developed a second test where the researchers fixed a tile to the ground so the bowerbird could not remove it. The cleverest birds would realise they could not remove it and would cover the tile with leaf litter.
When the scientists looked at the mating success of the birds, they discovered those that came high in the test scores also achieved the highest mating success.
There are a number of potential reasons why it might be better to mate with a male who is a better problem solver, Keagy explains.
"It helps to think of the brain as a window on the genetic quality of a male due to the brain's complexity," he says.
For example, says Keagy, other studies show that sickly individuals with lots of parasites tend not to do well in cognitive tests and those same males are generally parasitised because of poor immune systems.
The red object won't budge, but how long till the bird realises?
"If you are a female, those aren't the kind of genes you want in the father of your children."
"Also if we are talking about a female in a species with parental care, males who are more 'intelligent' may be better able to find food and take care of the young, and so be a good choice for a mating partner," he says.
"At this point we can't be sure exactly how females are choosing to mate more 'intelligent' males, but there are two basic ways this could occur," says Keagy.
"Females may have evolved to choose males who are cognitively superior and so observe behaviours of males during courtship that indicate how 'intelligent' a male is."
Complex male bowerbird behaviour including dance routines, mimicking of other birds and the building of the bower may allow females to get an idea of a male's cognitive performance.
Another possibility suggests Keagy is that males use their brains to outwit the female, or at least convince her not to go elsewhere. They could do this by responding effectively to female signalling, which is already known to occur in bowerbirds.
"What is most likely, is that some sort of combination of these two things is occurring," he says.
Keagy hopes this study will get people thinking about how sexual selection influences cognitive evolution.
"Usually when the evolution of the brain is discussed, it is always assumed that it must have been some sort of naturally selected benefit such as being better at extracting food or getting along in big social groups," he says.
"We can't ignore, however, that unless a male gets a female to mate with him, he will not pass on his genes. If an animal is carrying around something as big and expensive as a brain, why not use it for increasing his likelihood of mating?"