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Showing papers in "Journal of Morphology in 2008"


Journal ArticleDOI
TL;DR: Ecomorphological analyses of various locomotor types revealed consistent differences in postcranial skeletal morphology that relate to functionally important traits, suggesting that the method could be applied to extinct rodents, regardless of ancestry, to accurately infer their locomotor ecologies.
Abstract: Living rodents show great diversity in their locomotor habits, including semiaquatic, arboreal, fossorial, ricochetal, and gliding species from multiple families. To assess the association between limb morphology and locomotor habits, the appendicular skeletons of 65 rodent genera from 16 families were measured. Ecomorphological analyses of various locomotor types revealed consistent differences in postcranial skeletal morphology that relate to functionally important traits. Behaviorally similar taxa showed convergent morphological characters, despite distinct evolutionary histories. Semiaquatic rodents displayed relatively robust bones, enlarged muscular attachments, short femora, and elongate hind feet. Arboreal rodents had relatively elongate humeri and digits, short olecranon processes of the ulnae, and equally proportioned fore and hind limbs. Fossorial rodents showed relatively robust bones, enlarged muscular attachments, short antebrachii and digits, elongate manual claws, and reduced hind limb elements. Ricochetal rodents displayed relatively proximal insertion of muscles, disproportionate limbs, elongate tibiae, and elongate hind feet. Gliding rodents had relatively elongate and gracile bones, short olecranon processes of the ulnae, and equally proportioned fore and hind limbs. The morphological differences observed here can readily be used to discriminate extant rodents with different locomotor strategies. This suggests that the method could be applied to extinct rodents, regardless of ancestry, to accurately infer their locomotor ecologies. When applied to an extinct group of rodents, we found two distinct ecomorphs represented in the beaver family (Castoridae), semiaquatic and semifossorial. There was also a progressive trend toward increased body size and increased aquatic specialization in the giant beaver lineage (Castoroidinae). J. Morphol., 2008. © 2008 Wiley-Liss, Inc.

225 citations


Journal ArticleDOI
TL;DR: It is shown that biomechanical stress and the hydrodynamic properties of the skull show a strong relationship with the distribution of crocodilians in skull morphospace, whereas phylogeny and biogeography show weak or no correlation.
Abstract: Extant and fossil crocodilians have long been divided into taxonomic and/or ecological groups based on broad patterns of skull shape, particularly the relative length and width of the snout. However, these patterns have not been quantitatively analyzed in detail, and their biomechanical and functional implications are similarly understudied. Here, we use geometric morphometrics and finite element analysis to explore the patterns of variation in crocodilian skull morphology and the functional implications of those patterns. Our results indicate that skull shape variation in extant crocodiles is much more complex than previously recognized. Differences in snout length and width are the main components of shape variation, but these differences are correlated with changes in other regions of the skull. Additionally, there is considerable disparity within general classes such as longirostrine and brevirostrine forms. For example, Gavialis and Tomistoma occupy different parts of morphospace implying a significant difference in skull shape, despite the fact that both are traditionally considered longirostrine. Skull length and width also strongly influence the mechanical performance of the skull; long and narrow morphotypes (e.g., Tomistoma) experience the highest amount of stress during biting, whereas short and broad morphotypes (e.g., Caiman latirostris) experience the least amount of stress. Biomechanical stress and the hydrodynamic properties of the skull show a strong relationship with the distribution of crocodilians in skull morphospace, whereas phylogeny and biogeography show weak or no correlation. Therefore, ecological specializations related to feeding and foraging likely have the greatest influence on crocodilian skull shape.

189 citations


Journal ArticleDOI
TL;DR: This investigation explores the development of the dermatocranium, gastralia, and osteoderms in the American alligator, Alligator mississippiensis, and concludes that all osteoderm may share a deep homology, connected by the structural and skeletogenic properties of the dermis.
Abstract: The dermal skeleton (=exoskeleton) has long been recognized as a major determinant of vertebrate morphology. Until recently however, details of tissue development and diversity, particularly among amniotes, have been lacking. This investigation explores the development of the dermatocranium, gastralia, and osteoderms in the American alligator, Alligator mississippiensis. With the exception of osteoderms, elements of the dermal skeleton develop early during skeletogenesis, with most initiating ossification prior to mineralization of the endoskeleton. Characteristically, circumoral elements of the dermatocranium, including the pterygoid and dentigerous elements, are among the first to form. Unlike other axially arranged bones, gastralia develop in a caudolateral to craniomedial sequence. Osteoderms demonstrate a delayed onset of development compared with the rest of the skeleton, not appearing until well after hatching. Osteoderm development is asynchronous across the body, first forming dorsally adjacent to the cervical vertebrae; the majority of successive elements appear in caudal and lateral positions. Exclusive of osteoderms, the dermal skeleton initiates osteogenesis via intramembranous ossification. Following the establishment of skeletal condensations, some preossified spicules become engorged with many closely packed clusters of chondrocyte-like cells in a bone-like matrix. This combination of features is characteristic of chondroid bone, a tissue otherwise unreported among nonavian reptiles. No secondary cartilage was identified in any of the specimens examined. With continued growth, dermal bone (including chondroid bone) and osteoid are resorbed by multinucleated osteoclasts. However, there is no evidence that these cells contribute to the rugose pattern of bony ornamentation characteristic of the crocodylian dermatocranium. Instead, ornamentation develops as a result of localized concentrations of bone deposited by osteoblasts. Osteoderms develop in the absence of osteoblastic cells, osteoid, and periosteum; bone develops via the direct transformation of the preexisting dense irregular connective tissue. This mode of bone formation is identified as metaplasia. Importantly, it is also demonstrated that osteoderms are not histologically uniform but involve a range of tissues including calcified and uncalcified dense irregular connective tissue. Between taxa, not all osteoderms develop by homologous processes. However, it is concluded that all osteoderms may share a deep homology, connected by the structural and skeletogenic properties of the dermis.

169 citations


Journal ArticleDOI
TL;DR: A new model system for the integration of development, evolution, and ecology, the lizard genus Anolis, a diverse group of lizards whose ecology and evolution is well understood, and whose genome has recently been sequenced is introduced.
Abstract: Vertebrate developmental biologists typi- cally rely on a limited number of model organisms to understand the evolutionary bases of morphological change. Unfortunately, a typical model system for squa- mates (lizards and snakes) has not yet been developed leaving many fundamental questions about morphologi- cal evolution unaddressed. New model systems would ideally include clades, rather than single species, that are amenable to both laboratory studies of development and field-based analyses of ecology and evolution. Com- bining an understanding of development with an under- standing of ecology and evolution within and between closely related species has the potential to create a seam- less understanding of how genetic variation underlies eco- logically and evolutionarily relevant variation within pop- ulations and between species. Here we briefly introduce a new model system for the integration of development, evolution, and ecology, the lizard genus Anolis, a diverse group of lizards whose ecology and evolution is well understood, and whose genome has recently been se- quenced. We present a developmental staging series for Anolis lizards that can act as a baseline for later com- parative and experimental studies within this genus. J.

