Evolution of bite force in Darwin's finches: a key role for head width

Many-to-many mapping: A simulation study of how the number of traits and tasks affect the evolution of form and function

Many-to-many mapping of form-to-function posits that multiple morphological and physiological traits affect the performance of multiple tasks in an organism, and that redundancy and multitasking occur simultaneously to shape the evolution of an organism's phenotype. Many-to-many mapping is expected to be ubiquitous in nature, yet little is known about how it influences the evolution of organismal phenotype. The F-matrix is a powerful tool to study these issues because it describes how multiple traits affect multiple tasks. We undertook a simulation study using the F-matrix to test how the number of traits and the number of tasks affect trait integration and evolvability, as well as the relationships among tasks. We found that as the number of traits and/or tasks increases, the relationships between the tasks and the integration between the traits become weaker, and that the evolvability of the traits increases, all resulting in a system that is freer to evolve. We also found that as the number of traits increases, performance tradeoffs tend to become weaker, but only to a point. Our work shows that it is important to consider not only multiple traits, but also the multitude of tasks that those traits carry out when studying form-function relationships. We suggest that evolution of these relationships follows functional lines of least resistance, which are less defined in more complex systems, resulting in a mechanism for diversification.

Habituation to a predatory stimulus in a harvester (Arachnida, Opiliones)

Several studies have investigated habituation in a defensive context, but few have addressed responses to dangerous stimuli. In such cases, animals should not habituate since this could cost their lives. Here we have stimulated individuals of the harvester Mischonyx squalidus with a predatory stimulus (squeezing with tweezers) in repeated trials within and between days, and measured the occurrence and magnitude of nipping, a defensive behavior. Contrary to our expectations, they did habituate to this stimulus. The probability and magnitude of response declined over trials during each of three days of testing in a typical habituation pattern. During the trials we also observed other defensive behaviors. We discuss our results mainly considering alternative defensive responses. Our data show that we lack information on (1) the role played by the ambiguity of stimuli, (2) the role played by subsequent stimuli and (3) the importance of the array of defensive behaviors of a species in understanding habituation. Although ubiquitous across animals and therefore expected, habituation is described for the first time in the order Opiliones.

A bite force database of 654 insect species

Bite force is a decisive performance trait in animals because it plays a role for numerous life history components such as food consumption, inter- and intraspecific interactions, and reproductive success. Bite force has been studied across a wide range of vertebrate species, but only for 32 species of insects, the most speciose animal lineage. Here we present the insect bite force database with bite force measurements for 654 insect species covering 476 genera, 111 families, and 13 orders with body lengths ranging from 3.76 to 180.12 mm. In total we recorded 1906 bite force series from 1290 specimens, and, in addition, present basal head, body, and wing metrics. As such, the database will facilitate a wide range of studies on the characteristics, predictors, and macroevolution of bite force in the largest clade of the animal kingdom and may serve as a basis to further our understanding of macroevolutionary processes in relation to bite force across all biting metazoans.

Coping with abrasive food: diverging composition of radular teeth in two Porifera-consuming nudibranch species (Mollusca, Gastropoda)

Molluscs forage with their radula, a chitinous membrane with teeth. Adaptations to hard or abrasive ingesta were well studied in Polyplacophora and Patellogastropoda, but for other taxa there are large gaps in knowledge. Here, we investigated the nudibranch gastropods Felimare picta and Doris pseudoargus, both of which feed on Porifera. Tooth morphologies were documented by scanning electron microscopy, and mechanical properties were tested by nanoindentation. We found that these parameters are rather similar in both species, indicating that teeth are similar in their function. To study the composition, teeth were visualized using confocal laser scanning microscopy (CLSM), to determine the degree of tanning, and analysed with energy-dispersive X-ray spectroscopy, to test the elemental composition. The emitted autofluorescence signal and the inorganic content differed between the species. This was especially prominent when studying the inner and outer tooth surfaces (leading and trailing edges). In F. picta, we detected high proportions of Si, whereas teeth of D. pseudoargus contained high amounts of Ca, which influenced the autofluorescence signal in CLSM. Employing nanoindentation, we determined high Young's modulus and hardness values for the leading edges of teeth, which relate to the Si and Ca content. This highlights that teeth with a similar morphology and mechanical properties can be mechanically enhanced via different chemical pathways in Nudibranchia.

