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.
Trabecular bone is modelled throughout an animal’s life in response to its mechanical environment, but like other skeletal anatomy, it is also subject to evolutionary influences. Yet the relative strengths of factors that affect trabecular bone architecture are little studied. We investigated these influences across the Philippine endemic murine rodent clade Chrotomyini. These mammals have robustly established phylogenetic relationships, exhibit a range of well-documented substrate-use types, and have a body size range spanning several hundred grammes, making them ideal for a tractable study of extrinsic and intrinsic influences on trabecular bone morphology. We found slight differences in vertebral trabecular bone among different substrate-use categories, with more divergent characteristics in more ecologically specialized taxa. This suggests that the mechanical environment must be relatively extreme to affect trabecular bone morphology in small mammals. We also recovered allometric patterns that imply that selective pressures on bone may differ between small and large mammals. Finally, we found high intrataxonomic variation in trabecular bone morphology, but it is not clearly related to any variable we measured, and may represent a normal degree of variation in these animals rather than a functional trait. Future studies should address how this plasticity affects biomechanical properties and performance of the skeleton.
more » « less- Award ID(s):
- 2010756
- NSF-PAR ID:
- 10502119
- Publisher / Repository:
- The Linnean Society
- Date Published:
- Journal Name:
- Biological Journal of the Linnean Society
- Volume:
- 140
- Issue:
- 1
- ISSN:
- 0024-4066
- Page Range / eLocation ID:
- 1 to 25
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Objectives Trabecular bone adapts to the strains placed upon the skeleton during life. Anthropological research has largely focused on linking variation in primate trabecular bone to locomotor mode, to provide a context for interpreting fossil morphology. However, intraspecific variation and its underlying mechanisms are still poorly understood. Trabecular bone is influenced by a variety of factors including body mass, age, diet, temperature, genetics, sex, and behavior. Before trabecular structure can be used to infer habitual behavior in the past, the effects of these factors need to be understood. In this article, we examine variation in trabecular structure in the human foot in four archaeological groups in relation to inferred levels of terrestrial mobility and sex.
Materials and methods We use high‐resolution μCT scanning to examine variation in trabecular structure in the human calcaneus, talus, and first metatarsal in two relatively mobile and two relatively sedentary archaeological groups.
Results The four population samples show similar patterns of trabecular variation throughout the foot, influenced by mechanical loading. Greater inferred terrestrial mobility is associated with greater bone volume fraction and thicker, more widely spaced, and less interconnected trabeculae. However, contrary to diaphyseal rigidity, only limited sexual dimorphism was found in trabecular structure.
Discussion This work demonstrates that trabecular bone may serve as a useful proxy of habitual behavior in the fossil and archaeological record when other factors are carefully considered. However, the mechanisms underlying sexual dimorphism are not well understood. As such, inferring sex differences in habitual behavior is currently challenging.
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The regionalized vertebral column is a hallmark of mammalian morphology and reflects functional differentiation of the vertebral regions. Mammalian vertebrae are serially homologous and morphologically patterened by Hox expression, but also vary in number and gross morphology across species. The trabecular bone inside vertebral centra is more plastic than gross vertebral bone, and structurally adapts to better withstand forces it experiences during life. However, the functional regionalization of vertebral trabecular bone is poorly examined. Are there trabecular "regions” reflecting the differing functions and in-vivo stress patterns of gross morphological vertebral regions? Or is trabecular morphology homogeneous throughout the spine, suggesting that differences in functional demands are borne exclusively by external characteristics? To address these questions, we collected μCT scans and linear measurements of cervical, thoracic, and lumbar vertebrae in four species of large shrews, including two species of the hero shrew Scutisorex, which has a highly modified vertebral column. We compared linear measurements and trabecular bone characteristics of the cranial and caudal ends of each centrum across species. To detect unique vertebral regions, we executed principal coordinates analysis and segmented regression on three versions of our data set: trabecular bone data only, external measurements only, and the two combined. We found that some regionalization is recovered using only trabecular bone data, but trabecular bone regions do not correspond exactly to gross vertebral regions. This reflects divergence between the functional signals of internal and external vertebral bone morphology, which should be further examined in a kinematic context.more » « less
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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.more » « less
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null (Ed.)Captive specimens in museum collections facilitate study of rare taxa, but the lifestyles, diets, and lifespans of captive animals differ from their wild counterparts. Trabecular bone architecture adapts to in vivo forces, and may reflect interspecific variation in ecology and behavior as well as intraspecific variation between captive and wild specimens. We compared trunk vertebrae bone microstructure in captive and wild xenarthran mammals to test the effects of ecology and captivity. We collected μCT scans of the last six presacral vertebrae in 13 fossorial, terrestrial, and suspensorial xenarthran species (body mass: 120 g to 35 kg). For each vertebra, we measured centrum length; bone volume fraction (BV.TV); trabecular number and mean thickness (Tb.Th); global compactness (GC); cross-sectional area; mean intercept length; star length distribution; and connectivity and connectivity density. Wild specimens have more robust trabeculae, but this varies with species, ecology, and pathology. Wild specimens of fossorial taxa (Dasypus) have more robust trabeculae than captives, but there is no clear difference in bone microstructure between wild and captive specimens of suspensorial taxa (Bradypus, Choloepus), suggesting that locomotor ecology influences the degree to which captivity affects bone microstructure. Captive Tamandua and Myrmecophaga have higher BV.TV, Tb.Th, and GC than their wild counterparts due to captivity-caused bone pathologies. Our results add to the understanding of variation in mammalian bone microstructure, suggest caution when including captive specimens in bone microstructure research, and indicate the need to better replicate the habitats, diets, and behavior of animals in captivity.more » « less
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1093/biolinnean/blad033
