The courtship displays of tropical birds are fascinating showcases of evolutionary adaptations and sexual selection. New evidence shows how speedy wing muscles can be adapted for more than flight in two species of South American birds – they can play an important role in flirting, too.

Male red-capped and golden-crowned manakins woo females with showy displays – dancing up and down branches whilst clapping their wings together to create loud noises. In a way, the birds are flexing their muscles.

A recent study led by Matthew Fuxjager from Wake Forest University investigated the contractile properties of the major humeral retractor muscle in the wing responsible for these courtship claps and compared them to the same muscles from three non-clapping birds: the blue-crowned manakin, the dusky antbird, and the house wren.

“Of the species studied, only the golden-collared and red-capped manakins produce exceptionally rapid wing movements as part of their acrobatic courtship displays,” Fuxjager explains.

The team found that the humeral retractor of the two clapping species was capable of contracting at frequencies of 80-100 Hz, which is faster than the human eye can detect. For comparison, top Olympic sprinters can generate leg stride frequencies of about 8 Hz.

It was estimated that contraction frequencies required for flight in manakin-sized birds are only 25-30 Hz, suggesting that the superfast properties of these muscles are likely to have evolved to impress females rather than for improving flight performance.

The researchers also compared the contractile properties of the two major flight muscles in the two clapping species and compared them with the non-clapping species. They found that there was no significant difference in their contracting abilities, suggesting that these courtship adaptations are limited to the humeral wing muscles.

“The discovery of the superfast wing muscle in these birds paves the way for further studies into what has to change, or what can change, in a muscle to make it drive faster movements,” says Fuxjager.

There is often a trade-off between generating high contraction frequencies and generating the force required for locomotion. However, the team believe these birds have evolved adaptations that allow them to avoid this trade-off, and can generate the high-speeds for the courtship display as well as generating the force needed for flight.

Fuxjager suggests that understanding these adaptations may help future studies in treating human muscle diseases: “This could be important for developing therapies for motor disorders, particularly those characterized by decreases in muscle performance that result from diseases such as cancer and HIV.”

Fuxjager concludes by stating: “Further studies could now explore how this one muscle can create such superfast wing movements and whether male hormones, such as testosterone, play a role in regulating the muscle’s speed.”

Reference

Fuxjager, M, Goller, F, Dirkse, A. et al. (2016) Select forelimb muscles have evolved superfast contractile speed to support acrobatic social displays. eLife, DOI: 10.7554/eLife.13544

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