Data for Journal of Anatomy article: Comparative morphology and soft tissue histology of the remote-touch bill-tip organ in three ibis species of differing foraging ecology

Ibises (order: Pelecaniformes, family: Threskiornithidae) are probe‐foraging birds that use ‘remote‐touch’ to locate prey items hidden in opaque substrates. This sensory capability allows them to locate their prey using high‐frequency vibrations in the substrate in the absence of other sensory cues. Remote‐touch is facilitated by a specialised bill‐tip organ, comprising high densities of mechanoreceptors (Herbst corpuscles) embedded in numerous foramina in the beak bones. Each foramen and its associated Herbst corpuscles make up a sensory unit, called a ‘sensory pit’. These sensory pits are densely clustered in the distal portion of the beak. Previous research has indicated that interspecific differences in the extent of sensory pitting in the beak bones correlate with aquatic habitat use of ibises, and have been suggested to reflect different levels of remote‐touch sensitivity. Our study investigates the interspecific differences in the bone and soft tissue histology of the bill‐tip organs of three species of southern African ibises from different habitats (mainly terrestrial to mainly aquatic). We analysed the external pitting pattern on the bones, as well as internal structure of the beak using micro‐CT scans and soft tissue histological sections of each species' bill‐tip organs. The beaks of all three species contain remote‐touch bill‐tip organs and are described here in detail. Clear interspecific differences are evident between the species' bill‐tip organs, both in terms of bone morphology and soft tissue histology. Glossy Ibises, which forage exclusively in wetter substrates, have a greater extent of pitting but lower numbers of Herbst corpuscles in each pit, while species foraging in drier substrates (Hadeda and Sacred Ibises) have more robust beaks, fewer pits and higher densities of Herbst corpuscles. Our data, together with previously published histological descriptions of the bill‐tip organs of other remote‐touch foraging bird species, indicate that species foraging in drier habitats have more sensitive bill‐tip organs (based on their anatomy). The vibrations produced by prey (e.g., burrowing invertebrates) travel poorly in dry substrates compared with wetter ones (i.e., dry soil vs. mud or water), and thus we hypothesise that a more sensitive bill‐tip organ may be required to successfully locate prey in dry substrates. Furthermore, our results indicate that the differences in bill‐tip organ anatomy between the species reflect complex trade‐offs between morphological constraints of beak shape and remote‐touch sensitivity requirements, both of which are likely related to each species' foraging behaviour and substrate usage. Our study suggests that structures in the bone of the bill‐tip organ could provide valuable osteological correlates for the associated soft tissues, and consequently may provide information on the sensory ecology and habitat usage of the birds in the absence of soft tissues. This study describes interspecific differences in the anatomy of the remote‐touch bill‐tip organ in ibises which correlate with the birds' foraging habitats. Our results indicate that species foraging in drier substrates have more sensitive bill‐tip organs, and that several structures in the beak bones may be valuable osteological correlates for the associated soft tissues and consequently for the birds' foraging behaviour and sensory ecology.

鹈鹕科（order: Pelecaniformes, family: Threskiornithidae）的白鹳为探查式觅食鸟类，它们运用‘远距离触觉’来定位隐藏在不透明基质中的猎物。这种感官能力使它们能够在缺乏其他感官线索的情况下，通过基质中的高频振动来发现猎物。远距离触觉得以通过一种特殊的喙端器官实现，该器官由大量嵌入喙骨众多孔隙中的机械感受器（Herbst 纤维球）组成。每个孔隙及其相关的 Herbst 纤维球共同构成一个感官单元，称为‘感官凹坑’。这些感官凹坑在喙的远端部分密集分布。先前的研究表明，喙骨上感官凹坑的种间差异与白鹳的栖息地利用（主要是陆地至主要是水域）相关，并被认为是反映不同远程触觉敏感度的指标。我们的研究调查了来自不同栖息地（主要是陆地至主要是水域）的南非三种白鹳喙端器官的骨和软组织组织学差异。我们分析了骨骼表面的外部凹坑模式，以及使用微CT扫描和软组织组织切片研究了每个物种喙端器官的内部结构。所有三种物种的喙都含有远程触觉喙端器官，并在此详细描述。物种间喙端器官在骨骼形态和软组织组织学方面均存在明显的种间差异。在湿润基质中专门觅食的闪光鹳，其凹坑程度更高，但每个凹坑中的 Herbst 纤维球数量较少，而在干燥基质中觅食的物种（如黑鹳和圣鹳）则具有更为坚固的喙，凹坑较少，Herbst 纤维球的密度更高。我们的数据，连同先前发表的关于其他远程触觉觅食鸟类物种喙端器官的组织学描述，表明在干燥栖息地觅食的物种具有更为敏感的喙端器官（基于其解剖结构）。猎物（如挖掘的无脊椎动物）产生的振动在干燥基质中的传播比在湿润基质中差（例如，干土与泥或水相比），因此我们假设在干燥基质中成功定位猎物可能需要一个更为敏感的喙端器官。此外，我们的结果还表明，物种间喙端器官解剖结构的不同反映了喙形态的形态学限制与远程触觉敏感度需求之间的复杂权衡，这两者很可能与每种物种的觅食行为和基质使用有关。我们的研究表明，喙端器官的骨骼结构可能为相关的软组织提供有价值的骨学相关特征，从而可能为鸟类在缺乏软组织的情况下提供有关其感官生态学和栖息地利用的信息。本研究描述了白鹳远程触觉喙端器官的种间差异，这些差异与鸟类的觅食栖息地相关。我们的结果表明，在干燥基质中觅食的物种具有更为敏感的喙端器官，并且喙骨中的几个结构可能对相关的软组织以及鸟类的觅食行为和感官生态学具有价值。