06/13/2024
With special thanks from my team led by Dr Emma Schachner, we just got an incredible publication in Nature. The following is summarized by Dr Schachner.
Extremely pleased to share our team’s latest work in Nature - hawks and other soaring birds use their respiratory system to enhance their flight muscles. https://www.nature.com/articles/s41586-024-07485-y
Here is a temporary link to download the PDF if you do not have access: https://we.tl/t-UDsipZ1CHG
It has long been known that breathing is functionally linked to locomotion, and it has been demonstrated that flapping enhances ventilation. This project shows the opposite is also true in some species: a component of the respiratory system is influencing and modifying the performance of the flight apparatus. Here are the highlights:
1. The SPD: Branching off the lung is a large air-filled sac, the subpectoral diverticulum (SPD), that dives between the flight muscles in various birds, including Red-tailed hawks. Specifically, the SPD lies between the pectoralis (the downstroke muscle) and the supracoracoideus (the upstroke muscle).
2. What is it not for: We found that the SPD does not play a critical role in ventilation in the Red-tailed hawk or Swainson's hawk by looking at CT scans of live sedated birds while they breathed. They can voluntarily collapse the SPD and still breathe, and independently open and close it on one side only. These birds were imaged for clinical purposes prior to this project (for unrelated reasons).
3. Who has it: We then looked at 68 species of birds (57 via CT or microCT - including a previously collected clinical dataset) and found that the SPD independently evolved at least 7 times in different clades of soaring birds, but does not appear in non-soaring species. This strongly suggests a functional and adaptive link between having an SPD and using soaring flight.
4. What is it for: To assess its functional significance, we evaluated the impact of an inflated SPD on the pectoralis muscle using a method called MDA modeling (with a computational open-source gait modeling program: Gaitsym) and analyzed muscle fascicle length in soarers and non-soarers. We found that the SPD and muscle architecture together improve the torque generating capacity of the wing when held in the extended "soaring" position.
5. Takeaway: The respiratory system is modified in specific groups of birds to mechanically enhance the flight muscles for soaring
6. Another takeaway: This work demonstrates a previously unknown role for avian air sacs and diverticula. The respiratory system in birds is probably doing a lot more interesting non-ventilatory activities that we have not yet discovered. 🥳
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