Data from: Massive, solidified bone in the wing of a volant courting bird

One pervasive morphological feature of tetrapods is the pipe-like, often marrow-filled, structure of the limb or long bones. This “hollow” form maximizes flexural strength and stiffness with the minimum amount of bony material, and is exemplified by truly hollow (air-filled), or pneumatic, humeri in many modern birds. High-resolution microCT scans of the wings of two male Club-winged Manakins (Machaeropterus deliciosus) uncovered a notable exception to the hollow-tube rule in terrestrial vertebrates; males exhibited solidified ulnae >3x the volume of birds of comparable body size, with significantly higher tissue mineral densities. The humeri exhibited similar (but less extreme) modifications. Each of the observed osteological modifications increases the overall mass of the bone, running counter to pervasive weight-reducing optimizations for flight in birds. The Club-winged Manakin is named for a pair of unique wing feathers found in adult males; these enlarged feathers attach directly to the ulna and resonate to produce a distinctive sound used in courtship displays. Given that the observed modifications probably assist in sound production, the Club-winged Manakin represents a case in which sexual selection by female choice has generated an ecologically “costly” forelimb morphology, unique in being specialized for sound production at a presumed cost in flight efficiency.四足动物普遍存在的形态特征之一，是四肢长骨呈管状、且通常内部充盈骨髓的结构。这种“中空”结构可通过最少的骨组织用量实现抗弯强度与刚度的最大化，诸多现代鸟类的肱骨（humerus）便为真正中空（充盈空气）的充气骨（pneumatic bone），即为该特征的典型例证。研究人员对两只雄性团翅侏儒鸟（Club-winged Manakin，学名*Machaeropterus deliciosus*）的翅膀进行显微CT（microCT）扫描后，发现了陆生脊椎动物中空管状骨演化规则的一个显著例外：该雄性个体的尺骨（ulna）呈实心状态，其体积是体型相近鸟类尺骨的3倍以上，且组织矿化密度显著更高。肱骨也呈现出类似的（但程度稍弱的）结构改造。所有观测到的骨骼结构改造均会增加骨骼的整体质量，这与鸟类为适应飞行而普遍演化出的减重优化策略背道而驰。团翅侏儒鸟之名源于成年雄性个体特有的一对翅羽：这些膨大的翅羽直接附着于尺骨之上，可通过共振产生独特的鸣响，用于求偶展示。鉴于观测到的骨骼改造或有助于发声，团翅侏儒鸟便成为一个典型案例：由雌性选择介导的性选择塑造了一种在生态层面"代价高昂"的前肢形态，其独特之处在于，该形态特化为发声功能，但却以牺牲飞行效率为代价。