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1. A Shrewd Inspection of Vertebral Regionalization in Large Shrews (Soricidae: Crocidurinae)

   https://doi.org/10.1093/iob/obac006  

   Smith, Stephanie M. ; Angielczyk, Kenneth D. ( February 2022 , Integrative Organismal Biology)

   The regionalization of the mammalian spinal column is an important evolutionary, developmental, and functional hallmark of the clade. Vertebral column regions are usually defined using transitions in external bone morphology, such as the presence of transverse foraminae or rib facets, or measurements of vertebral shape. Yet the internal structure of vertebrae, specifically the trabecular (spongy) bone, plays an important role in vertebral function, and is subject to the same variety of selective, functional, and developmental influences as external bone morphology. Here, we investigated regionalization of external and trabecular bone morphology in the vertebral column of a group of shrews (family Soricidae). The primary goals of this study were to: (1) determine if vertebral trabecular bone morphology is regionalized in large shrews, and if so, in what configuration relative to external morphology; (2) assess correlations between trabecular bone regionalization and functional or developmental influences; and (3) determine if external and trabecular bone regionalization patterns provide clues about the function of the highly modified spinal column of the hero shrew Scutisorex. Trabecular bone is regionalized along the soricid vertebral column, but the configuration of trabecular bone regions does not match that of the external vertebral morphology, and is less consistent across individuals and species. The cervical region has the most distinct and consistent trabecular bone morphology, with dense trabeculae indicative of the ability to withstand forces in a variety of directions. Scutisorex exhibits an additional external morphology region compared to unmodified shrews, but this region does not correspond to a change in trabecular architecture. Although trabecular bone architecture is regionalized along the soricid vertebral column, and this regionalization is potentially related to bone functional adaptation, there are likely aspects of vertebral functional regionalization that are not detectable using trabecular bone morphology. For example, the external morphology of the Scutisorex lumbar spine shows signs of an extra functional region that is not apparent in trabecular bone analyses. It is possible that body size and locomotor mode affect the degree to which function is manifest in trabecular bone, and broader study across mammalian size and ecology is warranted to understand the relationship between trabecular bone morphology and other measures of vertebral function such as intervertebral range of motion.

    

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2. Vertebral number and spinal regionalization in large shrews (Soricidae)

   Smith, SM ; Angielczyk, KD ; Kerbis Peterhans, JC ( March 2020 , Integrative and comparative biology)

   In addition to having unique extra articulations on its vertebrae, the hero shrew (Scutisorex) is unusual in having almost twice as many lumbar vertebrae as other shrews of its size. Other than being noted in descriptive literature, this increase in vertebral number has received little attention; there has been no investigation of how it might reflect the elusive function of the highly modified Scutisorex spine. Comparisons of individual vertebrae and whole-column characteristics between Scutisorex and other large shrews are also lacking, despite the fact that such studies could give insight into i) function of particular vertebral regions in shrews with and without external vertebral modifications, and ii) developmental patterns driving regional proportions. We collected μCT scans and linear measurements of cervical, thoracic, and lumbar vertebrae in two species of Scutisorex and three other species of large shrews. We compared a variety of linear vertebra measurements, and trabecular bone characteristics of each centrum, across species. Further, using this combined suite of measurements, we executed principal coordinates analysis and segmented regression to detect unique vertebral regions in each taxon. Our results show that relative to other large shrews, Scutisorex has a shorter thoracic region and longer lumbar region, and, despite having more dorsal vertebrae than other species, does not have a proportionally longer body length. Regionalization signals vary within and across the five species, but generally suggest that functional regions may not correspond exactly with traditionally recognized anatomical regions of the column, and that the extended lumbar region in Scutisorex may afford it an additional functional region. 

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3. Deciphering an extreme morphology: bone microarchitecture of the hero shrew backbone (Soricidae: Scutisorex )

   https://doi.org/10.1098/rspb.2020.0457  

   Smith, Stephanie M. ; Angielczyk, Kenneth D. ( May 2020 , Proceedings of the Royal Society B: Biological Sciences)

   Biological structures with extreme morphologies are puzzling because they often lack obvious functions and stymie comparisons to homologous or analogous features with more typical shapes. An example of such an extreme morphotype is the uniquely modified vertebral column of the hero shrew Scutisorex , which features numerous accessory intervertebral articulations and massively expanded transverse processes. The function of these vertebral structures is unknown, and it is difficult to meaningfully compare them to vertebrae from animals with known behavioural patterns and spinal adaptations. Here, we use trabecular bone architecture of vertebral centra and quantitative external vertebral morphology to elucidate the forces that may act on the spine of Scutisorex and that of another large shrew with unmodified vertebrae ( Crocidura goliath ). X-ray micro-computed tomography (µCT) scans of thoracolumbar columns show that Scutisorex thori is structurally intermediate between C. goliath and S. somereni internally and externally, and both Scutisorex species exhibit trabecular bone characteristics indicative of higher in vivo axial compressive loads than C. goliath. Under compressive load, Scutisorex vertebral morphology is adapted to largely restrict bending to the sagittal plane (flexion). Although these findings do not solve the mystery of how Scutisorex uses its byzantine spine in vivo , our work suggests potentially fruitful new avenues of investigation for learning more about the function of this perplexing structure. 

