Department of Ecology and Evolutionary Biology, Cornell University Museum of Vertebrates, Cornell University, Ithaca, NY 14850, USA.
Biol Lett. 2012 Oct 23;8(5):760-3. doi: 10.1098/rsbl.2012.0382. Epub 2012 Jun 13.
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 more than three times 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.
四足动物的一个普遍形态特征是管状的，通常充满骨髓的，肢体或长骨的结构。这种“空心”形式最大限度地提高了弯曲强度和刚性，同时使用的骨材料最少，许多现代鸟类中真正的空心（充气）或充气的肱骨就是这种形式的例子。对两只雄性阔嘴凤头鹦鹉（Machaeropterus deliciosus）翅膀的高分辨率微 CT 扫描揭示了一个陆地脊椎动物中空管规则的显著例外；雄性的尺骨比体型相当的鸟类的尺骨凝固，体积大 3 倍以上，组织矿物质密度显著更高。肱骨也表现出类似的（但不那么极端）的变化。观察到的每一种骨骼变化都增加了骨骼的整体质量，与鸟类为飞行而进行的普遍减重优化背道而驰。阔嘴凤头鹦鹉因其在成年雄性中发现的一对独特的翅膀羽毛而得名；这些羽毛直接附着在尺骨上，通过共振产生一种独特的声音，用于求偶展示。鉴于观察到的变化可能有助于产生声音，阔嘴凤头鹦鹉代表了一种由雌性选择的性选择产生的生态“昂贵”前肢形态的情况，其独特之处在于专门用于声音产生，而在飞行效率方面可能存在代价。
