Ontogenetic skull variation in a shovel-headed amphisbaenian species

Leposternon microcephalum is a species belonging to the Amphisbaenia, a group of burrowing reptiles. Amphisbaenia present various morphological and physiological adaptations that allow them to penetrate the ground and live underground, through a system of galleries and permanent chambers that they build themselves. Among the morphological adaptations in this group, those of the skull stand out as it serves as the main excavation tool. Four basic skull shapes are recognized: rounded, keeled, shovel-shaped, and spade-shaped. The skull of L. microcephalum belongs to this last type, which is considered the most specialized. The species inhabits soils that are highly compacted and difficult to penetrate. Among the species of Leposternon present in South America, L. microcephalum has the widest distribution, being found in all Brazilian biomes and neighboring countries such as Bolivia, Argentina, Paraguay, and Uruguay. The analysis of the skull of this species was carried out using three-dimensional geometric morphometrics (3D-GMM), a technique that allows comparative analysis, through robust statistical methods, of shape and its variations, using Cartesian coordinate data from a configuration of homologous landmarks. The technique allows the size and shape components of a structure to be analyzed separately. From an ontogenetic point of view, this methodology had also been used to investigate variations in Cynisca leucura, a member of the Amphisbaenidae with a rounded head. Our hypothesis is that the patterns of morphological differentiation in the skull, mainly in the intermediate and occipital regions, are similar in different Amphisbaenia species. Therefore, the objective of this study was to analyze cranial morphological variations in an ontogenetic series of L. microcephalum using 3D-GMM. Computed Tomographic scans of 13 specimens were analyzed: juveniles (N = 8) and adults (N = 5), based on 20 landmarks that characterize the skull. Principal components and regression analyses between shape (dependent variable) and size (independent variable) showed a clear difference between the cranial morphological pattern of juvenile individuals and that of adults. For instance, young specimens tend to have a dorsoventrally tall neurocranium, with the tip of the snout more anteriorly oriented and its dorsal border subtly curved. Dorsally, the parietal region is thicker and smoothly dome-shaped in juveniles. As in C. leucura, the variation was strongly correlated with the size change from juvenile to adult, indicating a dominant role for ontogenetic allometry in determining skull shape.

Asymmetry in the length of human humerus and radius during ontogeny

Bilateral body structures usually present some kind of asymmetry. In humans, long bones are suitable to study asymmetries because they are subject to different stimulus that can influence their development and shape. This study focuses on asymmetries of humerus and radius length during ontogeny, with a sample of 1,421 people from Barcelona ranging in age from 0 to 22 years. Data were obtained from dual-energy X-ray absorptiometry images. The humerus results showed the existence of significant directional asymmetry to the left in neonates that progressively turns to right through ontogeny, being significant in people from 11 to 16 years. The radius results are not so evident, but it can be deduced directional asymmetry in children and adolescents, with little delay from humerus. Regarding fluctuating asymmetry, higher values have been seen in neonates and they decrease during ontogeny.

Cross-Sectional Geometry and Scaling in the Baculum of Cuban Hutias (Rodentia: Capromyidae)

Bacula from 61 individual hutia (Rodentia) from five species were studied. The purpose was to investigate cross-sectional geometry as an indicator of mechanical behavior in order to answer questions around the origin and maintenance of the mammalian baculum. From images of the apical and basal cross sections, the following variables were calculated: perimeter, cross-sectional area, maximum second moment of area, and polar moment. An allometric analysis showed that these variables were related to body size. The orientation of the maximum second moment of area was analyzed by means of circular statistics. This orientation was transverse in both the apical and basal cross sections. Values for the second moment of area and polar moment, obtained from the predicted value of the allometric equations, showed that either the bending moment or the twisting moment of the baculum must be relatively low in hutias, compared with those of the radius in the same species. The results of the second moment of area predict that the main bending stress acting on the baculum is transverse. At the same time, shear stress would not be negligible. Anat Rec, 303:1346-1353, 2020. © 2019 American Association for Anatomy.