136 citations


Journal ArticleDOI
TL;DR: In this paper, the limb bones of the erethizontid Steiromys duplicatus, one of the most abundant Miocene porcupines from Patagonia, were analyzed to infer their locomotor behavior.
Abstract: Functional analysis of the limb bones of the erethizontid Steiromys duplicatus, one of the most abundant Miocene porcupines from Patagonia, provides evidence to infer their locomotor behavior. Remains of the giant Neosteiromys pattoni (Late Miocene of Northeast Argentina) are also analyzed. Osteological and myological features of extant porcupines were evaluated and used as a model to interpret the functional significance of Miocene species' limbs. Several features in erethizontids are compatible with the ability to climb: the low humeral tuberosities indicate a mobile gleno-humeral joint; the prominent and distally extended deltopectoral crest indicates a powerful pectoral muscle, which is particularly active when climbing; the humero-ulnar and humero-radial joints are indicative of pronation-supination movements; the well-developed lateral epicondylar ridge and the medially protruding entepicondyle are in agreement with an important development of the brachioradialis, supinator, flexor digitorum profundus, and pronator teres muscles, acting in climbing and grasping functions; the mechanical advantage of the biceps brachii would be emphasized because of its distal attachment on the bicipital tuberosity. As with extant porcupines, in Miocene species, the large femoral head would have permitted a broad range of abduction of the femur, and the medially protruding lesser trochanter would have emphasized the abduction and outward rotation of the femur by the action of the ilio-psoas complex. In S. duplicatus, the shape of the hip, knee, and cruro-astragalar, calcaneo-astragalar, and astragalo-navicular joints would have allowed lateral and rotational movements, although probably to a lesser degree than in extant porcupines. Foot features of S. duplicatus (e.g., great medial sesamoid bone, medial astragalar head, complete hallux) indicate that this species would have had grasping ability, but would not have achieved the high degree of specialization of Coendou. Steiromys duplicatus would have been a semiarboreal dweller, resembling Erethizon dorsatum.

111 citations


Journal ArticleDOI
TL;DR: In this paper, the relationship between olfactory morphology, habitat occupancy, and lifestyle in 21 elasmobranch species in a phylogenetic context was investigated using generalized least squares phylogenetic regression.
Abstract: This study investigated the relationship between olfactory morphology, habitat occupancy, and lifestyle in 21 elasmobranch species in a phylogenetic context. Four measures of olfactory capability, that is, the number of olfactory lamellae, the surface area of the olfactory epithelium, the mass of the olfactory bulb, and the mass of the olfactory rosette were compared between individual species and groups, comprised of species with similar habitat and/or lifestyle. Statistical analyses using generalized least squares phylogenetic regression revealed that bentho-pelagic sharks and rays possess significantly more olfactory lamellae and larger sensory epithelial surface areas than benthic species. There was no significant correlation between either olfactory bulb or rosette mass and habitat type. There was also no significant difference between the number of lamellae or the size of the sensory surface area in groups comprised of species with similar diets, that is, groups preying predominantly on crustaceans, cephalopods, echinoderms, polychaetes, molluscs, or teleosts. However, some groups had significantly larger olfactory bulb or rosette masses than others. There was little evidence to support a correlation between phylogeny and morphology, indicating that differences in olfactory capabilities are the result of functional rather than phylogenetic adaptations. All olfactory epithelia exhibited microvilli and cilia, with microvilli in both nonsensory and sensory areas, and cilia only in sensory areas. Cilia over the sensory epithelia originated from supporting cells. In contrast to teleosts, which possess ciliated and microvillous olfactory receptor types, no ciliated olfactory receptor cells were observed. This is the first comprehensive study comparing olfactory morphology to several aspects of elasmobranch ecology in a phylogenetic context.

107 citations


Journal ArticleDOI
TL;DR: The relationship of skull shape to skull length, phylogenetic grouping, habit, and characters relating to the feeding apparatus are explored here with geometric morphometric analysis on two‐dimensional landmarks and principle components analysis demonstrates that there are significant differences between phylogenetic groups.
Abstract: The Rhynchocephalia are a group of small diapsid reptiles that were globally distributed during the early Mesozoic. By contrast, the only extant representatives, Sphenodon punctatus and S. guntheri (Tuatara), are restricted to New Zealand off-shore islands. The Rhynchocephalia are widely considered to be morphologically uniform but research over the past 30 years has revealed unexpected phenotypic and taxonomic diversity. Phylogenetically "basal taxa" generally possess relatively simple conical or columnar teeth whereas more derived taxa possessed stouter flanged teeth and sophisticated shearing mechanisms: orthal in some (e.g., Clevosaurus hudsoni) and propalinal in others (e.g., S. punctatus). This variation in feeding apparatus suggests a wide range of feeding niches were exploited by rhynchocephalians. The relationship of skull shape to skull length, phylogenetic grouping, habit, and characters relating to the feeding apparatus are explored here with geometric morphometric analysis on two-dimensional landmarks. Principle components analysis demonstrates that there are significant differences between phylogenetic groups. In particular, Sphenodon differs significantly from all well known fossil taxa including the most phylogenetically basal forms. Therefore, it is not justifiable to use Sphenodon as a solitary outgroup when studying skull shape and feeding strategy in squamates; rhynchocephalian fossil taxa also need to be considered. There are also significant differences between the skull shapes of aquatic taxa and those of terrestrial taxa. Of the observed variation in skull shape, most variation is subsumed by variation in dentary tooth base shape, the type of jaw movement employed (e.g., orthal vs. propalinal) and the number of palatal tooth rows. By comparison, the presence or absence of flanges, dentary tooth number and palatal tooth row orientation subsume much less. Skull length was also found to be a poor descriptor of overall skull shape. Compared to basal rhynchocephalians members of more derived terrestrial radiations possess an enlarged postorbital area, a high parietal, and a jaw joint positioned ventral to the tooth row. Modification of these features is closely associated with increased biting performance and thus access to novel food items. Some of these same trends are apparent during Sphenodon ontogeny where skull growth is allometric and there is evidence for ontogenetic variation in diet.

