Livezey Bradley C, Humphrey Philip S
Museum of Natural History and Department of Systematics and Ecology, University of Kansas, Lawrence, KS, 66045.
Evolution. 1986 May;40(3):540-558. doi: 10.1111/j.1558-5646.1986.tb00506.x.
Flightlessness in Tachyeres is caused by wing-loadings in excess of 2.5 g·cm , which result from the large body size and small wing areas of the flightless species. Reduced wing areas of flightless species are related to absolutely shorter remiges, and to relatively or absolutely shortened wing bones, although these reductions differ among species. Reduced lengths of the ulna, radius, and carpometacarpus are associated most strongly with flightlessness. Pectoral muscles and the associated sternal keel are well developed in all species of Tachyeres, largely because of the use of wings in "steaming," an important locomotor behavior. Relative size of these muscles was greatest in largely flighted T. patachonicus; however, sexual dimorphism in wing-loadings results in flightlessness in some males of this species. Proportions in the wing skeleton, intraspecific allometry, and limited data on growth indicate that the relatively short wing bones and remiges of flightless Tachyeres are produced developmentally by a delay in the growth of wing components, and that this heterochrony may underlie, in part, skeletal sexual dimorphism. Increased body size in flightless steamer-ducks is advantageous in territorial defense of food resources and young, and perhaps diving in cold, turbulent water; reductions in wing area probably reflect refinements for wing-assisted locomotion and combat. Flightlessness in steamer-ducks is not related to relaxed predation pressure, but instead was permitted selectively by the year-round habitability of the southern South American coasts. These conditions not only permitted the success of the three flightless species of Tachyeres, but at present may be moving marine populations of T. patachonicus toward flightlessness.
南美硬尾鸭的飞行能力丧失是由超过2.5克·厘米的翼负荷引起的，这是由于不会飞的物种体型大而翼面积小。不会飞的物种翼面积减小与初级飞羽绝对长度较短以及翼骨相对或绝对缩短有关，尽管这些减少在不同物种间存在差异。尺骨、桡骨和腕掌骨长度的缩短与飞行能力丧失的关联最为紧密。在所有南美硬尾鸭物种中，胸肌和相关的胸骨龙骨都很发达，这主要是因为在“划水”这种重要的运动行为中翅膀的使用。这些肌肉的相对大小在主要会飞的巴塔哥尼亚硬尾鸭中最大；然而，翼负荷的性别二态性导致该物种的一些雄性丧失飞行能力。翼骨骼的比例、种内异速生长以及有限的生长数据表明，不会飞的南美硬尾鸭相对较短的翼骨和初级飞羽是由翼部组件生长延迟在发育上产生的，这种异时性可能部分是骨骼性别二态性的基础。不会飞的硬尾鸭体型增大有利于在食物资源和幼雏的领地防御中，或许也有利于在寒冷、湍急的水中潜水；翼面积的减小可能反映了对翼辅助运动和战斗的优化。硬尾鸭的飞行能力丧失与捕食压力的减轻无关，而是由南美洲南部海岸全年适宜居住的环境选择性地促成的。这些条件不仅使三种不会飞的南美硬尾鸭物种得以成功，而且目前可能正使巴塔哥尼亚硬尾鸭的海洋种群走向飞行能力丧失。