Biomechanics illuminates form-function relationships in bird bills

The field of comparative biomechanics examines how form, mechanical properties and environmental interactions shape the function of biological structures. Biomechanics has advanced by leaps and bounds as rapid technological progress opens up new research horizons. In this Review, I describe how our understanding of the avian bill, a morphologically diverse multifunctional appendage, has been transformed by employing a biomechanical perspective. Across functions from feeding to excavating hollows in trees and as a vocal apparatus, the study of the bill spans both solid and fluid biomechanics, rendering it useful to understand general principles across disciplines. The different shapes of the bill across bird species result in functional and mechanical trade-offs, thus representing a microcosm of many broader form-function questions. Using examples from diverse studies, I discuss how research into bird bills has been shaped over recent decades, and its influence on our understanding of avian ecology and evolution. Next, I examine how bill material properties and geometry influence performance in dietary and non-dietary contexts, simultaneously imposing trade-offs on other functions. Following an examination of the interactions of bills with fluids and their role as part of the vocal apparatus, I end with a discussion of the sensory biomechanics of the bill, focusing specifically on the bill-tip mechanosensory organ. With these case studies, I highlight how this burgeoning and consequential field represents a roadmap for our understanding of the function and evolution of biological structures.

A biomechanical model for the relation between bite force and mandibular opening angle in arthropods

Bite forces play a key role in animal ecology: they affect mating behaviour, fighting success, and the ability to feed. Although feeding habits of arthropods have a significant ecological and economical impact, we lack fundamental knowledge on how the morphology and physiology of their bite apparatus controls bite performance, and its variation with mandible gape. To address this gap, we derived a biomechanical model that characterizes the relationship between bite force and mandibular opening angle from first principles. We validate this model by comparing its geometric predictions with morphological measurements on the muscoloskeletal bite apparatus of Atta cephalotes leaf-cutter ants, using computed tomography (CT) scans obtained at different mandible opening angles. We then demonstrate its deductive and inductive utility with three examplary use cases: Firstly, we extract the physiological properties of the leaf-cutter ant mandible closer muscle from in vivo bite force measurements. Secondly, we show that leaf-cutter ants are specialized to generate extraordinarily large bite forces, equivalent to about 2600 times their body weight. Thirdly, we discuss the relative importance of morphology and physiology in determining the magnitude and variation of bite force. We hope that a more detailed quantitative understanding of the link between morphology, physiology, and bite performance will facilitate future comparative studies on the insect bite apparatus, and help to advance our knowledge of the behaviour, ecology and evolution of arthropods.

Head Shape Heritability in the Hungarian Meadow Viper Vipera ursinii rakosiensis

Understanding heritability patterns in functionally relevant traits is a cornerstone for evaluating their evolutionary potential and their role in local adaptation. In this study, we investigated patterns of heritability in the head shape of the Hungarian meadow viper (Vipera ursinii rakosiensis). To this end, we used geometric morphometric data from 12 families composed of 8 mothers, 6 fathers and 221 offspring, bred in captivity at the Hungarian Meadow Viper Conservation Centre (Hungary). We separately evaluated maternal and paternal contributions to the offspring phenotype, in addition to additive genetic effects, all determined using a mixed animal model. Our results indicate a strong genetic and maternal contribution to head shape variations. In contrast, the paternal effects-which are rarely evaluated in wild-ranging species-as well as residual environmental variance, were minimal. Overall, our results indicate a high evolutionary potential for head shape in the Hungarian meadow viper, which suggests a strong contribution of this ecologically important trait in shaping the ability of this endangered species to adapt to changing conditions and/or habitats. Furthermore, our results suggest that maternal phenotypes should be carefully considered when designing captive breeding parental pairs for reinforcing the adaptive capacity of threatened populations, whereas the paternal phenotypes seem less relevant.