100 citations


Journal ArticleDOI
TL;DR: The results support the concept that ruminant species that ingest more grass have smaller salivary glands, possibly indicating a reduced requirement for the production ofsalivary tannin‐binding proteins.
Abstract: In the ongoing debate about divergent evolutionary morphophysiological adaptations of grazing and browsing ruminants, the size of the salivary glands has received special attention. Here, we report the most comprehensive dataset on ruminant salivary glands so far, with data on the Glandula parotis (n=62 species), Gl. mandibularis (n=61), Gl. buccalis ventralis (n=44), and Gl. sublingualis (n=30). All four salivary gland complexes showed allometric scaling with body mass (BM); in all cases, the 95% confidence interval for the allometric exponent included 0.75 but did not include 1.0 (linearity); therefore, like other parameters linked to the process of food intake, salivary gland mass appears to be correlated to metabolic body weight (BM0.75), and comparisons of relative salivary gland mass between species should rather be made on the basis of BM0.75 than as a percentage of BM. In the subsequent analyses, the percentage of grass (%grass) in the natural diet was used to characterize the feeding type; the phylogenetic tree used for a controlled statistical evaluation was entirely based on mitochondrial DNA information. Regardless of phylogenetic control in the statistical treatment, there was, for all four gland complexes, a significant positive correlation of BM and gland mass, and a significant negative correlation between %grass in the natural diet and gland mass. If the Gl. parotis was analyzed either for cervid or for bovid species only, the negative correlation of gland mass and %grass was still significant in either case; an inspection of certain ruminant subfamilies, however, suggested that a convergent evolutionary adaptation can only be demonstrated if a sufficient variety of ruminant subfamilies are included in a dataset. The results support the concept that ruminant species that ingest more grass have smaller salivary glands, possibly indicating a reduced requirement for the production of salivary tannin-binding proteins.

85 citations


Journal ArticleDOI
TL;DR: A larval staging scheme is defined, intended as a guide for determining the degree of larval development, and for identifying larval and adult structures, and is of use to investigators studying gene expression and morphogenesis in postembryonic larvae.
Abstract: The adult body plan of Strongylocentrotus purpuratus is established within the imaginal rudiment during the larval stages. To facilitate the study of these stages, we have defined a larval staging scheme, which consists of seven stages: Stage I, four-arm stage; Stage II, eight-arm stage; Stage III, vestibular invagination stage; Stage IV, rudiment initiation stage; Stage V, pentagonal disc stage; Stage VI, advanced rudiment stage; and Stage VI, tube-foot protrusion stage. Each stage is characterized by significant morphological features observed for the first time at that stage. This scheme is intended as a guide for determining the degree of larval development, and for identifying larval and adult structures. Larval anatomy was visualized using light and confocal microscopy as required on living material, whole mount fixed specimens, and serial sections. Antibody staining to localize specific gene products was also used. Detailed analysis of these data has furthered our understanding of the morphogenesis of the rudiment, and has suggested provocative questions regarding the molecular basis for these events. We intend this work to be of use to investigators studying gene expression and morphogenesis in postembryonic larvae.

78 citations


Journal ArticleDOI
TL;DR: The results demonstrate that the embryonic organs exhibit body plans that are primarily bilateral and antero‐posteriorly elongated at stereotyped positions, and suggest that extensions of the collar‐funnel compartment and free epidermal folds derived from multiple topological foot regions may play an important role in forming the head complex, which is thought to be an important feature during the body plan transition.
Abstract: Cephalopod head parts are among the most complex occurring in all invertebrates. Hypotheses for the evolutionary process require a drastic body-plan transition in relation to the life-style changes from ben- thos to active nekton. Determining these transitions, however, has been elusive because of scarcity of fossil records of soft tissues and lack of some of the early de- velopmental stages of the basal species. Here we report the first embryological evidence in the nautiloid cephalo- pod Nautilus pompilius for the morphological develop- ment of the head complex by a unique assembly of mul- tiple archetypical molluscan body parts. Using a special- ized aquarium system, we successfully obtained a series of developmental stages that enabled us to test previous controversial scenarios. Our results demonstrate that the embryonic organs exhibit body plans that are pri- marily bilateral and antero-posteriorly elongated at ster- eotyped positions. The distinct cephalic compartment, foot, brain cords, mantle, and shell resemble the body plans of monoplacophorans and basal gastropods. The numerous digital tentacles of Nautilus develop from sim- ple serial and spatially-patterned bud-like anlagen along the anterior-posterior axis, indicating that origins of digital tentacles or arms of all other cephalopods develop not from the head but from the foot. In middle and late embryos, the primary body plans largely change to those of juveniles or adults, and finally form a ''head'' complex assembled by anlagen of the foot, cephalic hood, collar, hyponome (funnel), and the foot-derived epidermal cov- ers. We suggest that extensions of the collar-funnel com- partment and free epidermal folds derived from multiple topological foot regions may play an important role in forming the head complex, which is thought to be an important feature during the body plan transition.

77 citations


Journal ArticleDOI
TL;DR: Profiles with all orientations have been used to visualize the 3D structure of ivory from tusks of elephant, mammoth, walrus, hippopotamus, pig,bush, boar, and warthog, sperm whale, killer whale, and narwhal, with main findings relating to the way that dentinal tubules align in sheets to form microlaminae in the length of the tusk.
Abstract: Profiles with all orientations have been used to visualize the 3D structure of ivory from tusks of elephant, mammoth, walrus, hippopotamus, pig (bush, boar, and warthog), sperm whale, killer whale, and narwhal. Polished, forming, fractured, aged, and stained surfaces were prepared for microscopy using epi-illumination. Tusks have a minor peripheral component, the cementum, a soft derivative of the enamel layer, and a main core of dentine = ivory. The dentine is composed of a matrix of particles 5–20 μm in diameter in a ground substance containing dentinal tubules about 5 μm in diameter with a center to center spacing of 10–20 μm. Dentinal tubules may be straight (most) or curly (pigs). The main findings relate to the way that dentinal tubules align in sheets to form microlaminae in the length of the tusk. Microlaminae are sheets of laterally aligned dentinal tubules. They are axial but may be radial (most), angled to the forming face (pigs and hippopotamus canines), or radial but helical (narwhals). Within the microlaminae the dentinal tubules may be radial, angled to the axis (whales, walrus, and pigs), or may change their orientation from one microlamina to the next in helicoids (canines of hippopotamuses, incisors of proboscidea). In the nonbanded, featureless ivories from the hippopotamus incisors, the dentinal tubules form radial microlamina from which the arrangements in other ivories can be derived. In the canines of hippopotamuses and incisors of proboscidea, the dentinal tubule orientation changes incrementally from one microlamina to the next in a helicoid, a stack of dentinal tubules that change their orientation by 180° anticlockwise. Dentinal tubules having different orientations are laid down concurrently, not layer by layer as in most examples of helicoidal architecture (e.g., insect cuticle). In proboscidean ivory, the microlaminae are radial, normal to the banding of growth layers marking the plane of deposition. They form radial segments with each 180° turn in the orientation of their constituent dentinal tubules. Below the cementum they are almost complete 180° helicoids, but nearer to the core they become narrower with the loss of radially oriented dentinal tubules. These truncated helicoidal patterns appear in longitudinal profile as VVVV feather patterns rather than ∩∩∩∩, each V or ∩ being the side view of a partial or complete helicoid. The Schreger pattern in proboscidean ivory consists of these helicoids divided tangentially into columns in the length of the tusk. Narwhals have the most abundant matrix particles with their radial/helical dentinal tubules having a twist opposite to that in the cementum. J. Morphol., 2008. © 2007 Wiley-Liss, Inc.

