Intraspecific Allometry Research Articles

Length-mass allometries of the larvae of aquatic dipterans: differences between taxa, morphological traits, and methods.

Body mass underpins many ecological processes at the level of individuals, populations, and communities. Often estimated in arthropods from linear morphological traits such as body length or head width, these relationships can vary even between closely related taxa. Length-mass relationships of mosquito (Diptera: Culicidae) larvae are poorly known despite the importance of this family to disease and aquatic ecology. To fill this gap, we measured ontogenetic changes in linear traits (body length, head width, and thorax width) and dry and wet masses and estimated length- and width-mass relationships in larvae of 3 culicid species inhabiting different niches: the tropical Aedes albopictus (Skuse, 1894), the temperate Culex pipiens (Linnaeus, 1758), and the snowmelt Ochlerotatus punctor (Kirby, 1837). We compared our results with published length-mass allometries of other aquatic dipteran larvae. We showed that thorax width and body length, but not head width, reliably predicted body mass for our 3 species. The length-mass allometry slopes in aquatic dipterans varied considerably between and within families but were independent of phylogeny, specimen handling, preservation techniques, and data fitting methods. Slope estimates became less precise with decreasing sample size and size range. To obtain reliable estimates of the allometric slopes, we have thus recommended using data on all larval stages for intraspecific allometries and a wide range of species for interspecific allometries. We also cautioned against the indiscriminate use of length-mass allometries obtained for other taxa or collected at lower taxonomic resolutions, e.g., when using length-mass relationships to estimate biomass production at a given site.

Evolution along allometric lines of least resistance: morphological differentiation in Pristurus geckos.

Species living in distinct habitats often experience unique ecological selective pressures, which can drive phenotypic divergence. However, how ecophenotypic patterns are affected by allometric trends and trait integration levels is less well understood. Here we evaluate the role of allometry in shaping body size and body form diversity in Pristurus geckos utilizing differing habitats. We found that patterns of allometry and integration in body form were distinct in species with different habitat preferences, with ground-dwelling Pristurus displaying the most divergent allometric trend and high levels of integration. There was also strong concordance between intraspecific allometry across individuals and evolutionary allometry among species, revealing that differences in body form among individuals were predictive of evolutionary changes across the phylogeny at macroevolutionary scales. This suggested that phenotypic evolution occurred along allometric lines of least resistance, with allometric trajectories imposing a strong influence on the magnitude and direction of size and shape changes across the phylogeny. When viewed in phylomorphospace, the largest rock-dwelling species were most similar to the smallest ground-dwelling species, and vice versa. Thus, in Pristurus, phenotypic evolution along the differing habitat-based allometric trajectories resulted in similar body forms at differing body sizes in distinct ecological habitats.

Genetic variability and plasticity of plant allometry

Abstract The metabolic scaling theory (MST) predicts quasi‐universal trait–size relationships in plants, characterised by a unique allometric exponent within and across large taxonomic scales. However, recent studies have identified variability in allometric relationships, without a clear understanding of the modulating role played by genetic variation and environment. Here, we investigated (1) the allometric relationships for two central traits of MST, namely total leaf area and plant growth rate, in the model species Arabidopsis thaliana, (2) the variability of plant allometries between genotypes and (3) the plastic responses of plant allometries under water deficit, high temperature and their interaction. Using a population of 120 genotypes, we found that intraspecific allometries adhered on average with MST predictions. However, a broad variability but a moderate plasticity in the allometric exponents was observed across genotypes and environments. Allometric exponents were impacted significantly, yet weakly, by water deficit, but not by high temperature. Moreover, genotypes that deviated from MST predictions exhibited more plasticity in trait–size relationships than genotypes that followed MST predictions. Our study suggests that plant allometry is genetically variable and might be related to different adaptive strategies to cope with stressing conditions. Thus, our results highlights the need of assessing trait–size relationships within species to understand the mechanisms of plant adaptation to contrasted environments. Read the free Plain Language Summary for this article on the Journal blog.