Decoupled evolution of the cranium and mandible in carnivoran mammals

The relationship between skull morphology and diet is a prime example of adaptive evolution. In mammals, the skull consists of the cranium and the mandible. Although the mandible is expected to evolve more directly in response to dietary changes, dietary regimes may have less influence on the cranium because additional sensory and brain-protection functions may impose constraints on its morphological evolution. Here, we tested this hypothesis by comparing the evolutionary patterns of cranium and mandible shape and size across 100+ species of carnivoran mammals with distinct feeding ecologies. Our results show decoupled modes of evolution in cranial and mandibular shape; cranial shape follows clade-based evolutionary shifts, whereas mandibular shape evolution is linked to broad dietary regimes. These results are consistent with previous hypotheses regarding hierarchical morphological evolution in carnivorans and greater evolutionary lability of the mandible with respect to diet. Furthermore, in hypercarnivores, the evolution of both cranial and mandibular size is associated with relative prey size. This demonstrates that dietary diversity can be loosely structured by craniomandibular size within some guilds. Our results suggest that mammal skull morphological evolution is shaped by mechanisms beyond dietary adaptation alone.

The terroir of the finch: How spatial and temporal variation shapes phenotypic traits in DARWIN'S finches

The term terroir is used in viticulture to emphasize how the biotic and abiotic characteristics of a local site influence grape physiology and thus the properties of wine. In ecology and evolution, such terroir (i.e., the effect of space or “site”) is expected to play an important role in shaping phenotypic traits. Just how important is the pure spatial effect of terroir (e.g., differences between sites that persist across years) in comparison to temporal variation (e.g., differences between years that persist across sites), and the interaction between space and time (e.g., differences between sites change across years)? We answer this question by analyzing beak and body traits of 4388 medium ground finches (Geospiza fortis) collected across 10 years at three locations in Galápagos. Analyses of variance indicated that phenotypic variation was mostly explained by site for beak size (η² = 0.42) and body size (η² = 0.43), with a smaller contribution for beak shape (η² = 0.05) and body shape (η² = 0.12), but still higher compared to year and site‐by‐year effects. As such, the effect of terroir seems to be very strong in Darwin's finches, notwithstanding the oft‐emphasized interannual variation. However, these results changed dramatically when we excluded data from Daphne Major, indicating that the strong effect of terroir was mostly driven by that particular population. These phenotypic results were largely paralleled in analyses of environmental variables (rainfall and vegetation indices) expected to shape terroir in this system. These findings affirm the evolutionary importance of terroir, while also revealing its dependence on other factors, such as geographical isolation.

Estimating bite force in extinct dinosaurs using phylogenetically predicted physiological cross-sectional areas of jaw adductor muscles

I present a Bayesian phylogenetic predictive modelling (PPM) framework that allows the prediction of muscle parameters (physiological cross-sectional area, A _Phys) in extinct archosaurs from skull width (W _Sk) and phylogeny. This approach is robust to phylogenetic uncertainty and highly versatile given its ability to base predictions on simple, readily available predictor variables. The PPM presented here has high prediction accuracy (up to 95%), with downstream biomechanical modelling yielding bite force estimates that are in line with previous estimates based on muscle parameters from reconstructed muscles. This approach does not replace muscle reconstructions but one that provides a powerful means to predict A _Phys from skull geometry and phylogeny to the same level of accuracy as that measured from reconstructed muscles in species for which soft tissue data are unavailable or difficult to obtain.