Journal ArticleDOI
TL;DR: Hair cell polarization was rostro‐caudal in most superficial neuromasts on trunk scales (with the exception of those on the lateral line scales), or on the tail fin, and the most frequent hair cell polarized was dorso‐ventral in front of the eyes and on the ventral mandible.
Abstract: Distribution, morphology, and orientation of superficial neuromasts and polarization of the hair cells within superficial neuromasts of the goldfish (Carassius auratus) were examined using fluorescence labeling and scanning electron microscopy On each body side, goldfish have 1,800–2,000 superficial neuromasts distributed across the head, trunk and tail fin Each superficial neuromast had about 14–32 hair cells that were arranged in the sensory epithelium with the axis of best sensitivity aligned perpendicular to the long axis of the neuromast Hair cell polarization was rostro-caudal in most superficial neuromasts on trunk scales (with the exception of those on the lateral line scales), or on the tail fin On lateral line scales, the most frequent hair cell polarization was dorso-ventral in 45% and rostro-caudal in 20% of the superficial neuromasts On individual trunk scales, superficial neuromasts were organized in rows which in most scales showed similar orientations with angle deviations smaller than 45° In about 16% of all trunk scales, groups of superficial neuromasts in the dorsal and ventral half of the scale were oriented orthogonal to each other On the head, most superficial neuromasts were arranged in rows or groups of similar orientation with angle deviations smaller than 45° Neighboring groups of superficial neuromasts could differ with respect to their orientation The most frequent hair cell polarization was dorso-ventral in front of the eyes and on the ventral mandible and rostro-caudal below the eye and on the operculum J Morphol, 2008 © 2008 Wiley-Liss, Inc

Journal ArticleDOI
TL;DR: The morphology and mechanical strength of the upper canines in all eight extant species of ursids is analyzed, and the findings are discussed in relation to feeding ecology.
Abstract: The morphology and mechanical strength of the upper canines in all eight extant species of ursids is analyzed, and the findings are discussed in relation to feeding ecology. Ursids have proportionally smaller canines than other large carnivores with a specialized feeding ecology, such as large felids, and the upper canine morphology is both canid-like and felid-like. The giant panda is the most divergent species, and its short, blunt, and cone-like canines appear well adapted for tearing into bamboo. The almost equally herbivorous spectacled bear has a less derived canine morphology. The large canines of the sun bear are divergent from other ursine ursids, and may be an adaptation for tearing open tree trunks in search of insects. Discriminant Analysis is successful in separating ursid species on the basis of canine morphology, but the canines of ursine ursids, and also of the spectacled bear, show greater resemblance among the species than the marked differences in feeding ecology would suggest. This could be in part due to a short evolutionary history, and in part due to canines not having been subjected to much evolutionary selection as has been the case among other large carnivores, such as large felids. Ursids are probably evolutionarily and ecologically successful due to physical size and strength rather than a derived craniodental anatomy. J. Morphol., 2008. © 2008 Wiley-Liss, Inc.

Journal ArticleDOI
TL;DR: A noticeable increase of overlapping sutures was observed in the snout of the smallest sphaerodactylids compared to other gekkotans, which may be attributed to adaptations for decreasing mechanical resistance of the cranium during feeding or burrowing.
Abstract: A detailed description of the skull and jaw of the gecko Sphaerodactylus roosevelti is presented. The bones are described articulated and isolated with special consideration given to the type of suture among joining elements. S. roosevelti was compared with 109 gekkotan species to evaluate the osteological variation and to find characters for cladistic analysis. Changes in the skull associated with the miniaturization process are discussed within the sphaerodactylid geckos. A noticeable increase of overlapping sutures was observed in the snout of the smallest sphaerodactylids compared to other gekkotans. This pattern is convergent with that in miniaturized pygopodids and may be attributed to adaptations for decreasing mechanical resistance of the cranium during feeding or burrowing. New cranial characters support Sphaerodactylinae as a monophyletic group and should be useful for resolving questions such as their relationship with other gekkotans.

Journal ArticleDOI
TL;DR: It is shown that threespine stickleback sampled from anadromous and lacustrine populations are differentiated gonochorists and the data suggest that an increase in PGC number may direct the undifferentiated gonad toward ovarian differentiation.
Abstract: Gonadal sex differentiation is increasingly recognized as a remarkably plastic process driven by species-specific genetic or environmental determinants. Among aquatic vertebrates, gonadal sex differentiation is a frequent endpoint in studies of endocrine disruption with little appreciation of underlying developmental mechanisms. Work in model organisms has highlighted the diversity of master sex-determining genes rather than uncovering any broad similarities prompting the highly conserved developmental decision of testes versus ovaries. Here we use molecular genetic markers of chromosomal sex combined with traditional histology to examine the transition of the bipotential gonads to ovaries or testes in threespine stickleback (Gasterosteus aculeatus). Serially-sectioned threespine stickleback fry were analyzed for qualitative and quantitative indications of sexual differentiation, including changes in gonadal morphology, number of germ cells and the incidence of gonadal apoptosis. We show that threespine stickleback sampled from anadromous and lacustrine populations are differentiated gonochorists. The earliest sex-specific event is a premeiotic increase in primordial germ cell number followed by a female-specific spike in apoptosis in the undifferentiated gonad of genetic females. The data suggest that an increase in PGC number may direct the undifferentiated gonad toward ovarian differentiation.