Interspecific allometric scaling in eDNA production among northwestern Atlantic bony fishes reflects physiological allometric scaling

AbstractRelating environmental DNA (eDNA) signal strength to organism abundance requires a fundamental understanding of eDNA production. A number of studies have demonstrated that eDNA production may scale allometrically—that is, larger organisms tend to exhibit lower mass‐specific eDNA production rates, likely due to allometric scaling in key processes related to eDNA production (e.g., surface area, excretion/egestion). While most previous studies have examined intraspecific allometry, physiological rates and organism surface area also scale allometrically across species. We therefore hypothesize that eDNA production will similarly exhibit interspecific allometric scaling. To evaluate this hypothesis, we reanalyzed previously published eDNA data from Stoeckle et al. (ICES Journal of Marine Science, 78(1), 293–304, 2021) which compared metabarcoding read count to organism count and biomass data obtained from trawl surveys off the New Jersey coast. Using a Bayesian model, we empirically estimated the value of the allometric scaling coefficient (“b”) for Northwestern Atlantic bony fishes to be 0.77 (credible interval = 0.64–0.92), although our model failed to converge for Chondrichthyan species. We found that integrating allometry significantly improved correlations between organism abundance and metabarcoding read count relative to traditional metrics of abundance (density and biomass) for bony fishes. Although substantial unexplained variation remains in the relationship between read count and organism abundance, our study provides evidence that eDNA production may scale allometrically across species in some contexts. Future studies investigating the relationship between eDNA signal strength and metrics of fish abundance could potentially be improved by accounting for allometry; to this end, we developed an online tool that can facilitate the integration of allometry in eDNA/abundance relationships.

Morphological variation of the maxilla in modern humans and African apes

Differences in morphology among modern humans and African apes are frequently used when assessing whether hominin fossils should be attributed to a single species or represent evidence for taxic diversity. A good understanding of the degree and structure of the intergeneric, interspecific, and intraspecific variation, including aspects such as sexual dimorphism and age, are key in this context. Here we explore the variation and differences shown by the maxilla of extant hominines, as maxillary morphology is central in the diagnosis of several hominin taxa. Our sample includes adults of all currently recognized hominine species and subspecies, with a balanced species sex ratio. In addition, we compared the adults with a small sample of late juveniles. The morphology of the maxillae was captured using three-dimensional landmarks, and the size and shape were analyzed using geometric morphometric methods. Key observations are that 1) the maxillae of all extant hominine species and subspecies show statistically significant differences, but complete separation in shape is only seen at the genus level; 2) the degree of variation is not consistent between genera, with subspecies of Gorilla being more different from each other than are species of Pan; 3) the pattern of sexual shape dimorphism is different in Pan, Gorilla, and Homo, often showing opposite trends; and 4) differentiation between maxillary shapes is increased after adjustment for static intraspecific allometry. These results provide a taxonomically up-to-date comparative morphological framework to help interpret the hominin fossil record, and we discuss the practical implications in that context.

Ontogeny, phylogeny and mechanisms of adaptive changes in evaporative water loss in geckos.

Body size dependence of metabolic rate, body surface and scale morphology complicate disentangling the contribution of these characteristics to adaptive changes in total evaporative water loss (TEWL) of reptiles. To separate adaptive changes from size-related dependence, we compared intra- and interspecific scaling of several candidate traits in eyelid geckos (Eublepharidae), a group exhibiting large variation in body size and TEWL. The intraspecific allometry of TEWL of a eublepharid species fits the geometric surface-mass relationship. However, evolutionary shifts to both higher and lower evaporation were strongly correlated with habitat aridity and cannot be explained by shifts in body size alone. The intraspecific allometry of standard metabolic rate is nearly the same as the interspecific allometry. Unlike for mammals and birds, this pattern rules out respiratory water loss as a driver of the adaptive changes in TEWL among eublepharids. Scale morphology was independent of TEWL variation as well, but the correlation between cutaneous water loss and TEWL suggests a crucial role of skin permeability in adaptation to habitat aridity. Our analyses demonstrate how powerful a comparison between intra- and interspecific allometries can be for detecting body size-dependent mechanisms of adaptive changes in ecophysiological traits correlated with body size.