Muscle fibre types in the reduced forelimb and enlarged hindlimb of the quokka (Setonix brachyurus, Macropodidae)

The quokka (Setonyx brachyurus) is restricted to two offshore islands and small isolates on the mainland of south-western Australia. It displays a tendency to saltatorial locomotion and moves at speed by bipedal hopping, although it also uses its forelimbs at low speed. Its bipedal adaptation involves enlarged hind limbs, with elongated feet. The fibre type distribution of the elbow and knee extensors, and the ankle plantar flexors, in comparison with two eutherians, the quadrupedal rhesus monkey, as a locomotor generalist, and the jerboa, a small eutherian hopping species morphologically similar to the quokka, were studied. The quokka’s forelimb showed the same characteristics as that of the jerboa, lacking the fatigue-resistant Type I fibres that are used to sustain posture. As in the jerboa, the gastrocnemius lateralis was the muscle head with the highest proportion of fast twitch fibres. Muscular fibre pattern is not identical in the quokka and the jerboa hindlimb, but it appears that both species have similar anatomical adaptations to saltatorial locomotion. Differences in muscle fibre proportions could be due to several factors including, resting posture, body size and the propensity for elastic energy storage, the burrowing behaviour of the jerboa, but also to phylogenetic constraints where the adaptation to hop on the hindlimbs is a shared behaviour of the Macropodoidea (jerboas are the only Dipodidae to have elongated hindlimbs).

Scaling and mechanics of the felid calcaneus: geometric similarity without differential allometric scaling

Six mechanically significant skeletal variables were measured on the calcanei from 60 Felidae specimens (22 species) to determine whether these variables were scaled to body mass, and to assess whether differential scaling exists. The power equation (y = a · x(b) ) was used to analyse the scaling of the six variables to body mass; we compared traditional regression methods (standardised major axis) to phylogenetically independent contrasts. In agreement with previous studies that compared these methodologies, we found no significant differences between methods in the allometric coefficients (b) obtained. Overall, the scaling pattern of the felid calcaneus conformed to the predictions of the geometric similarity hypothesis, but not entirely to those of the elastic similarity hypothesis. We found that the moment arm of the ankle extensors scaled to body mass with an exponent not significantly different from 0.40. This indicated that the tuber calcanei scaled to body mass faster than calcaneus total length. This explained why the effective mechanical advantage of the ankle extensors increased with body mass, despite the fact that limb posture does not change in felid species. Furthermore, this finding was consistent with the hypothesis of the isometric scaling of ground reaction forces. No evidence for differential scaling was found in any of the variables studied. We propose that this reflected the similar locomotor pattern of all felid species. Thus, our results suggested that the differences in allometric coefficients for 'large' and 'small' mammals were in fact caused by different types of locomotion among the species included in each category.

Bite force and jaw biomechanics in the subterranean rodent Talas tuco-tuco (Ctenomys talarum) (Caviomorpha: Octodontoidea)

The Talas tuco-tuco ( Ctenomys talarum Thomas, 1898) is a South American subterranean rodent that digs using both forelimbs and incisors, the latter being used when animals face hard soils and fibrous roots. In this rodent, the incisors are also used during intermale competition for mates. Bite forces were measured on wild females (n = 21) and males (n = 21) (both adult and young individuals) using a force transducer. Bite force was significantly higher in adult males than in females (32 vs. 27 N, respectively). Bite forces calculated on the physiological cross-section of jaw adductor muscles in dissected specimens were slightly higher than in vivo measurements. Regressions against body mass showed that bite force scaled with positive allometry, with slopes of 0.89 (females) and 0.99 (males). No significant differences were observed, neither in the slope nor in the y intercept of both sexes’ equations; therefore intersexual differences in bite forces observed in adults should mainly be due to size dimorphism. Considering that soil hardness of C. talarum’s typical habitat averages 100 N/cm2, and taking into account incisor’s cross-section, it was assessed that the pressure exerted by jaw adductor muscles at the incisors level is three times higher than that required for soil penetration.