Journal ArticleDOI
TL;DR: This study elucidates how differences in jaw and hindlimb musculoskeletal morphology of accipiters and falcons are reflected in Differences in their killing modes, and through differences in their force‐generating capacities.
Abstract: Accipiters (Accipiter spp.) and falcons (Falco spp.) both use their feet to seize prey, but falcons kill primarily with their beaks, whereas accipiters kill with their feet. This study examines the mechanistic basis to differences in their modes of dispatching prey, by focusing on the myology and biomechanics of the jaws, digits, and distal hindlimb. Bite, grip, and distal hindlimb flexion forces were estimated from measurements of physiological cross-sectional area (PCSA) and indices of mechanical advantage (MA) for the major jaw adductors, and digit and tarsometatarsal flexors. Estimated bite force, total jaw adductor PCSA, and jaw MA (averaged over adductors) tended to be relatively and absolutely greater in falcons, reflecting their emphasis on biting for dispatching their prey. Differences between genera in estimated grip force, total digit flexor PCSA, and digit MA (averaged over inter-phalangeal joints and digits) were not as clear-cut; each of these parameters scaled positively allometric in accipiters, which may reflect the scaling of both prey size, and the proportion of mammalian prey consumed by this lineage with increasing body size. Estimated tarsometatarsal force was greater in falcons than in accipiters, due to their greater MA, which may reflect selection for incurring greater forces during prey strikes. Conversely, the comparatively lower tarsometatarsal MA in accipiters reflects their capacity for greater foot speed potentially necessary for grasping elusive prey. Thus, this study elucidates how differences in jaw and hindlimb musculoskeletal morphology of accipiters and falcons are reflected in differences in their killing modes, and through differences in their force-generating capacities.

Journal ArticleDOI
TL;DR: An alternative structural hypothesis for the neural reduction and, ultimately, the complete loss of the neural series is proposed and the complete reduction of neurals in Emydura spp.
Abstract: Although we are starting to understand the molecular basis of shell development based on the study of cryptodires, basic comparative ontogenetic data for the other major clade of living turtle, the pleurodires, are largely missing. Herein, the developmental and phylogenetic relation between the bony shell and endoskeleton of Pleurodira are examined by studying histological serial sections of nine specimens of three different species, including an ontogenetic series of Emydura subglobosa. Emphasis is given to the portion of the carapace in which ribs and vertebral spinous processes become part of the carapace. Central questions are how neurals and costals are formed in pleurodiran turtles, whether costals and neurals are of endoskeletal or exoskeletal origin, and what ontogenetic factors relate to neural reduction of some Pleurodira. The neurals and costals do not develop as independent ossification centers, but they are initial outgrowths of the periosteal collar of endoskeletal ribs and neural arches. Slightly later in development, the ossification of both shell elements continues without a distinct periosteum but by metaplastically ossifying precondensed soft-tissue integumentary structures. Through ontogeny, ribs of the turtles studied are closely associated with the hypaxial intercostalis musculature while epaxial interspinalis musculature connects the neural arches. We here propose an alternative structural hypothesis for the neural reduction and, ultimately, the complete loss of the neural series. The complete reduction of neurals in Emydura spp. may be linked to heterochrony, accompanied by a restricted influence of epaxial musculature and epidermal–dermal interaction in shell bone formation. J. Morphol., 2008. © 2008 Wiley-Liss, Inc.

Journal ArticleDOI
TL;DR: It is demonstrated that ontogeny, geography, and reproductive state may influence morphological parameters such as structural fiber densities and adipocyte numbers and sizes, measured in bottlenose dolphin blubber.
Abstract: This study investigated blubber morphology and correlations of histological measurements with ontogeny, geography, and reproductive state in live, wild bottlenose dolphins (Tursiops truncatus) from the southeastern United States. Surgical skin-blubber biopsies (N=74) were collected from dolphins during capture-release studies conducted in two geographic locations: Charleston, SC (N=38) and Indian River Lagoon, FL (N=36). Histological analysis of blubber revealed stratification into superficial, middle, and deep layers. Adipocytes of the middle blubber were 1.6x larger in Charleston subadults than in Indian River Lagoon subadults (4,590+/-340 compared to 2,833+/-335 microm2 per cell). Charleston subadult dolphins contained higher levels of total blubber lipids than Charleston adult animals (49.3%+/-1.9% compared to 34.2%+/-1.7%), and this difference was manifested in more adipocytes in the middle blubber layer (19.2+/-0.9 compared to 14.9+/-0.5 cells per field). However, dolphins from Indian River Lagoon did not exhibit this pattern, and the adipocyte cell counts of subadults were approximately equal to those of the adults (16.0+/-1.4 compared to 13.4+/-0.8 cells per field). The colder year-round water temperatures in Charleston compared to Indian River Lagoon may explain these differences. Adipocytes in the deep blubber layer were significantly smaller in lactating and simultaneously pregnant and lactating animals compared to pregnant dolphins (840+/-179, 627+/-333, and 2,776+/-586 microm2 per cell, respectively). Total blubber lipid content and adipocyte size in the deep blubber of mothers with calves decreased linearly with calf length. Lactating females may utilize lipids from the deep blubber during periods of increased energetic demands associated with offspring care. This study demonstrates that ontogeny, geography, and reproductive state may influence morphological parameters such as structural fiber densities and adipocyte numbers and sizes, measured in bottlenose dolphin blubber.

Journal ArticleDOI
TL;DR: Oviducal gland morphology, the microscopic organization of the terminal zone, and sperm storage were described in the female gummy shark (Mustelus antarcticus), a nonplacental viviparous hound shark, which displays minimal histotrophy during embryonic development.
Abstract: Oviducal gland morphology, the microscopic organization of the terminal zone, and sperm storage were described in the female gummy shark (Mustelus antarcticus). Mustelus antarcticus is a nonplacental viviparous hound shark, which displays minimal histotrophy during embryonic development. The animals examined represented all stages of maturity and gestation. The oviducal gland was found to have the same fundamental zonation as in most chondrichthyans. Using recent terminology, the oviducal gland of chondrichthyans has an anterior club zone, followed by a papillary zone, both of which produce jelly that surrounds the egg, a baffle zone that elaborates the tertiary egg envelope and a terminal zone, where sperm storage occurs. Each zone is composed of simple tubular glands that connect to transverse grooves, which extend the full width of the gland. The exception is the terminal zone, which does not have transverse grooves but consists of individual tubules. The microscopic organization and histochemical nature of the zones display similar patterns to those of other chondrichthyan genera. Tubules of the terminal zone contain four types of cell: ciliated cells, alcian blue-positive secretory cells, periodic acid-Schiff and alcian blue-negative secretory cells, and secretory columnar cells. These tubules end in recesses, the sperm storage tubules, which extend beyond the periphery of the baffle zone. Sperm were stored in the sperm storage tubules of all maturing and mature animals examined. Of note is the observation of stored sperm in an animal 1 year prior to first ovulation. Sperm were also observed throughout the uterine sphincter, body of the uterus, isthmus, and oviduct of maturing and mature animals, and in the uterine sphincter of an immature animal. These sperm represent immediately postcopulation aggregations of sperm and sperm in the process of migrating to the site of storage or to the site of fertilization.