Seasonal shape variations, ontogenetic shape changes, and sexual dimorphism in a population of land isopod Porcellionides pruinosus: a geometric morphometric study

BackgroundIsopods shape features are sensitive and respond to several selective pressures which may result in variations of these features. These pressures might reflect the heterogeneity of the environment where an animal lives. Land isopods Porcellionides pruinosus were collected from an agricultural field. Landmarks geometric morphometrics was applied to evaluate its shape changes during two different seasons. The present work aims to (i) assess and characterize morphological changes in body shape of P. pruinosus as a response to seasonal variations, (ii) determine differences in the body shape during ontogeny, (iii) examine the effect of intraspecific allometry to interpret the observed variations in the species, and (iv) clarify whether the body shape of P. pruinosus can be used as a sexual differentiating trait.ResultsJuveniles showed no seasonal variations in the body shape, which were detected among adults, females and males as shown by PCA, DFA, and MANOVA.The adult winter forms have large bodies, small heads, broad pereons, and short but wide telsons. The adult summer forms have small slender bodies, slightly stretched heads, and relatively long telsons. Juveniles’ growth to adulthood showed body shape changes in the head and pereon, that include shrank of the head in the anteroposterior direction and its level became slightly lower than the body. The pereon becomes broader and the two anterolateral projections of the first pereonite extend anteriorly, reaching a little beyond the posterior margin of the eyes. Present species showed a shape sexual dimorphism which includes the broader body and more convex pereon in females and a small waist between the second and third pleonites in males. Shape sexual dimorphism was attributed to reproductive activity. Both allometric trajectories of juveniles and adults (ontogenetic allometry) and of sexes (static allometry) were parallel.ConclusionsThe landmark geometric morphometric technique was able to reveal the seasonal shape variations in terrestrial isopod P. pruinosus. Also, this method provides information about shape variations between juveniles and adults, as well as about shape sexual dimorphism.

Intraspecific allometries reveal hyper‐slender stems in forest gaps and the impact on tree growth from insect herbivores

AbstractConspicuously absent from plant allometric models are insect herbivores, despite their ubiquity in pantropical forests as well as the importance of gauging their impact on plant growth for understanding tree regeneration dynamics. Conceivably, the scaling relationships of plant populations with and without exposure to insects could be compared to gain insight, but this has yet to be tried. Here, I examined the size allometry of three canopy‐statured species (Fabaceae) by fitting standard major axis regressions of stem height, number of leaves, and maximum area per leaf against stem diameter (root collar) of their juveniles (seedlings/saplings) in 41 canopy gaps after ca. 2 years of herbivore exclusion (using mesh cages with mesh‐rooftop controls), in a central African forest. Herbivores did not change the slope or intercept, nor shift the location of Tetraberlinia bifoliolata and T. korupensis populations, whose seedlings tolerate shade well. In stark contrast, all three size allometries were altered in the faster‐growing Microberlinia bisulcata, whose seedlings cannot tolerate shade indefinitely. More pronounced than either an altered slope (scaling exponent) or intercept was the strong rightward shift in location (to larger sizes) of protected M. bisulcata, whereas its normal (control) population was left stunted, with fewer and smaller leaves. Nevertheless, all species had higher than theoretically predicted scaling exponents, even in the absence of herbivores, suggesting a convergent response early in tree ontogeny across the shade‐tolerance spectrum prioritizing height gain over mechanical stability in forest gaps. But insect herbivores were able to significantly strengthen this deviation in M. bisulcata, and perhaps in T. korupensis. These results demonstrate an allometric approach may prove useful for revealing how insects differentially impact trees' growth and form (slenderness) when they are most vulnerable to herbivory.