The effects of skull flattening on suchian jaw muscle evolution

Jaw muscles are key features of the vertebrate feeding apparatus. The jaw musculature is housed in the skull whose morphology reflects a compromise between multiple functions, including feeding, housing sensory structures, and defense, and the skull constrains jaw muscle geometry. Thus, jaw muscle anatomy may be suboptimally oriented for the production of bite force. Crocodylians are a group of vertebrates that generate the highest bite forces ever measured with a flat skull suited to their aquatic ambush predatory style. However, basal members of the crocodylian line (e.g., Prestosuchus) were terrestrial predators with plesiomorphically tall skulls, and thus the origin of modern crocodylians involved a substantial reorganization of the feeding apparatus and its jaw muscles. Here, we reconstruct jaw muscles across a phylogenetic range of crocodylians and fossil suchians to investigate the impact of skull flattening on muscle anatomy. We used imaging data to create 3D models of extant and fossil suchians that demonstrate the evolution of the crocodylian skull, using osteological correlates to reconstruct muscle attachment sites. We found that jaw muscle anatomy in early fossil suchians reflected the ancestral archosaur condition but experienced progressive shifts in the lineage leading to Metasuchia. In early fossil suchians, musculus adductor mandibulae posterior and musculus pterygoideus (mPT) were of comparable size, but by Metasuchia, the jaw musculature is dominated by mPT. As predicted, we found that taxa with flatter skulls have less efficient muscle orientations for the production of high bite force. This study highlights the diversity and evolution of jaw muscles in one of the great transformations in vertebrate evolution.

The Terrific Skink bite force suggests insularity as a likely driver to exceptional resource use

Natural history museum collections hold extremely rare, extinct species often described from a single known specimen. On occasions, rediscoveries open new opportunities to understand selective forces acting on phenotypic traits. Recent rediscovery of few individuals of Bocourt´s Terrific Skink Phoboscincus bocourti, from a small and remote islet in New Caledonia allowed to genetically identify a species of land crab in its diet. To explore this further, we CT- and MRI-scanned the head of the holotype, the only preserved specimen dated to about 1870, segmented the adductor muscles of the jaw and bones, and estimated bite force through biomechanical models. These data were compared with those gathered for 332 specimens belonging to 44 other skink species. Thereafter we recorded the maximum force needed to generate mechanical failure of the exoskeleton of a crab specimen. The bite force is greater than the prey hardness, suggesting that predation on hard-shelled crabs may be an important driver of performance. The high bite force seems crucial to overcome low or seasonal variations in resource availability in these extreme insular environments. Phoboscincus bocourti appears to be an apex predator in a remote and harsh environment and the only skink known to predate on hard-shelled land crabs.

Geometry and dynamics link form, function, and evolution of finch beaks

Darwin's finches are a classic example of adaptive radiation, exemplified by their adaptive and functional beak morphologies. To quantify their form, we carry out a morphometric analysis of the three-dimensional beak shapes of all of Darwin's finches and find that they can be fit by a transverse parabolic shape with a curvature that increases linearly from the base toward the tip of the beak. The morphological variation of beak orientation, aspect ratios, and curvatures allows us to quantify beak function in terms of the elementary theory of machines, consistent with the dietary variations across finches. Finally, to explain the origin of the evolutionary morphometry and the developmental morphogenesis of the finch beak, we propose an experimentally motivated growth law at the cellular level that simplifies to a variant of curvature-driven flow at the tissue level and captures the range of observed beak shapes in terms of a simple morphospace. Altogether, our study illuminates how a minimal combination of geometry and dynamics allows for functional form to develop and evolve.

Testing the influence of crushing surface variation on seed-cracking performance among beak morphs of the African seedcracker Pyrenestes ostrinus

Extreme phenotypic polymorphism is an oft-cited example of evolutionary theory in practice. Although these morphological variations are assumed to be adaptive, few studies have biomechanically tested such hypotheses. Pyrenestes ostrinus (the African seedcracker finch) shows an intraspecific polymorphism in beak size and shape that is entirely diet driven and allelically determined. Three distinct morphs feed upon soft sedge seeds during times of abundance, but during lean times switch to specializing on three different species of sedge seeds that differ significantly in hardness. Here, we test the hypothesis that beak morphology is directly related to consuming seeds of different hardness. We used a novel experimental analysis to test how beak morphology affects the efficiency of cracking sedge seeds of variable hardness, observing that neither mandibular ramus width nor crushing surface morphology had significant effects on the ability to crack different seed types. It is likely that feeding performance is correlated with other aspects of beak size and shape, such as beak depth and strength, muscle force or gape. Our results highlight how even seemingly straightforward examples of adaptive selection in nature can be complex in practice.