Journal ArticleDOI
TL;DR: The results suggest that the occurrence and density of bone vascular canals are basically dependant on specific size, not phylogenetic relationships; vascular density reflects the absolute growth rates of bone cortices, and the orientation of vascularCanal is a variable feature independent of phylogeny or growth rate.
Abstract: Bone vascular canals occur irregularly in tetrapods; however, the reason why a species has or lacks bone canals remains poorly understood. Basically, this feature could depend on phylogenetic history, or result from diverse causes, especially cortical accretion rate. The Varanidae, a monophyletic clade that includes species with impressive size differences but similar morphologies, is an excellent model for this question. Cortical vascularization was studied in 20 monitor species, on three bones (femur, fibula, and tibia) that differ in their shaft diameters, and in the absolute growth speed of their diaphyseal cortices. In all species smaller than 398 mm SVL (133–397 mm in sample), bone cortices lack vascular canals, whereas all larger species (460–1,170 mm in sample) display canals. The size 398–460 mm SVL is thus a threshold for the appearance of the canals. The distribution of vascular and avascular bone tissues among species does not precisely reflect phylogenetic relationships. When present, vascular canals always occur in the femur and tibia, but are less frequent, sparser, and thinner in the fibula. Vascular density increases linearly with specific size but decreases exponentially during individual growth. In most species, canal orientation varies between individuals and is diverse in a single section. No clear relationship exists between canal orientation and vascular density. These results suggest that: a) the occurrence and density of bone vascular canals are basically dependant on specific size, not phylogenetic relationships; b) vascular density reflects the absolute growth rates of bone cortices; c) the orientation of vascular canals is a variable feature independent of phylogeny or growth rate. J. Morphol., 2008. © 2007 Wiley-Liss, Inc.

Journal ArticleDOI
TL;DR: Those of the female ring‐tailed lemur are characterized as moderately “masculinized,” highlighting certain morphological similarities and differences between ring‐tails lemurs and the most male‐like of female mammals, the spotted hyena, and calling attention to a potential hormonal mechanism of “Masculinization” in female lemur development.
Abstract: The extravagance and diversity of external genitalia have been well characterized in male primates; however, much less is known about sex differences or variation in female form. Our study represents a departure from traditional investigations of primate reproductive anatomy because we 1) focus on external rather than internal genitalia, 2) measure both male and female structures, and 3) examine a strepsirrhine rather than an anthropoid primate. The subjects for morphological study were 21 reproductively intact, adult ring-tailed lemurs (Lemur catta), including 10 females and 11 males, two of which (one per sex) subsequently died of natural causes and also served as specimens for gross anatomical dissection. Male external genitalia presented a typical masculine configuration, with a complex distal penile morphology. In contrast, females were unusual among mammals, presenting an enlarged, pendulous external clitoris, tunneled by the urethra. Females had a shorter anogenital distance and a larger urethral meatus than did males, but organ diameter and circumference showed no sex differences. Dissection confirmed these characterizations. Noteworthy in the male were the presence of a "levator penis" muscle and discontinuity in the corpus spongiosum along the penile shaft; noteworthy in the female were an elongated clitoral shaft and glans clitoridis. The female urethra, while incorporated within the clitoral body, was not surrounded by erectile tissue, as we detected no corpus spongiosum. The os clitoridis was 43% the length and 24% the height of the os penis. On the basis of these first detailed descriptions of strepsirrhine external genitalia (for either sex), we characterize those of the female ring-tailed lemur as moderately "masculinized." Our results highlight certain morphological similarities and differences between ring-tailed lemurs and the most male-like of female mammals, the spotted hyena (Crocuta crocuta), and call attention to a potential hormonal mechanism of "masculinization" in female lemur development.

Journal ArticleDOI
TL;DR: The combined results suggest that dolphins utilize muscles, similar to those used by galloping mammals, to power their explosive ventilation.
Abstract: Most mammals possess stamina because their locomotor and respiratory (i.e. ventilatory) systems are mechanically coupled. These systems are decoupled, however, in bottlenose dolphins (Tursiops truncatus) as they swim on a breath-hold. Locomotion and ventilation appear to be coupled only during their brief surfacing event, when they respire explosively (up to 90% of total lung volume in approximately 0.3s) (Ridgway et al., 1969). The predominantly slow-twitch fiber profile of their diaphragm (Dearolf, 2003) suggests that this muscle does not likely power their rapid ventilatory event. Based upon Bramble’s (1989) biomechanical model of locomotor-respiratory coupling in galloping mammals, I hypothesized that muscles located within the cranial-cervical and lumbo-pelvic units, which act upon the thoracic unit, function to power ventilation in bottlenose dolphins. I also hypothesized that these muscles would be composed predominantly of fast-twitch fibers to facilitate the bottlenose dolphin’s rapid ventilation. The gross morphology (n=6) of cranio-cervical (sternomastoid, sternohyoid, scalenes), thoracic (intercostals), and lumbo-pelvic (rectus abdominis, abdominal obliques, hypaxialis) muscles and the fiber-type profiles (n=6) of selected muscles (sternohyoid, sternomastoid, and rectus abdominis) of bottlenose dolphins were investigated. Physical manipulations of excised thoracic units were carried out to investigate potential actions of these muscles. Results suggest that the cranio-cervical muscles act to draw the sternum and associated ribs cranio-dorsally, which flares the ribs laterally, and increases thoracic cavity volume required for inspiration. The thoracic muscles physically link the ribs to create a single functional unit; these muscles can also act to control the size of the intercostal space. The lumbo-pelvic muscles act to draw the sternum and caudal ribs caudally, which decreases the volume of the thoracic and abdominal cavities required for expiration. All muscles investigated were composed predominantly of fast-twitch fibers (range 72-88% by area) and appear histochemically poised for rapid contraction. These combined results suggest that dolphins utilize muscles, similar to those used by galloping mammals, to power their explosive ventilation. However, the mechanisms that permit dolphins to selectively couple and uncouple their locomotor and ventilatory systems, depending upon whether they are respiring at the surface or swimming on a breath-hold, remain unknown.