Investigating the gill-oxygen limitation hypothesis in fishes: intraspecific scaling relationships of metabolic rate and gill surface area.

Many ectotherms have shown a reduction in maximum body size in the past decades in parallel with climate warming. Indeed, some models forecast a maximum body size decline of 14%-24% by 2050 for numerous fish species. The gill-oxygen limitation (GOL) hypothesis is perhaps the most prominent concept regarding the physiological mechanisms underlying the observed trends, implicating oxygen uptake limitations in driving the decline in fish body size with warming. Current scientific debates, however, demonstrate a clear need for a synthesis of existing empirical evidence to test the fundamental assumptions of the GOL hypothesis. Here, we perform a systematic literature review of the intraspecific allometry of gill surface area (GSA) and metabolic rate. Additionally, we introduce a new parameter, the ratio S, which provides a measure of GSA in relation to the metabolic requirements for maintenance (S SMR) and maximum activity (S AMR). Support for the GOL hypothesis would be evidenced by a universal decline in S with increasing body mass within each species, such that gills become less equipped to supply metabolic requirements as fish grow. In contrast to the predictions of the GOL hypothesis, we show that the scaling exponents for S SMR and S AMR are consistently close to zero, with only a few exceptions where S either increased or decreased. These findings suggest that the GSA of each species is sufficient to meet its oxygen requirements throughout life, and that growth is not universally restricted by oxygen uptake limitations across the gills. We identify the need to investigate hypotheses other than the GOL hypothesis to help explain the observed declines in maximum fish body sizes concurrent with climate warming, in order to facilitate accurate predictions of fish community structure and manage fisheries in the face of climate change.

A quadratic approach to allometry yields promising results for the study of growth

Julian Huxley (1924) came to the conclusion that intra-specific growth usually follows a sequence of power curves. So Huxley claimed that during growth sudden changes in the growth rate can occur. The restudy of his material, however, reveals that his observations closely follow single quadratic curves. As a result the intra-specific allometry studied by Huxley is comparable to ontogenetic allometry. The quadratic factor of the quadratic equations obtained, represents the growth rate; it shows the constant increase (positive factor) or decrease (minus factor) of one of the measurements for a constant increase in the other measurement with which it is compared. The quadratic factor explains the entire growth process and is the same for the smaller (younger) and larger (older) specimens. It could probably permit the prediction of the shape of larger and/or smaller animals not yet found, or give a clue to some evolutionary changes. By using the quadratic parabola there is no need to postulate “sudden changes in the growth curve” and so it appears that Huxley’s power curve can be abandoned.

The biomechanical origin of extreme wing allometry in hummingbirds

Flying animals of different masses vary widely in body proportions, but the functional implications of this variation are often unclear. We address this ambiguity by developing an integrative allometric approach, which we apply here to hummingbirds to examine how the physical environment, wing morphology and stroke kinematics have contributed to the evolution of their highly specialised flight. Surprisingly, hummingbirds maintain constant wing velocity despite an order of magnitude variation in body weight; increased weight is supported solely through disproportionate increases in wing area. Conversely, wing velocity increases with body weight within species, compensating for lower relative wing area in larger individuals. By comparing inter- and intraspecific allometries, we find that the extreme wing area allometry of hummingbirds is likely an adaptation to maintain constant burst flight capacity and induced power requirements with increasing weight. Selection for relatively large wings simultaneously maximises aerial performance and minimises flight costs, which are essential elements of humming bird life history.

Ontogenetic intraspecific allometry of autochthonous and reintroduced Eurasian beaver Castor fiber populations

Ontogenetic allometry.of the cranium was studied in 580 individuals of the Eurasian beavers Castorfiber in two subspecies out of six autochthonous and reintroduced populations using the methods of multivariate nonparametric statistics. It was noted that the growth dynamics demonstrates a correspondence to trajectories of ontogenetic rodent models with the difference that intensive beaver growth finishes by the age of four years. Significant differences in the dynamics ofage variability were detected in the beavers of two subspecies; with somewhat smaller initial postnatal sizes, C.fiber belorussicus grows faster than C.fiber orientoeuropaeus.

