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（tetrapods）普遍具备的形态学特征之一，是四肢长骨呈管状且通常充满骨髓。这种"中空"结构能够以最少的骨组织用量实现抗弯强度与刚度的最大化，诸多现代鸟类的充气肱骨（pneumatic humeri，即真正中空、充满空气的骨骼）便是该特征的典型范例。研究人员对两只雄性团翅侏儒鸟（Club-winged Manakin，学名*Machaeropterus deliciosus*）的翅膀开展高分辨率显微CT（microCT）扫描后，发现了陆生脊椎动物中空管状骨骼规则的一处显著例外：该物种雄性的尺骨（ulna，复数形式ulnae）呈实心结构，体积为同等体型鸟类尺骨的3倍以上，且组织矿物密度显著更高。其肱骨也呈现出类似但程度稍弱的形态特化。每一处观测到的骨骼形态特化（osteological modifications）都会提升骨骼整体质量，这与鸟类为适配飞行而普遍演化出的减重优化策略背道而驰。团翅侏儒鸟的命名源自成年雄性特有的一对翅羽：这些膨大的翅羽直接附着于尺骨之上，可通过共振产生用于求偶炫耀的独特声响。鉴于本次观测到的形态特化可能有助于发声，团翅侏儒鸟便成为了由雌性选择介导的性选择（sexual selection by female choice）塑造出生态层面"代价高昂"前肢形态的典型案例：该形态特化专门用于发声，却以牺牲飞行效能为代价，在鸟类中独树一帜。