Lumbar ontogenetic allometry and dimorphism in humans. A case for comparison between interspecific and intraspecific scaling

The ontogenetic allometry of the lumbar region of 1913 humans (1228 females and 685 males), ranging from newborn to 21-year-old individuals, was studied by means of length, width, projected surface area and bone mineral density of the segment L2 - L4, obtained by dual X-ray absorptiometry (DXA). All these parameters were regressed to body mass and height of the individuals, considered alternatively as the independent variable. Firstly, we addressed the comparison between the results obtained on both sexes in order to elucidate whether ontogenetic differences existed. Length of the segments increased significantly faster in females than in males, independently whether the regression was made against body mass or height, while in both types of regression width scaled in males faster than in females. Regarding bone mineral density, although males increased bone mineral density faster than females, slope differences were not significant. However, y-interception was significantly higher in females than in males when bone mineral density was regressed to body mass. Results on length and width are compared with others from previous research on allometry. Finally, global results are discussed as regards the slope predictions for interspecific scaling.

Regional variation in cortical bone properties from broiler fowl--a first look

1. We used microhardness testing as a probe for fine-scale regional variation in the mechanical performance of bone and present data showing the extent of regional variation in the femora and humeri of 7-week-old broiler birds. 2. Ash content of dry bone was broadly correlated with microhardness, although there is some evidence that the relationship linking the two differs between the femur and the humerus. 3. Regional variations in the properties of bone from poultry are widely overlooked in the literature. Awareness of them is vital and existing measures of bone 'strength' may be misleading if local variation in properties is not taken into account when exploring the effects of nutrition and husbandry practices on bone mechanical performance.

Heterochronic detection through a function for the ontogenetic variation of bone shape

Heterochrony, evolutionary modifications in the rates and/or the timing of development, is widely recognized as an important agent of evolutionary change. In this paper, we are concerned with the detection of this evolutionary mechanism through the analysis of long bone growth. For this, we provide a function sigma (t) for the ontogenetic variation of bone shape by taking the ratio of two Gompertz curves explaining, respectively, the relative contribution to long bone growth of (a) endochondral ossification and (b) periosteal ossification. The significance of the fitting of this function to empirical data was tested in Anas platyrhynchos (Anseriformes). In this function sigma (t), the time t(m) at which periosteal growth rate first equalizes endochondral growth rate was taken as the timing parameter to be compared between taxa. On the other hand, the maximum rate of ontogenetic change in bone shape (maximum slope, beta) from hatching to t(m) was taken as the rate parameter to be compared. Comparisons of these parameters between the plesiomorphic condition and the derived character state would provide evidence for hypomorphosis (earlier occurrence of t(m)), hyper-morphosis (delayed occurrence of t(m)), deceleration (smaller beta) or acceleration (higher beta). Regarding the phylogenetic context, the ancestral condition for the character of interest should be estimated to polarize the direction of the heterochronic change. We have quantified the influence of the phylogenetic history on the variation of adult bone shape in a sample of 13 species of Anseriformes and 17 species from other neornithine orders of birds by using permutational phylogenetic regressions. Phylogenetic effects are significant, and this fact allows the optimization of bone shape onto a phylogenetic tree of Anseriformes to estimate the ancestral condition for Anas platyrhynchos.

Avian long bones, flight and bipedalism

Morphological, geometrical, chemical and mechanical characteristics of avian long bones are reviewed. Important differences exist between long bones of birds and mammals. Differences are also present in appendicular bones of birds, either between wing bones and leg bones or proximal (stylopodial) long bones and distal (zeugopodial) long bones. Special emphasis is put on pneumatization, in terms of both phylogenetic origin and geometrical and mechanical characteristics linked to it. Cortical thickness, bending strength and flexural Young's modulus were significantly lower in pneumatized bones than in marrow-filled bones. Possible adaptive reasons for the differences shown are discussed.