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4. Vertebral Trabecular Bone Mechanical Properties Vary Among Functional Groups of Cetaceans

   https://doi.org/10.1093/iob/obab036  

   Ingle, D. N. ; Porter, M. E. ( January 2022 , Integrative Organismal Biology)

   Since their appearance in the fossil record 34 million years ago, modern cetaceans (dolphins, whales, and porpoises) have radiated into diverse habitats circumglobally, developing vast phenotypic variations among species. Traits such as skeletal morphology and ecologically linked behaviors denote swimming activity; trade-offs in flexibility and rigidity along the vertebral column determine patterns of caudal oscillation. Here, we categorized 10 species of cetaceans (families Delphinidae and Kogiidae; N = 21 animals) into functional groups based on vertebral centra morphology, swimming speeds, diving behavior, and inferred swimming patterns. We quantified trabecular bone mechanical properties (yield strength, apparent stiffness, and resilience) among functional groups and regions of the vertebral column (thoracic, lumbar, and caudal). We extracted 6 mm3 samples from vertebral bodies and tested them in compression in 3 orientations (rostrocaudal, dorsoventral, and mediolateral) at 2 mm min−1. Overall, bone from the pre-fluke/fluke boundary had the greatest yield strength and resilience, indicating that the greatest forces are translated to the tail during caudal oscillatory swimming. Group 1, composed of 5 shallow-diving delphinid species, had the greatest vertebral trabecular bone yield strength, apparent stiffness, and resilience of all functional groups. Conversely, Group 3, composed of 2 deep-diving kogiid species, had the least strong, stiff, and resilient bone, while Group 2 (3 deep-diving delphinid species) exhibited intermediate values. These data suggest that species that incorporate prolonged glides during deep descents in the water column actively swim less, and place relatively smaller loads on their vertebral columns, compared with species that execute shallower dives. We found that cetacean vertebral trabecular bone properties differed from the properties of terrestrial mammals; for every given bone strength, cetacean bone was less stiff by comparison. This relative lack of material rigidity within vertebral bone may be attributed to the non-weight-bearing locomotor modes of fully aquatic mammals.

    

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5. Zoo Versus Wild: Trabecular Bone Architecture in Captive and Wild Xenarthra

   Zack, EH ; Smith, SM ; Angielczyk, KD ( March 2020 , Integrative and comparative biology)

   Captive (zoo) specimens in natural history collections allow researchers to inspect the morphologies of rare or CITES-listed taxa, but the lifestyles, diets, and lifespans of captive animals differ from those of their wild counterparts. To quantify these differences, we compared trabecular bone architecture (TBA) of dorsal vertebrae in captive and wild specimens of xenarthran mammals (anteaters, armadillos, and sloths). Because TBA develops following in-vivo bone force regimes, it reflects ecology and behavior, but this also means that it may differ between captive and wild specimens of the same species. We collected μCT scans of the last six presacral vertebrae in 15 species of fossorial, terrestrial, and suspensorial xenarthrans ranging in body mass from 120g (Chlamyphorus) to 35kg (Myrmecophaga). For each vertebra, we measured bone volume fraction (BVF), trabecular number (TbN), mean trabecular thickness (TbTh), degree of anisotropy, and trabecular orientation. We found that wild specimens generally have a greater BVF, TbN, and TbTh than captive specimens, but that these metrics differ by species, vertebral position, ecology, and pathology. Wild specimens of Dasypus have greater BVF, TbN, and TbTh than captive specimens in the three most posterior lumbar vertebrae, but have much closer metrics in the anterior three vertebrae. In Choloepus, BVF, TbN, and TbTn are greater in wild specimens in the anterior vertebrae and more similar in the posterior vertebrae. Arthritis in captive Tamandua increased BVF and TbTh, whereas wild specimens had greater TbN. Our results add to overall understanding of variation in mammalian vertebral trabecular bone, and suggest caution when including captive specimens in research on the relationship between TBA and ecology. 

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