(CN) — In a new study, researchers say soaring birds like eagles, vultures and falcons that can glide effortlessly by flapping their wings only occasionally have evolved to use their lungs to aid in such graceful flights.
Led by University of Florida evolutionary biologist Emma Schachner, an international research team sheds light on potential new functions of bird lungs in their study published Wednesday in the journal Nature.
Unlike mammalian lungs, bird lungs do more than just facilitate breathing. Researchers believe an air-filled sac within their lungs increases the force used to power flight muscles while flying.
"It has long been known that breathing is functionally linked to locomotion," Schachner said in a statement. "But our findings demonstrate that the opposite is also true in some species. We have shown that a component of the respiratory system is influencing and modifying the performance of the flight apparatus in soaring birds, who are using their lungs to modify the biomechanics of their flight muscles.”
While mammalian lungs are flexible and air flows in and out along the same path, birds breathe differently, with a stationary lung where air is pumped through in one constant direction by a series of balloon-like air pockets. These pockets expand and deflate, with many small extensions called diverticula branching off from them.
The functions of these diverticula are not well understood, but the discovery of the unique air sac, known as a subpectoral diverticulum, happened by accident.
While studying CT scans of red-tailed hawks, Schachner noticed a huge bulge between two of the bird's flight muscles. This led her to hypothesize that the subpectoral diverticulum might be important for soaring mechanics.
The researchers surveyed 68 bird species and found the air sac in all soaring birds, though it was absent in non-soaring birds.
"This evolutionary pattern strongly suggests that this unique structure is functionally significant for soaring flight," Schachner said.
To understand how the subpectoral diverticulum air sac affects flight, Schachner collaborated with Karl Bates of the University of Liverpool. Bates used digital models to study the air sac's impact on the pectoralis muscle in hawks.
"Measuring the behavior of the SPD in a real hawk as it soars in the sky is close to impossible," Bates said in a statement. "So instead, we built a computer model."
The models showed that the air sac increases the lever arm of the pectoralis muscle, much like using a screwdriver to open a paint can provide better leverage than using a coin. This helps soaring birds keep their wings in a static, horizontal position.
Further analysis revealed that the pectoralis muscle in soaring birds is significantly different from that in non-soaring birds, improving force generation.
"Part of what makes this such an important discovery is that it reshapes how we think about the interaction between locomotion and respiration," Schachner said. "From previous studies, we know that locomotion, like running or wing flapping, enhances lung ventilation. But now we’ve shown the inverse — the lung is also able to fundamentally modify the way that locomotion works in soaring birds."
Schachner and the rest of the team ruled out other possible functions of the air sac by observing CT scans of a live, sedated red-tailed hawk while it breathed. They found that the hawk could voluntarily collapse the air sac and still breathe, and could also independently open and close it.
"The evolutionary story here couldn’t be clearer," Andrew Moore, an evolutionary biologist at Stony Brook University in New York said in a statement. "Our data indicate that the SPD only evolves in birds that soar, and did so at least seven times independently across distantly related soaring lineages."
The research also suggests that bird lungs may have other unknown non-respiratory functions.
"Birds are wildly diverse. Think about how different an ostrich is from a hummingbird or a penguin," Schachner said. "It is likely that their lungs are involved in an array of really fascinating functional and behavioral activities that are waiting to be discovered."
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