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.

四足动物的一项普遍形态特征，是四肢长骨呈管状结构，且通常内部充满骨髓。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，诸多现代鸟类的肱骨（humerus）即为真正中空（充气）的气腔化骨骼，是该特征的典型范例。对两只雄性斑翅侏儒鸟（Club-winged Manakin，学名Machaeropterus deliciosus）翅膀开展的高分辨率显微CT（microCT）扫描，发现了陆生脊椎动物中空管状骨骼规则的一个显著例外：雄性个体的尺骨（ulna）呈实心结构，体积达到同等体型鸟类的3倍以上，且组织矿化密度显著更高。肱骨也呈现出类似（但程度稍轻）的结构改造。每一处观测到的骨骼结构改造都会提升骨骼整体质量，这与鸟类为适应飞行而普遍采用的减重优化策略背道而驰。斑翅侏儒鸟因成年雄性特有的一对翼部羽毛得名：这些膨大的羽毛直接附着于尺骨之上，通过共振产生独特声响，用于求偶炫耀。鉴于观测到的骨骼改造或可辅助声音产生，斑翅侏儒鸟便是雌性选择介导的性选择造就了生态层面“代价高昂”的前肢形态的典型案例：其前肢特化用于声音产生，而这是以牺牲飞行效率为代价的，该特征在鸟类中独一无二。