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）普遍存在的一类形态特征，是四肢骨或长骨呈管状、通常充满骨髓的结构。这种“中空”构型以最少的骨组织用量实现抗弯强度（flexural strength）与刚度（stiffness）的最大化，多数现代鸟类的气腔肱骨（pneumatic humeri，即充满空气的中空肱骨）便是该结构的典型代表。研究人员对两只雄性斑翅娇鹟（Club-winged Manakins，*Machaeropterus deliciosus*）的翅膀进行高分辨率显微CT扫描（microCT scans）后，发现了陆生脊椎动物中空管状骨规则的一处显著例外：该物种雄性个体的尺骨（ulnae）体积达到同等体型鸟类的3倍以上，且组织矿物密度（tissue mineral densities）显著更高；肱骨也呈现出类似但程度更缓和的特化改造。每一种观测到的骨骼特化（osteological modifications）都会提升骨骼整体质量，这与鸟类为适配飞行而普遍存在的减重优化方向截然相反。斑翅娇鹟因成年雄性特有的一对翼羽而得名：这些膨大的羽毛直接附着于尺骨之上，通过共振产生用于求偶炫耀（courtship displays）的独特声响。鉴于本次观测到的骨骼特化可能有助于声音产生，斑翅娇鹟便成为了雌性选择介导的性选择造就生态“高成本”前肢形态的典型案例——其前肢特化用于声音产生，且以牺牲飞行效率（flight efficiency）为代价，这在鸟类中独一无二。