Clifton Glenna T, Carr Jennifer A, Biewener Andrew A
Concord Field Station, Harvard University, Bedford, MA, USA.
Department of Biology, Tufts University, Medford, MA, USA.
J Anat. 2018 Jan;232(1):105-123. doi: 10.1111/joa.12710. Epub 2017 Nov 3.
Several groups of birds have convergently evolved the ability to swim using their feet despite facing trade-offs with walking. However, swimming relative to terrestrial performance varies across these groups. Highly specialized divers, such as loons and grebes, excel at swimming underwater but struggle to stand on land, whereas species that primarily swim on the water surface, such as Mallards, retain the ability to move terrestrially. The identification of skeletal features associated with a swimming style and conserved across independent groups suggests that the hindlimb of foot-propelled swimming birds has adapted to suit the physical challenges of producing propulsive forces underwater. But in addition to skeletal features, how do hindlimb muscles reflect swimming ability and mode? This paper presents the first comparative myology analysis associated with foot-based swimming. Our detailed dissections of 35 specimens representing eight species reveal trends in hindlimb muscle size and attachment location across four independent lineages of extant swimming birds. We expand upon our dissections by compiling data from historical texts and provide a key to any outdated muscle nomenclature used in these sources. Our results show that highly diving birds tuck the femur and proximal tibiotarsus next to the ribcage and under the skin covering the abdomen, streamlining the body. Several hindlimb muscles exhibit dramatic anatomical variation in diving birds, including the flexor cruris lateralis (FCL) and iliofibularis (IF), which reduce in size and shift distally along the tibiotarsus. The femorotibialis medius (FTM) extends along an expanded cnemial crest. The resulting increased moment arms of these muscles likely help stabilize the hip and knee while paddling. Additionally, distal ankle plantarflexors, including the gastrocnemius and digital flexors, are exceptionally large in diving birds in order to power foot propulsion. These patterns exist within distantly related lineages of diving birds and, to a lesser extent, in surface swimmers. Together, our findings verify conserved muscular adaptations to a foot-propelled swimming lifestyle. The association of muscle anatomy with skeletal features and biomechanical movement demands can inform functional interpretation of fossil birds and reveal selective pressures underlying avian diversification.
尽管在行走方面面临权衡，但有几组鸟类已经趋同进化出了用脚游泳的能力。然而，相对于陆地运动能力而言，这些鸟类群体的游泳能力各不相同。高度特化的潜水鸟类，如潜鸟和䴙䴘，擅长在水下游泳，但在陆地上站立困难，而主要在水面游泳的物种，如绿头鸭，仍保留在陆地上移动的能力。识别与游泳方式相关且在不同独立群体中保守的骨骼特征表明，用脚推进游泳的鸟类的后肢已经适应了应对在水下产生推进力的物理挑战。但除了骨骼特征外，后肢肌肉如何反映游泳能力和方式呢？本文首次呈现了与基于脚的游泳相关的比较肌学分析。我们对代表八个物种的35个标本进行了详细解剖，揭示了现存游泳鸟类四个独立谱系后肢肌肉大小和附着位置的趋势。我们通过汇编历史文献中的数据来扩展解剖研究，并为这些文献中使用的任何过时的肌肉命名法提供了关键信息。我们的结果表明，高度潜水的鸟类会将股骨和近端胫跗骨紧靠胸腔并置于覆盖腹部的皮肤下方，使身体更加流线型。几种后肢肌肉在潜水鸟类中表现出显著的解剖学差异，包括外侧屈肌（FCL）和髂腓肌（IF），它们的尺寸减小并沿着胫跗骨向远端移动。股胫中肌（FTM）沿着扩大的胫嵴延伸。这些肌肉由此增加的力臂可能有助于在划水时稳定髋部和膝盖。此外，包括腓肠肌和趾屈肌在内的远端踝关节跖屈肌在潜水鸟类中异常大，以便为足部推进提供动力。这些模式存在于亲缘关系较远的潜水鸟类谱系中，在水面游泳者中程度较轻。总之，我们的研究结果证实了对用脚推进游泳生活方式的保守肌肉适应。肌肉解剖结构与骨骼特征和生物力学运动需求之间的关联可以为化石鸟类的功能解释提供信息，并揭示鸟类多样化背后的选择压力。