A biomechanical study of the long bones in platyrrhines

The long bones of 72 individuals of extant platyrrhines, belonging to 17 species (11 genera) were studied by regressions of length, diameters and curvature. Cross-sectional shapes at midshaft and axial and bending strength indicators were also calculated. Results show that forelimb bones scale faster than hindlimb bones, for both length and diameters. Curvature scales faster in the femur than in other bones. Strength indicators showed a high variability in the relative importance of axial and bending loadings. Results are consistent with field observations of locomotor behaviour, mainly as regards quadrupedalism versus suspensory locomotion.

Tensile fatigue in bone: are cycles-, or time to failure, or both, important?

In life, bones are subjected to fatigue loading which has different frequency and amplitude components, as well as various kinds of loading modes like tension, compression, shear and combinations of them. Considerable variability is observed in fatigue results of bone, which may be caused by these experimental variables or by the bone itself. In past studies the effect of magnitude and mode of loading have been examined in standard fatigue strength (stress vs. cycles to failure) diagrams. The effect of frequency is not clear, but there is clear evidence (from Carter & co-workers) that, at least in human bone, tension "fatigue" failure was determined solely by time rather than by cycles. We sought to confirm these results in the same and a different species. We cycled human and bovine bone in tension at two frequencies: 0.5 and 5 Hz. There was no cycle number effect; the results from the tests at the two frequencies were different if plotted and analysed as a function of cycles to failure, but were not separable if plotted and analysed as a function of time to failure. In this respect bone differs from tendon, in which failure in tension is a function of both cycles and time.

Exploring the effects of hypermineralisation in bone tissue by using an extreme biological example

The properties of bone tissue with very high or very low mineral levels attract attention because they allow researchers to comprehend more fully the mechanics, interaction and effects of mineral on collagen through a greater range of compositions than that found in the "ordinary". The bone tissue of the rostrum of the whale Mesoplodon densirostris is the densest bone known. We examined the composition, static and fatigue strength, hardness and toughness of this tissue and compared them to those of other less mineralised analogues. The rostrum bone has remarkably little organic matter and retains very little water in its native state, but its basic mineral stoichiometry is very similar to that of other bones. We present here updated versions of the microhardness vs. modulus and microhardness vs. mineral fraction relationships, which thanks to the rostrum have been produced for a considerably wider range than in the past. We found the rostrum to be extremely brittle with a toughness ratio in two perpendicular directions (along and across its length) similar to that of tissue of other "ordinary" long bones and we discuss the possible significance of our findings.

Mechanical properties and chemical composition of avian long bones

We have studied the mechanical behaviour of avian long bones as whole structures, by calculating mechanical parameters such as maximum load, stiffness, bending strength and flexural Young's modulus; bones were always tested in three-point bending. Furthermore composition in several chemical elements and amino acids related to collagen content was also analysed. Correlations were established between body mass, mechanical parameters and chemical contents. Both bending strength and Young's modulus were negatively correlated to body mass. Significant correlations were found between nitrogen content and both strength and Young's modulus, with negative slopes in both cases. Magnesium and phosphorus appear to be the most important inorganic elements to the understanding of the mechanical behaviour of avian long bones.

Cumulative damage and the response of human bone in two-step loading fatigue

It has already been shown that in fatigue tests in vitro human cortical bone accumulates damage in the form of microcracks and that the total number of microcracks generated prior to the creation of the fatal macrocrack, and their effect (softening) on the material properties, depends on the level of applied stress. At each stress level the amount of accumulated damage has also been shown to be a non-linear function of the cycle number (Zioupos et al., 1996a, b; Pattin et al., 1996). The theoretical implications of the previous findings and two possible models for cumulative damage were put to the test here by performing tensile fatigue tests in two-step level (high/low or low/high) loading on human cortical bone specimens. The results indicate that the accumulation of damage in-vitro is highly dependent on the level of stress and the stress history. Usual linear expressions for fatigue lifetime predictions, like the Palmgren-Miner rule, substantially over or underestimate the outcome depending on whether the stress was applied in a high/low or a low/high sequence, respectively. In view of these discrepancies we conclude that predicting the fatigue lifetime of any bone in vivo under variable loading and complex history regimes is an extremely difficult task to which the study of accumulation of damage can offer a significant but, perhaps, still limited contribution.

