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Microstructure-function relationships remain poorly understood in primary bone tissues. The relationship between bone growth rate and bone tissue type, although documented in some species by previous works, remains somewhat unclear and controversial. We assessed this relationship in a species with extreme adaptations, the king penguin (Aptenodytes patagonicus). These birds have a peculiar growth, interrupted 3·months after hatching by the austral winter. Before this interruption, chicks undergo extremely rapid statural and ponderal growth. We recorded experimentally (by means of fluorescent labelling) the growth rate of bone tissue in four long bones (humerus, radius, femur and tibiotarsus) of four king penguin chicks during their fastest phase of growth (3-5·weeks after hatching) and identified the associated bone tissue types ('laminar', 'longitudinal', 'reticular' or 'radial' fibro-lamellar bone tissue). We found the highest bone tissue growth rate known to date, up to 171·µm·day -1 (mean 55·µm·day -1 ). There was a highly significant relationship between bone tissue type and growth rate (P<10 -6 ). Highest rates were obtained with the radial microarchitecture of fibro-lamellar bone, where cavities in the woven network are aligned radially. This result supports the heuristic value of a relationship between growth rate and bone primary microstructure. However, we also found that growth rates of bone tissue types vary according to the long bone considered (P<10 -5 ) (e.g. growth rates were 38% lower in the radius than in the other long bones), a result that puts some restriction on the applicability of absolute growth rate values (e.g. to fossil species). The biomechanical disadvantages of accelerated bone growth are discussed in relation to the locomotor behaviour of the chicks during their first month of life.

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Here we study the occurrence of torsion-resisting morphological and histological features (thin bone walls, circular shaft cross-section, oblique collagen fibers, and laminar tissue arrangement) in a sample of 168 long bones from wings and legs of 22 bird species. These structural parameters were measured in mid diaphyseal undemineralized cross-sections and analyzed using uni-, bi-, and multivariate (principal components analysis) data analysis techniques. We found that the four variables are significantly and positively correlated, and that covariation between variables accounts for as much as 58% of the total variation. These results suggest that torsion is a main determinant of the macro-and microstructural design of long bones in birds. Humerus, ulna, and femur generally possess torsion-resisting features, while other bones (radius, carpometacarpus, tibiotarsus, tarsometatarsus, and foot phalanx) rather show bending/axial load-resisting structural properties. These results are congruent with in vivo strain data from the literature, which reported high torsional loading in humerus and ulna during flapping flight, but also in the subhorizontal avian femur during terrestrial locomotion. The precise function of the laminar tissue spatial arrangement, the role of pneumatization, and the influence of flight mode are discussed.

In primates, age determination using lines of arrested growth (LAGs) from bones has rarely been attempted, and the reliability of these structures has never been experimentally validated. In order to test skeletochronology in primates, LAGs were studied mainly in the long bones of known age Microcebus murinus, a small primate, whose potential longevity may reach 12 years. LAGs were extensively studied in 43 males and 23 females ranging from juveniles to 11-year-old adults. All individuals were born and reared in captivity. Some young individuals were injected with fluorescent dyes to quantify bone growth rates. LAGs in the diaphysis of the tibia are well correlated with age and this skeletal element appears to be the best for assessing skeletochronology in Microcebus murinus. There is strong evidence that the seasonal cycle of photoperiodicity is more important than age alone in producing LAGs.

In spite of the fact that the potential usefulness of bone histology in systematics has been discussed for over one and a half centuries, the presence of a phylogenetic signal in the variation of histological characters has rarely been assessed. A quantitative assessment of phylogenetic signal in bone histological characters could provide a justification for performing optimizations of these traits onto independently generated phylogenetic trees (as has been done in recent years). Here we present an investigation on the quantification of the phylogenetic signal in the following bone histological, microanatomical, and morphological traits in a sample of femora of 35 species of sauropsids: vascular density, vascular orientation, index of Haversian remodeling, cortical thickness, and cross-sectional area (bone size). For this purpose, we use two methods, regressions on distance matrices tested for significance using permutations (a Mantel test) and random tree length distribution. Within sauropsids, these bone microstructural traits have an optimal systematic value in archosaurs. In this taxon, a Mantel test shows that the phylogeny explains 81.8% of the variation of bone size and 86.2% of the variation of cortical thickness. In contrast, a Mantel test suggests that the phylogenetic signal in histological traits is weak: although the phylogeny explains 18.7% of the variation of vascular density in archosaurs, the phylogenetic signal is not significant either for vascular orientation or for the index of Haversian remodeling. However, Mantel tests seem to underestimate the proportion of variance of the dependent character explained by the phylogeny, as suggested by a PVR (phylogenetic eigenvector) analysis. We also deal with some complementary questions. First, we evaluate the functional dependence of bone vascular density on bone size by using phylogenetically independent contrasts. Second, we perform a variation partitioning analysis and show that the phylogenetic signal in bone vascular density is not a by-product of phylogentic signal in bone size. Finally, we analyze the evolution of cortical thickness in diapsids by using an optimization by squared change parsimony and discuss the functional significance of this character in terms of decreased buoyancy in crocodiles and mass saving in birds. These results are placed in the framework of the constructional morphology model, according to which the variation of a character in a clade has a historical (phylogenetic) component, a functional (adaptive) component, and a structural (architectural) component.

After a brief recall of the classical meaning of the concept of longevity, the first part of this chapter describes and summarises the main current technique for the estimation of age in reptiles. Among them, sclerochronology is primarily taken into account. The cautious analysis of seasonal growth cycles recorded in hard tissues, although not as rigorous as the mark-release-recapture method of animals in their natural conditions, now appears as a rapid and reliable chronological tool already successfully used in individuals of many reptile species. Sclerochronology is especially efficient for the comparison of several populations, and it is the only method for fossils. The second part presents a synthetic review of known longevities and records in the different groups of reptiles. A short discussion about the significance of longevity shows that for reptiles, because of their thermic metabolism (ectothermy), the physiological longevity must be strongly distinguished from the chronological longevity, especially for a comparison with that known for birds and most mammals.