Journal ArticleDOI
TL;DR: In vitro culture of the isogenic marbled crayfish is recommended for broader use in research because it enables not only time and stage‐specific sampling but also precisely timed experimental manipulations.
Abstract: The late embryonic and early post-embryonic life period of freshwater crayfish, which is the main time period of organogenesis, is poorly investigated because of the protective brooding behavior of crayfish mothers. A combination of in vitro culture, behavioral observations, and microscopic investigations of organs involved in hatching, attachment, exploration of the environment, and searching and processing of food yielded deeper insights in this important period of life. Experiments were performed with the robust parthenogenetic marbled crayfish. The following results were obtained: (1) Marbled crayfish can be raised in simple in vitro systems from 80% embryonic development to juvenile Stage 4 with up to 100% survival; (2) Hatching is prepared by chemical weakening of the egg shell and completed by levering actions of the hatchling's appendages; (3) The telson thread, a safety line that keeps the hatchling secured to the mother, is formed by secretions from the telson and the detaching inner layer of the egg case; (4) Molting Stage-1 juveniles are secured by an anal thread that results from delayed molting of the hindgut; (5) Active attachment of the hatchlings to the maternal pleopods with their 1st pereiopods is achieved by an innate fixed action pattern; (6) In vitro, juveniles are motile from Stage 2 despite incomplete development of their balance controlling statocysts. Movement pattern and social behavior vary greatly among individuals; and (7) Feeding starts in Stage 3, when the mouthparts and the gastric mill are fully developed. Onset of feeding is innate and does not require maternal contributions. In vitro culture of the isogenic marbled crayfish is recommended for broader use in research because it enables not only time and stage-specific sampling but also precisely timed experimental manipulations.

Journal ArticleDOI
TL;DR: The nurse shark, Ginglymostoma cirratum, is an obligate suction feeder that preys on benthic invertebrates and fish that exhibits a suite of structural and functional modifications that facilitate this mode of prey capture.
Abstract: The nurse shark, Ginglymostoma cirratum, is an obligate suction feeder that preys on benthic inverte- brates and fish. Its cranial morphology exhibits a suite of structural and functional modifications that facilitate this mode of prey capture. During suction-feeding, subambient pressure is generated by the ventral expansion of the hyoid apparatus and the floor of its buccopharyngeal cavity. As in suction-feeding bony fishes, the nurse shark exhibits ex- pansive, compressive, and recovery kinematic phases that produce posterior-directed water flow through the bucco- pharyngeal cavity. However, there is generally neither a preparatory phase nor cranial elevation. Suction is gener- ated by the rapid depression of the buccopharyngeal floor by the coracoarcualis, coracohyoideus, and coracobran- chiales muscles. Because the hyoid arch of G. cirratum is loosely connected to the mandible, contraction of the rectus cervicis muscle group can greatly depress the floor of the buccopharyngeal cavity below the depressed mandible, resulting in large volumetric expansion. Suction pressures in the nurse shark vary greatly, but include the greatest subambient pressures reported for an aquatic-feeding ver- tebrate. Maximum suction pressure does not appear to be related to shark size, but is correlated with the rate of buc- copharyngeal expansion. As in suction-feeding bony fishes, suction in the nurse shark is only effective within approxi- mately 3 cm in front of the mouth. The foraging behavior of this shark is most likely constrained to ambushing or stalking due to the exponential decay of effective suction in front of the mouth. Prey capture may be facilitated by for- aging within reef confines and close to the substrate, which can enhance the effective suction distance, or by foraging at night when it can more closely approach prey. J. Mor-

Journal ArticleDOI
TL;DR: This study is the first to identify that each caudal melon terminates in a lipid cup that envelopes the echolocation sound generators, and suggests that these muscles could function to change the frequency, beam width, and directionality of the emitted sound beam in bottlenose dolphins.
Abstract: The melon is a lipid rich structure located in the odontocete forehead that functions to propagate echolocation sounds into the surrounding aquatic environment. To date, the melon’s abilities to guide and impedance match biosonar sounds to water has been attributed to its unique fatty acid composition. The melon, though, is also acted upon by facial muscles (rostral, pars anterointernus, and nasal plug muscles). The goals of this study were to investigate the gross morphology of the melon in the bottlenose dolphin (Tursiops truncatus) and to describe how it is tendinously connected to these facial muscles. Standard gross dissection (n = 8) and serial sectioning (n = 3) techniques were used to describe the melon and to identify the connections between it and the surrounding muscles and blubber in three orthogonal body planes. The melon was also thin-sectioned in three body planes (n = 3) and polarized light was used to reveal the birefringent collagen fibers within and surrounding the melon. This study identified two distinct regions of the melon, the main body and the caudal melon. The caudal melon is bilaterally asymmetrical. This study is the first to identify three regions of the right caudal melon, termed the cup, stem, and cone and two regions of the left caudal melon, named the cup and branch. All regions of the melon vary in shape and display locally specific muscle-tendon morphologies. The entire length of the melon, from the initial site of sound generation to its rostral end, is acted upon by facial muscles. These muscles have highly organized tendon populations that traverse the melon and insert into either the surrounding blubber, the connective tissue matrix of the nasal plug, or the connective tissue sheath surrounding the sound generators. The facial muscles and tendons also lie within multiple orthogonal planes, which suggest that the melon is capable of changing its shape in multiple planes. Thus, the sound propagation pathway from the echolocation sound generators to the surrounding water appears to be a tunable system. Specifically, the facial muscles are hypothesized to be able to change the frequency, beam width, and directionality of the emitted sound beam in bottlenose dolphins (Tursiops truncatus).

Journal ArticleDOI
TL;DR: Gonad development and sex differentiation from embryos to 594‐day‐old individuals were investigated in farmed Acipenser naccarii using light and transmission electron microscopy and Anatomical differentiation of the gonad precedes cytological differentiation and female differentiation largely precedes that of the male.
Abstract: Gonad development and sex differentiation from embryos to 594-day-old individuals were investigated in farmed Acipenser naccarii using light and transmission electron microscopy. The migrating primordial germ cells first appear along the dorsal wall of the body cavity in embryos 1.5 days before hatching. The gonadal ridge, containing a few primary primordial germ cells (PGC-1) surrounded by enveloping cells, appears in 16-day-old larvae. At 60 days, the undifferentiated gonad is lamellar and PGC-1 multiply, producing PGC-2. In 105-day-old juveniles, a distinct germinal area with advanced PGC-2 appears on the lateral side near the mesogonium and the first blood vessels are visible. At 180 days, putative ovaries with a notched gonadal epithelium and putative testes with a smooth one appear, together with adipose tissue on the distal side. In 210-day-old juveniles, active proliferation of germ cells begins in the putative ovaries, whereas putative testes still contain only a few germ cells. The onset of meiosis and reorganization of stromal tissue occurs in ovaries of 292-day-old individuals. Ovaries with developed lamellae enclosing early oocyte clusters and follicles with perinucleolar oocytes occur at 594 days. Meiotic stages are never found, even in anastomozing tubular testes of 594-day-old individuals. Steroid producing cells are detected in the undifferentiated gonad and in the differentiated ones of both sexes. Anatomical differentiation of the gonad precedes cytological differentiation and female differentiation largely precedes that of the male. Gonad development and differentiation are also associated with structural changes of connective tissue, viz. collagen-rich areas are massive in developing testes and reduced in ovaries. J. Morphol., 2008. © 2008 Wiley-Liss, Inc.