Life History Features and Aging Rates: Insights from Intra-specific Patterns in the Cricket Acheta domesticus

We carried out 42 lifetime studies of male and female crickets (Acheta domesticus) treated with different diets, nutritional supplements and dietary restriction (84 groups). Maximal longevity of controls averaged ~118 days and various treatments obtained lifespans of 86–257 days. Overall, greater longevity was most strongly associated with slower growth and delayed maturation. Early nymphal mortality and early fasting stress were also associated with life extension. The opposite associations of growth rate and stress resistance with maximal life span are consistent with modulation of aging by the target of rapamycin (growth) and forkhead transcription factors (stress resistance). Unlike inter-specific comparisons or intra-specific comparisons of species differentiated into strains or breeds, body size did not emerge as a strong predictor of cricket longevity. Rather, targeted body size was strongly compensated for in crickets whereas most genetically differentiated breeds of mice, rats, horses and dogs express breed-specific sizes and growth rates. Both extension of the juvenile period and adult lifetime contributed to increases in maximal longevity of crickets, although for extreme life extension, extended immaturity was a greater factor than adult life span. We discuss the inter- and intra-specific allometry of life histories from a perspective of aging and suggest that inter-specific variance may particularly reflect exogenous mortality risks whereas intra-specific aging is dominated by constraints on productivity.

Ontogenetic phase shifts in metabolism: links to development and anti-predator adaptation

The allometric relationships between resting metabolism (VO(2)) and body mass (M), VO(2) = a(i)M(b), are considered a fundamental law of nature. A distinction though needs to be made between the ontogeny (within a species) and phylogeny (among species) of metabolism. However, the nature and significance of the intraspecific allometry (ontogeny of metabolism) have not been established in fishes. In this study, we present experimental evidence that a puffer fish ranging 0.0008-3 g in wet body mass has four distinct allometric phases in which three stepwise increases in scaling constants (a(i), i = 1-4), i.e. ontogenetic phase shifts in metabolism, occur with growth during its early life stages at around 0.002, 0.01 and 0.1 g, keeping each scaling exponent constant in each phase (b = 0.795). Three stepwise increases in a(i) accompanied behavioural and morphological changes and three peaks of severe cannibalism, in which the majority of predation occurred on smaller fish that had a lower value of a(i). Though fishes are generally highly fecund, producing a large number of small eggs, their survivability is very low. These results suggest that individuals with the ability to rapidly grow and step up 'a(i)' develop more anti-predator adaptation as a result of the decreased predatory risk.

Limits to vertical force and power production in bumblebees (Hymenoptera: Bombus impatiens)

Maximum vertical forces produced by flying animals can be difficult to identify unequivocally, but potentially indicate general limits to aerodynamic force and muscle power output. We used two methods (i.e. incremental addition of supplemental mass and asymptotic load lifting) to determine both the intraspecific allometry of and methodological differences in estimates of maximum flight performance for the bumblebee Bombus impatiens. We found that incremental mass addition underestimated maximum lifting capacity by approximately 18% relative to values obtained by asymptotically increasing the applied load during a lifting bout. In asymptotic loading, bumblebees lifted on average 53% of their body weight, and demonstrated a significantly negative allometry of maximum aerodynamic force production relative to thoracic muscle mass. Estimates of maximum body mass-specific mechanical power output increased intraspecifically with body mass to the 0.38-0.50 power, depending on values assumed for the profile drag coefficient. We also found a significant reduction in vertical force production when both hindwings were removed. Limits to load-lifting capacity ultimately co-occur with an upper bound on stroke amplitude (approximately 145 deg.). Although thoracic muscle mass showed positive allometry, overall load-lifting performance exhibited significant size-dependent degradation.

