The straight flight of Alsomitra macrocarpa is associated with optimal aerodynamic efficiency - Figures, Scripts, and Data

Alsomitra macrocarpa produces exceptionally aerodynamically efficient winged seeds. While most specimens descend helically, a subset exhibits a rare straight flight mode associated with long-range dispersal. Here, we identify planform circularity as the principal morphological determinant distinguishing these flight behaviours: seeds with high circularity exhibit helical descent, whereas those seeds with low circularity display straight flight. Combined with the chordwise centre-of-mass eccentricity, circularity defines a morphological–aerodynamic parameter space that captures the full behavioural range. Free-fall experiments and reduced-order modelling, calibrated via heuristic optimisation, reveal two dynamic regimes within the straight-flying subset: stable gliding and dynamically unstable bounding. Numerical and steady-state analyses show that aerodynamic efficiency peaks within the bounding regime, with experimental eccentricity measurements clustered near this value. However, real seeds exhibit maximal efficiency during gliding, suggesting that the dynamic instability in our 2D model is stabilised by the 3D shape of real seeds. This alignment between morphology, efficiency, and stability is consistent with Alsomitra operating within an evolutionary niche finely tuned for maximal dispersal, exploiting aerodynamic trade-offs to achieve exceptional passive flight performance.

大果翼籽木（Alsomitra macrocarpa）能够产生空气动力学效率极高的带翼种子。尽管绝大多数样本呈螺旋状下落，但其中一小部分种子会呈现出罕见的直线飞行模式，该模式与长距离种子传播密切相关。本研究将展形圆度（planform circularity）确定为区分这两类飞行行为的核心形态学决定因素：圆度较高的种子呈现螺旋下落模式，而圆度较低的种子则表现为直线飞行。展形圆度与弦向质心偏心率（chordwise centre-of-mass eccentricity）相结合，共同构建了一个形态学-空气动力学参数空间，能够完整覆盖所有的飞行行为类型。通过启发式优化（heuristic optimisation）校准的自由下落实验与降阶建模（reduced-order modelling）研究，揭示了直线飞行种子子集内存在两类动态域：稳定滑翔与动态不稳定的边界振荡飞行。数值分析与稳态分析结果表明，空气动力学效率在边界振荡飞行域内达到峰值，实验测得的偏心率数据均聚集在该峰值附近。然而，真实种子却在滑翔状态下实现了最高效率，这表明我们二维模型中存在的动态不稳定性，会被真实种子的三维形态所稳定。形态特征、气动效率与飞行稳定性之间的这种匹配关系，与大果翼籽木所处的进化生态位高度契合——该生态位经过精细优化以实现最大化的种子传播，并通过利用空气动力学性能的权衡机制，获得了卓越的被动飞行性能。