Biomechanical significance of cross-sectional geometry of avian long bones

Cortical area, maximum second moment of area and polar moment were calculated for the long bones of 39 species of birds. Regressions of all these parameters to body mass were established. At the same time, the orientations of the maximum second moment of area were statistically tested. The parameters calculated on humerus and ulna scaled according to the predictions derived from the geometric similarity hypothesis, while those calculated for the long bones of the leg showed higher exponents, very close to the predictions of the elastic similarity hypothesis. Confidence intervals calculated for radius parameters appeared to agree with both predictions. Only the for tarsometarsus was it impossible to establish a global orientation pattern for the maximum second moment of area. In the other cases, the orientation was: sagittal in the radius, posteromedial-anterolateral in the proximal long bones (humerus and femur) and posterolateral-anteromedial in the distal long bones (ulna and tibiotarsus). The implications of the present findings are discussed in terms of the possible correlations between the orientation patterns produced in the cross-sectional geometry of avian long bones and the load carried.

The length of the vertebral column of primates: an allometric study

The length of the vertebral column of 425 primates (151 prosimians, 76 platyrrhines and 198 catarrhines) was related to body mass from bibliographic sources. Regressions were calculated for the whole sample and separately for the three taxonomic groups quoted above. In parallel, the lengths of the cervical, thoracic and lumbar regions were calculated in a sample of 105 primates (30 prosimians, 19 platyrrhines and 56 catarrhines) and partial correlations established. In all cases except one, the correlation coefficients were significant. Of these, 12 correlations (out of 16) scaled with negative allometry (< 0.33), 4 with positive allometry (> 0.33), and in 6 cases the exponents were not significantly different from the criterion for isometry (0.33). The lumbar region showed the highest variability, mainly in platyrrhines and catarrhines. Results from catarrhines are globally the closest to the expectations of elastic similarity. No obvious direct relationship was found between the length of the vertebral column and the number of vertebrae.

Allometry of the limb long bones of insectivores and rodents

In an attempt to investigate the relationships between allometry and locomotory adaptations, we studied the long limb bones of 45 species of insectivores and rodents. Animals ranged from a few grams to about 50 kilograms. Diameter and length of the bones and body mass (when known) were recorded. Regressions of diameter to length, diameter to body mass, and length to body mass were calculated by the least-squares and Model II, or major axis, methods. The results obtained do not agree with the predictions of either the theory of geometric similarity or the theory of elastic similarity. The discrepancies could be due to the fact that animals studied exhibit various modes of locomotion. Moreover, the allometric relationships of the different locomotor patterns are better reflected in insectivores and rodents than in other groups of mammals. The use of a single regression analysis seems to be inadequate when the sample includes a large range of body sizes.

On the allometry of long bones in dogs (Canis familiaris)

The allometric relations of diameter and length of humerus, ulna, femur, and tibia of 108 specimens, from 63 different breeds of dogs and 12 specimens of wolves, were calculated by means of model II of regression or major axis method. Only for the tibia were the values of wolves included in the cluster formed for dog breeds. Consequently, separate lines of regression were calculated for the other bones. Results agree in general with the exponents predicted by the theory of geometric similarity; however, the slope obtained for femur (0.865) differed significantly from this. Morphology of the long bones of the legs does not differentiate dogs and wolves; this probably reflects secondary convergence among wolves with relatively modern breeds of dogs.