Trophic specialisation reflected by radular tooth material properties in an "ancient" Lake Tanganyikan gastropod species flock

Background

Lake Tanganyika belongs to the East African Great Lakes and is well known for harbouring a high proportion of endemic and morphologically distinct genera, in cichlids but also in paludomid gastropods. With about 50 species these snails form a flock of high interest because of its diversity, the question of its origin and the evolutionary processes that might have resulted in its elevated amount of taxa. While earlier debates centred on these paludomids to be a result of an intralacustrine adaptive radiation, there are strong indications for the existence of several lineages before the lake formation. To evaluate hypotheses on the evolution and radiation the detection of actual adaptations is however crucial. Since the Tanganyikan gastropods show distinct radular tooth morphologies hypotheses about potential trophic specializations are at hand.

Results

Here, based on a phylogenetic tree of the paludomid species from Lake Tanganyika and adjacent river systems, the mechanical properties of their teeth were evaluated by nanoindentation, a method measuring the hardness and elasticity of a structure, and related with the gastropods’ specific feeding substrate (soft, solid, mixed). Results identify mechanical adaptations in the tooth cusps to the substrate and, with reference to the tooth morphology, assign distinct functions (scratching or gathering) to tooth types. Analysing pure tooth morphology does not consistently reflect ecological specializations, but the mechanical properties allow the determination of eco-morphotypes.

Conclusion

In almost every lineage we discovered adaptations to different substrates, leading to the hypothesis that one main engine of the flock’s evolution is trophic specialization, establishing distinct ecological niches and allowing the coexistence of taxa.

Elevation is a stronger predictor of morphological trait divergence than competition in a radiation of tropical lizards

Adaptations for efficient performance are expected to shape animal morphology based on selection for microhabitat use and ecological forces. The presence of competitor species is predicted to cause niches to contract and enhance trait divergence. Therefore, increased species richness is expected to lead to greater trait divergence, and to result in reduced overlap and similarity between morphologies of sympatric species. We examined patterns of morphospace occupancy and partitioning in the skink fauna of New Guinea, the world's largest tropical island. Because skink species richness is largely decoupled from elevation in New Guinea, we could examine the effects of both factors (as proxies for competition and abiotic conditions), on morphospace occupancy and partitioning. We measured 1,860 specimens from 79 species of skinks throughout Papua New Guinea, and examined their morphospace occupancy in a spatial context. We calculated, for each assemblage within equal-area cells, the volume of morphospace occupied by all skinks, the mean volume occupied per species, and the mean distance and overlap between all species pairs. We then examined whether these metrics are related to species richness and elevation. Elevation is a stronger predictor of morphospace occupancy than species richness. As elevation increases, intraspecific variation decreases and morphologies become more similar to each other such that overall morphospace occupancy decreases. Highland skinks are, on average, smaller, thinner and shorter limbed than lowland species. We hypothesise that harsh climates in the New Guinea highland habitats impose strong selection on skinks to occupy specific areas of morphospace that facilitate efficient thermoregulation in suboptimal thermal conditions. We conclude that the effect of competition on trait divergence on a community and assemblage scale is eclipsed by abiotic selection pressures in these harsh environments.