Shape and size variation: Growth and development of the dusky grouper (Epinephelus marginatus Lowe, 1834)

Demersal fishes have complex life cycles that involve an ontogenetic change in morphology, physiology, and behavior, as their pelagic larval stages colonize benthic habitats. The developmental transition between larvae and juveniles leads to very complex processes of morphogenesis and differentiation. These processes primarily determine changes in external morphology, which is shaped by selective pressures to optimize performance for basic activities such as swimming, escape from predators, and feeding. Fishes have provided fertile grounds for ecomorphological investigations throughout ontogeny, as the role of changing morphology in inducing ontogenetic niche shifts is not always clear. In this framework, some studies have demonstrated that certain species undergo gradual changes, whereas other species experience threshold effects in their ecomorphological relationships during ontogeny. In this study, the intraspecific allometry of the dusky grouper was examined. Geometric morphometric tools were used to quantify shape changes through the development, and a modular approach was also applied to analyze the pattern of covariation between three distinct blocks (head, trunk, and tail). For this purpose, a two-block Partial Least Square was computed. This method reveals that the pattern of changes in the overall body shape is the result of the modularized changes of these blocks.

Within and between species scaling in the weight, water, carbon and nitrogen contents of eggs and neonate larvae of twelve satyrine butterflies (Lepidoptera: Nymphalidae)

The fresh weight, dry weight, and C and N content of the eggs, egg shells and neonate larvae of several satyrines were measured. This was done in order to assess the specificity of the composition of the egg and larvae, the phylogenetic or ecological nature of the variation and the existence of structural constraints on the composition of the offspring. All the traits investigated were found to be highly species-specific. The nature of the variation was not primarily phylogenetic, suggesting that the composition of the offspring has an ecological meaning. However, only a slight association was detected between three life history traits or habitat features and the compositions of the eggs or larvae, namely: female egg dropping was associated with a high C content of the eggs, xerophily with a high C : N ratio, and a high content of N in the larvae with egg diapause. The evidence for intra-specific allometry between the traits investigated and egg weight varied among the species, suggesting that the slope of such relationship may be a specific feature. There was a close to isometric relationship between C and N contents in every species. Therefore simple C : N ratios are independent of egg size, hence they can be used directly in comparative studies. Across species analyses indicated that small offspring contained a proportionally low amount of carbon and had a high dry matter content, suggesting that selection for small eggs was accompanied by selection for an enhanced proportion of nitrogen per egg. Finally, the species with large adult females invested comparatively more nitrogen per egg, which indicates a potential, constraint-based advantage of large adult size.

Scaling of physiological performance during digestion

Meal digestion generates coordinated increases in the performance of the gastrointestinal, hepatic, renal, and cardiovascular systems, and a corresponding increase in metabolic rate. While the magnitudes of tissue and organ performance and metabolic rates during digestion have been documented for a variety of animals, relatively little is know of the impact of body size on physiological performance during digestion. I report here the intraspecific allometry of three indices of performance during digestion; metabolic rates for the toad Bufo marinus, small intestinal nutrient uptake capacity for the snake Nerodida rhombifer, and cardiac output for the snake Python molurus. For 102 B. marinus (2.4-881 g), peak rates of oxygen consumption during the digestion of rodent meals equaling 10% of toad body mass scaled with a mass exponent of 0.87, an exponent significantly greater than that of their standard metabolic rates (0.75). For 56 N. rhombifer (10-1530 g), small intestinal capacity to transport L-leucine, L-proline, and D-glucose scaled with mass exponents of 0.61, 0.59, and 0.80, respectively. These scaling exponents reflect the allometric scaling of small intestinal mass (0.82) and mass-specific rates of transport for L-leucine (−0.21), L-proline (−0.22), and D-glucose (−0.01). For 18 P. molurus (115-14700g), maximum cardiac output during the digestion of rodent meals equaling 25% of snake body mass scaled with a mass exponent of 0.70. For this snake, heart mass scales with an exponent of 0.76, and the two components of cardiac output, stroke volume and heart rate scale with exponents of 0.71 and −0.01, respectively. Physiological performance during digestion scales as a product of the allometry of morphology and function. Supported in part by NSF grant IOB-0446139.