Journal ArticleDOI
TL;DR: The extensive conformity of the SNS of both species suggests functional parallels and future electrophysiological studies of the motor circuits in the S NS of Drosophila will profit from parallel studies ofThe homologous but more accessible structures in Calliphora.
Abstract: The stomatogastric nervous system (SNS) associated with the foregut was studied in 3rd instar larvae of Drosophila melanogaster and Calliphora vicina (blowfly). In both species, the foregut comprises pharynx, esophagus, and proventriculus. Only in Calliphora does the esophagus form a crop. The position of nerves and neurons was investigated with neuronal tracers in both species and GFP expression in Drosophila. The SNS is nearly identical in both species. Neurons are located in the proventricular and the hypocerebral ganglion (HCG), which are connected to each other by the proventricular nerve. Motor neurons for pharyngeal muscles are located in the brain not, as in other insect groups, in the frontal ganglion. The position of the frontal ganglion is taken by a nerve junction devoid of neurons. The junction is composed of four nerves: the frontal connectives that fuse with the antennal nerves (ANs), the frontal nerve innervating the cibarial dilator muscles and the recurrent nerve that innervates the esophagus and projects to the HCG. Differences in the SNS are restricted to a crop nerve only present in Calliphora and an esophageal ganglion that only exists in Drosophila. The ganglia of the dorsal organs give rise to the ANs, which project to the brain. The extensive conformity of the SNS of both species suggests functional parallels. Future electrophysiological studies of the motor circuits in the SNS of Drosophila will profit from parallel studies of the homologous but more accessible structures in Calliphora.

Journal ArticleDOI
TL;DR: The author describes the ossification sequences of three ducks, the Common Eider Somateria mollissima dresseri, the Pekin Duck Anas platyrhynchos, and the Muscovy Duck Cairina moschata.
Abstract: Ossification sequences are poorly known for most amniotes, and yet they represent an important source of morphogenetic, phylogenetic, and life history information. Here, the author describes the ossification sequences of three ducks, the Common Eider Somateria mollissima dresseri, the Pekin Duck Anas platyrhynchos, and the Muscovy Duck Cairina moschata. Sequence differences exist both within and among these species, but are generally minor. The Common Eider has the most ossified skeleton prior to hatching, contrary to what is expected in a subarctic migrant species. This may be attributed to a tradeoff between growth rate and locomotory performance. Growth rate is higher in hatchlings with more cartilaginous skeletons, but this may compromise locomotion. No major ossification sequence differences were observed in the craniofacial skeleton when compared with Galliformes, which suggests that the influence of adult morphology on ossification sequence might be relatively minor in many taxa. Galliformes and Anseriformes, while both highly ossified at hatching, differ in the location of their late-stage ossification centers. In Anseriformes, these are most often located in the appendicular skeleton, whereas in Galliformes they are in the thoracic region and form the ventilatory apparatus. J. Morphol., 2008. © 2008 Wiley-Liss, Inc.

Journal ArticleDOI
TL;DR: On the basis of the configuration of the nervous system and the cuticle Diurodrilus is regarded to belong to Spiralia, possibly close to Annelida; however, until further evidence is acquired it should be regarded as incertae sedis in this large animal clade.
Abstract: Interstitial marine meiofaunal worms of the genus Diurodrilus have always been considered part of Annelida, either as basal or derived, though generally with reference to Dinophilidae. New evidence shows that Diurodrilus has a unique anatomy, and lacks key annelid features, possibly even segmentation. We assessed the systematic position of Diurodrilus among other protostome animals via light microscopy, confocal laser scanning microscopy, and transmission electron microscopy studies of anatomy, focusing on musculature, the nervous system, as well as molecular sequence data. We show that there is little morphological or molecular evidence to support a relationship with Dinophilidae or any other annelids. Diurodrilus has some similarities to Micrognathozoa, though the latter shows complex jaws. On the basis of the configuration of the nervous system and the cuticle we regard Diurodrilus to belong to Spiralia, possibly close to Annelida; however, until further evidence is acquired it should be regarded as incertae sedis in this large animal clade. J. Morphol., 2008. © 2008 Wiley-Liss, Inc.

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TL;DR: Femora of exercised mice were able to adapt to increased mechanical loading through increases in compressive, bending, and torsional rigidity, and no such adaptations were found in the tibia.
Abstract: The functional adaptation of juvenile mammalian limb bone to mechanical loading is necessary to maintain bone strength. Diaphyseal size and shape are modified during growth through the process of bone modeling. Although bone modeling is a well-documented response to increased mechanical stress on growing diaphyseal bone, the effect of proximodistal location on bone modeling remains unclear. Distal limb elements in cursorial mammals are longer and thinner, most likely to conserve energy during locomotion because they require less energy to move. Therefore, distal elements are hypothesized to experience greater mechanical loading during locomotion and may be expected to exhibit a greater modeling response to exercise. In this study, histomorphometric comparisons are made between femora and tibiae of mice treated with voluntary exercise and a control group (N = 20). We find that femora of exercised mice exhibit both greater bone growth rates and growth areas than do controls (P < 0.05). The femora of exercised mice also have significantly greater cortical area, bending rigidity, and torsional rigidity (P < 0.05), although bending and torsional rigidity are comparable when standardized by bone length. Histomorphometric and cross-section geometric properties of the tibial midshaft of exercised and control mice did not differ significantly, although tibial length was significantly greater in exercised mice (P < 0.05). Femora of exercised mice were able to adapt to increased mechanical loading through increases in compressive, bending, and torsional rigidity. No such adaptations were found in the tibia. It is unclear if this is a biomechanical adaptation to greater stress in proximal elements or if distal elements are ontogenetically constrained in a tradeoff of bone strength of distal elements for bioenergetic efficiency during locomotion. J. Morphol., 2008. © 2007 Wiley-Liss, Inc.