Measuring proboscis length in Lepidoptera: a review

Mouthpart morphologies relate to diet range. Differences among or within species may result in resource partitioning and speciation. In plant-pollinator interactions, mouthpart length has an important role in foraging efficiency, resource partitioning and pollination, hence measuring nectarivorous insect mouthparts’ morphological variation is important. Most adult lepidopterans feed on nectars and participate in pollination. Although a vast range of studies applied morphometric measurements on lepidopteran proboscis (tongue) length, general recommendations on methodologies are scarce. We review available proboscis length measurement methodologies for Lepidoptera. Focusing on how proboscides have been measured, how accurate the measurements were, and how were these constrained by sampling effort, we searched for research articles investigating lepidopteran proboscis length and extracted variables on the aims of measurements, preparation and measurement methodology, and descriptive statistics. Different methods were used both for preparation and measurements. Many of the 135 reviewed papers did not provide descriptions of the procedures applied. Research aims were different among studies. Forty-four percent of the studies measured dead specimens, 13% measured living specimens, and 43% were unclear. Fifteen percent of the studies used callipers, 9% rulers, 1% millimetre scales, 4% ocular micrometers, 3% drawings and 14% photographs; 55% were non-informative. We emphasise the importance to provide detailed descriptions on the methods applied. Providing guidelines for future sampling and measurements, we encourage fellow researchers planning measurements to take into account the effect of specimen preparation techniques on the results, define landmarks, consider resolution, accuracy, precision, choose an appropriate sample size and report details on methodology.

Bite force and its relationship to jaw shape in domestic dogs

Previous studies based on two-dimensional methods have suggested that the great morphological variability of cranial shape in domestic dogs has impacted bite performance. Here, we used a three-dimensional biomechanical model based on dissection data to estimate the bite force of 47 dogs of various breeds at several bite points and gape angles. In vivo bite force for three Belgian shepherd dogs was used to validate our model. We then used three-dimensional geometric morphometrics to investigate the drivers of bite force variation and to describe the relationships between the overall shape of the jaws and bite force. The model output shows that bite force is rather variable in dogs and that dogs bite harder on the molar teeth and at lower gape angles. Half of the bite force is determined by the temporal muscle. Bite force also increased with size, and brachycephalic dogs showed higher bite forces for their size than mesocephalic dogs. We obtained significant covariation between the shape of the upper or lower jaw and absolute or residual bite force. Our results demonstrate that domestication has not resulted in a disruption of the functional links in the jaw system in dogs and that mandible shape is a good predictor of bite force.

Independent evolution of song diversity and song motor performance in canaries, goldfinches and allies indicates clade-specific trade-offs in birdsong

The diversity and the motor performance of birdsongs can both be sexually selected. In wood warblers, most species with high motor performance sing a greater proportion of trills, presumably to advertise performance, and thus have lower syllable diversity. We tested if this trade-off between motor performance and syllable diversity extends to canaries, goldfinches and allies, a clade with much longer and more varied songs. We assembled a molecular phylogeny and inferred song motor performance based on the speed of frequency modulation either in trills or in within-song intervals. The two metrics of performance were positively, but only mildly, related across species. While performance evaluated in intervals had high phylogenetic signal, performance evaluated in trills changed independently of phylogeny and was constrained by body size. Species in densely vegetated habitats sang fewer trills, but did not differ in motor performance. Contrary to wood warblers, song motor performance did not predict the proportion of trilled syllables nor within-song syllable diversity, perhaps because large differences in the song duration of canaries, goldfinches and allies prevent trills from severely compromising syllable diversity. Opposed results in wood warblers and in these finches indicate the existence of clade-specific trade-offs in the evolution of birdsong.

Dietary niche and the evolution of cranial morphology in birds

Cranial morphology in birds is thought to be shaped by adaptive evolution for foraging performance. This understanding of ecomorphological evolution is supported by observations of avian island radiations, such as Darwin's finches, which display rapid evolution of skull shape in response to food resource availability and a strong fit between cranial phenotype and trophic ecology. However, a recent analysis of larger clades has suggested that diet is not necessarily a primary driver of cranial shape and that phylogeny and allometry are more significant factors in skull evolution. We use phenome-scale morphometric data across the breadth of extant bird diversity to test the influence of diet and foraging behaviour in shaping cranial evolution. We demonstrate that these trophic characters are significant but very weak predictors of cranial form at this scale. However, dietary groups exhibit significantly different rates of morphological evolution across multiple cranial regions. Granivores and nectarivores exhibit the highest rates of evolution in the face and cranial vault, whereas terrestrial carnivores evolve the slowest. The basisphenoid, occipital, and jaw joint regions have less extreme differences among dietary groups. These patterns demonstrate that dietary niche shapes the tempo and mode of phenotypic evolution in deep time, despite a weaker than expected form–function relationship across large clades.