Ontogenetic allometry, heterochrony, and interspecific differences in the skull of African apes, using tridimensional Procrustes analysis.

Ontogenetic studies of African ape skulls lead to an analysis of morphological differences in terms of allometry, heterochrony, and sexual dimorphism. The use of geometric morphometrics allows us 1) to define size and shape variations as independent factors (an essential but seldom respected condition for heterochrony), and 2) to calculate in percentage of shape changes and to graphically represent the parts of shape variation which are related to various biological phenomena: common allometry, intraspecific allometry, and allometric and nonallometric shape discrimination. Three tridimensional Procrustes analyses and the calculation of multivariate allometries, discriminant functions, and statistical tests are used to compare the skulls of 50 Pan troglodytes, and 50 Gorilla gorilla of different dental stages. The results both complement and modify classical results obtained from similar material but with different methods. Size and Scaling in Primate Morphology, New York: Plenum, p. 175-205). As previously described by Shea, the common growth allometric pattern is very important (64% of total shape variation). It corresponds to a larger increase of facial volume than of neurocranial volume, a more obliquely oriented foramen magnum, and a noticeable reshaping of the nuchal region (higher inion). However, the heterochronic interpretation based on common allometry is rather different from Shea. Gorillas differ from chimpanzees not only with a larger magnitude of allometric change (rate peramorphosis), as is classically said, but also grow more in size than in shape (size acceleration). In other words, for a similar stage of growth, gorillas have the size and shape corresponding to older chimpanzees, and for a similar shape, gorillas have a larger size than chimpanzees. In contrast, sexual dimorphism actually corresponds to allometric changes only, as classically demonstrated (time hypermorphosis). Sexual dimorphism is here significant in adult gorillas alone, and solely in terms of allometry (size-related shape and size, given that sagittal and nuchal crests are not taken into account). The study also permits us to differentiate two different shape variations that are classically confused in ontogenetic studies: a very small part of allometric shape change which is specific to each species (1% of the total shape variation), and nonallometric species-specific traits independent of growth (8% of total shape change). When calculated in terms of intraspecific allometries (including common allometry and noncommon allometry), shape changes are more extensive in gorillas (36% of total shape change) than in chimpanzees (29% of total shape change). The allometric differences mainly concern the inion, which becomes higher; the position of the foramen magnum, more dorsally oriented; and the palate, more tilted in adult gorillas than in adult chimpanzees. In contrast, nonallometric species-specific traits in gorillas are the long and flat vault characterized by a prominent occipital region, the higher and displaced backward glabella, and the protrusive nose. Biomechanical schemes built from shape partition suggest that the increased out-of-plumb position of the head during growth is partially compensated in gorillas by a powerful nuchal musculature due to the peculiar shape of the occipital region.

Comparing interspecific and intraspecific allometry in the Anatidae

Interspecific scaling relationships (e.g. of limb size with body mass) in vertebrates are usually assumed to be functional (e.g. biomechanical) attributes. In this paper on the Anatidae, we study the scaling of wing length and tarsus length with body mass, relationships that can be expected to have biomechanical significance. At an interspecific level, both wing length and tarsus are positively allometric, a finding consistent with results from previous comparative avian studies. These trends remained significant in regressions controlling for the effects of phylogeny, but interspecific slopes were less steep within tribes than in the whole family (a taxon-level effect). We are not aware of any biomechanical resasons that explain these patterns satisfactorily. Intraspecific (static) allometries in Green-winged Teal (Anas crecca) and Marbled Teal (Marmaronetta angustirostris) are different: wing length is negatively allometric and tarsus is isometric. These anomalies suggest that interspecific and intraspecific scaling relationships do not share common causes. Our results bring into question the significance of interspecific allometries in vertebrate morphology, which may to some extent be non-functional by-products of morphological optimisation processes within species and ecological differences between them.