Birds (Aves) exhibit exceptional and diverse locomotor behaviors, including the exquisite ability to balance on two feet. How birds so precisely control their movements may be partly explained by a set of intriguing modifications in their lower spine. These modifications are collectively known as the lumbosacral organ (LSO) and are found in the fused lumbosacral vertebrae called the synsacrum. They include a set of transverse canal-like recesses in the synsacrum that align with lateral lobes of the spinal cord, as well as a dorsal groove in the spinal cord that houses an egg-shaped glycogen body. Based on compelling but primarily observational data, the most recent functional hypotheses for the LSO consider it to be a secondary balance organ, in which the transverse canals are analogous to the semicircular canals of the inner ear. If correct, this hypothesis would reshape our understanding of avian locomotion, yet the LSO has been largely overlooked in the recent literature. Here, we review the current evidence for this hypothesis and then explore a possible relationship between the LSO and balance-intensive locomotor ecologies. Our comparative morphological dataset consists of micro-computed tomography (μ-CT) scans of synsacra from ecologically diverse species. We find that birds that perch tend to have more prominent transverse canals, suggesting that the LSO is useful for balance-intensive behaviors. We then identify the crucial outstanding questions about LSO structure and function. The LSO may be a key innovation that allows independent but coordinated motion of the head and the body, and a full understanding of its function and evolution will require multiple interdisciplinary research efforts.

中文翻译：

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禽腰lum器官平衡假说及其形态变异探讨

鸟类（Aves）具有异常多样的运动行为，包括精巧的两只脚平衡能力。鸟类如何如此精确地控制其运动可以部分通过其下脊柱的一系列有趣的修饰来解释。这些修饰统称为腰s器官（LSO），在称为腰s骨的融合腰s椎骨中发现。它们包括在突骨中与脊髓的侧叶对齐的一组横向运河状凹口，以及在脊髓中容纳卵形糖原体的背侧凹槽。根据引人注目的但主要是观察性数据，LSO的最新功能假设认为其是次要平衡器官，其中横向管类似于内耳的半圆形管。如果正确的话 这个假设将重塑我们对鸟类运动的理解，但是LSO在最近的文献中已被大大忽略。在这里，我们回顾了这一假设的当前证据，然后探讨了LSO与平衡密集型运动生态之间的可能关系。我们的比较形态学数据集包括来自生态多样性物种的合成骨的微计算机断层扫描（μ-CT）扫描。我们发现，栖息的鸟类倾向于具有更突出的横向运河，这表明LSO对于平衡密集型行为很有用。然后，我们确定有关LSO结构和功能的关键悬而未决的问题。LSO可能是一项关键创新，它可以实现头部和身体的独立但协调的运动，