By James Morgan
Science reporter, BBC News, Chicago
The tropical birds perform a courtship song and dance
It is the ultimate "gentleman's agreement". Rather than compete for females, male long-tailed manakins co-operate with their friends.
The tropical birds pair up to perform a courtship song and dance, but the alpha male gets the girl every time.
Meanwhile his "wingman" spends five years playing second fiddle. But he eventually inherits the mating site.
The dance, dubbed "backwards leapfrog", was filmed in Costa Rica by zoologists from the University of Wyoming.
At first glance, it appears like a competitive "dance-off".
But in fact it is a co-operative pact between buddies, says Dr David McDonald, of Wyoming University.
"As far as I know it is the only example of male-male [mating] co-operation in the animal kingdom," he told delegates at the American Association for the Advancement of Science (AAAS) meeting in Chicago.
"The male birds' partnership lasts up to five years. During that time, the beta male does not copulate.
"He has to wait until alpha male dies - he doesn't kick him out. So he may be waiting until he's 10, 15 or even older."
The wingman may be equally as good at dancing as the alpha.
Nevertheless, he agrees to forego sex and let his buddy take the spoils.
If he hits the jackpot he is one of the most successful vertebrates on the planet earth
Dr McDonald, Wyoming University
In return, he will eventually inherit the mating site and become the alpha himself.
The deal could be compared to Gordon Brown and Tony Blair's infamous "Granita pact".
At the London restaurant, Brown allegedly agreed to support Blair in his bid for Prime Minister, on condition that he would eventually inherit the reins.
"It's a rough life for a beta male manakin," concedes Dr McDonald.
"But if he hits the jackpot he is one of the most successful vertebrates on the planet earth."
The courtship duet is also highly unusual in evolutionary terms.
Most examples of co-operation in the animal kingdom involve either relatedness or kin selection, but neither is working here, says Dr McDonald.
"The way it works is he is helping establish a reputation for the dance site.
"The females don't know the males individually. They map the sites where males are doing really hot performances.
How well connected a young male is will predict how he will do - whether he becomes an alpha or a beta
Dr McDonald, Wyoming University
"Once a dance site has a strong reputation, females will keep coming back, even when it has a different alpha male.
"You don't go to a restaurant because you know the chef - you go because you know the meal is good.
"In the same way, the female manakins are happy as long as the singing and dancing is good. They let the males sort it all out."
But how do the males decide which of them is the alpha?
It is not a case of who is a better dancer, says Dr McDonald.
"Was Michelangelo's master a better artist than he was? Not necessarily," says Dr McDonald.
What it comes down to is how "well connected" he is among his buddies.
"As males grow up, they go through a complex network of social interactions," says Dr McDonald.
"How well connected a young male is will predict how he will do - whether he becomes an alpha or a beta.
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