Two species of small tropical birds called manakins can “clap” and “snap” their wings with incredible speed thanks to one of the fastest limb muscles ever known in vertebrates. And according to a new eLife study, they developed these superfast wing muscles for mating – not for flying.
From hares to fiddler crabs, many different species move their limbs rapidly as part of their showy, sometimes acrobatic courtship displays. But muscle performance is a trade-off between contraction speed and muscle force: The muscles that move limbs can contract with great strength (for, say, running and flying) or great speed, but not both. So exactly how animals have evolved the ability to produce both fast limb movement for displays and enough limb force to power locomotion is a mystery.
Male golden-collared manakins (Manacus vitellinus) perform what’s called “roll-snaps.” They hit their wings together above their backs at about 60 Hz to produce a loud mechanical sound. You can watch a video of a golden-collared manakin’s roll-snap here, or check out the animation below.
A high-speed clip of a golden-collared manakin performing a roll-snap. Credit: Barney Schlinger
Meanwhile, male red-capped manakins (Ceratopipra mentalis) produce “claps” by extending their wings slightly above the body and then immediately retracting them back to their sides in quick succession, at about 45 Hz.
To see how limb muscles can generate such superfast movements, a team led by Matthew Fuxjager of Wake Forest University in North Carolina compared the twitch speeds of forelimb muscles in golden-collared and red-capped manakins with that of three other species of wild-caught birds: blue-crowned manakins, dusky antbirds, and house wrens.
They found that golden-collared and red-capped manakins produce their quick courtship displays using one main wing muscle: Their main humeral retractor muscle (pictured to the right in red) evolved to move their wings at superfast speed. In fact, this muscle moves the wing at speeds more than twice of what's required for birds to fly.
Their other wing muscles – which generate the majority of their aerodynamic force for flying – are the same as that of other birds. That means these muscles have been preserved to power flight.
“Our study provides evidence not only for the emergence of the fastest known vertebrate limb muscle, but also a unique evolutionary design of the forelimb muscular system that enables both rapid movement for displaying and force-generating movement for locomotion,” the authors wrote.
Adult male golden-collared manakin. Nick Athanas
Image in the text: M.J. Fuxjager et al., eLife 2016