Temporally varying disruptive selection in the medium ground finch (Geospiza fortis)

Disruptive natural selection within populations exploiting different resources is considered to be a major driver of adaptive radiation and the production of biodiversity. Fitness functions, which describe the relationships between trait variation and fitness, can help to illuminate how this disruptive selection leads to population differentiation. However, a single fitness function represents only a particular selection regime over a single specified time period (often a single season or a year), and therefore might not capture longer-term dynamics. Here, we build a series of annual fitness functions that quantify the relationships between phenotype and apparent survival. These functions are based on a 9-year mark-recapture dataset of over 600 medium ground finches (Geospiza fortis) within a population bimodal for beak size. We then relate changes in the shape of these functions to climate variables. We find that disruptive selection between small and large beak morphotypes, as reported previously for 2 years, is present throughout the study period, but that the intensity of this selection varies in association with the harshness of environment. In particular, we find that disruptive selection was strongest when precipitation was high during the dry season of the previous year. Our results shed light on climatic factors associated with disruptive selection in Darwin's finches, and highlight the role of temporally varying fitness functions in modulating the extent of population differentiation.

Clutter negotiating ability in an ensemble of forest interior bats is driven by body mass

Differences in wing morphology are predicted to reflect differences in bat foraging strategies. Experimental tests of this prediction typically assess the relationship between wing morphology and a measures of flight performance on an obstacle course. However, studies have lacked measures of obstacle avoidance ability true scores, which may confound interpretation of ability across the range of presented tasks. Here, we used Rasch analysis of performance in a collision-avoidance experiment to estimate the ability of bat species to fly through vegetative clutter. We refer to this latent trait as 'clutter negotiating ability' and determined the relationships between clutter negotiating ability and wing morphology in 15 forest insectivorous bat species that forage in the densely cluttered rainforests of Malaysia. The clutter negotiating ability scores were quantified based on individual responses of each species to 11 different obstacle arrangements (four banks of vertical strings 10-60 cm apart). The tasks employed for the collision-avoidance experiment were reliable and valid, although Rasch analysis suggested that the experiment was too easy to discriminate completely among the 15 species. We found significant negative correlations between clutter negotiating ability and body mass, wingspan, wing loading and wing area but a positive significant correlation with wingtip area ratio. However, in stepwise multiple regression analyses, only body mass and wing loading were significant predictors of clutter negotiating ability. Species fell into clusters of different clutter negotiating ability, suggesting a potential mechanism for resource partitioning within the forest interior insectivorous ensemble.

Large feet are beneficial for eiders Somateria mollissima

Many waterbirds have fully (totipalmate) or partially webbed (palmate) feet that are used for locomotion in aquatic environments.If webbed feet and wings both contribute to efficient diving, we predicted a positive association between the area of webbed feet and the size of the frontal locomotor apparatus (wing area, heart mass, and breast muscle, after adjusting for any partial effects of body size). We predicted that individuals able to acquire more and better quality food due to larger webbed feet should have larger livers with higher concentrations of fat-soluble antioxidants such as vitamin E, and invest more in immune function as reflected by the relative size of the uropygial gland than individuals with small webbed feet.Here, we examine if the area of webbed feet is correlated with locomotion, diet, and body condition in a sea-duck, the eider (Somateria mollissima). We analyzed an extensive database of 233 eiders shot in Danish waters and at Åland, Finland during winter and early spring.Eiders with larger webbed feet had a larger locomotor apparatus, but did not have larger body size, they had larger uropygial glands that waterproof the plumage, they had larger beak volume and larger gizzards, and they had higher body condition.These findings imply that eiders with large webbed feet benefitted in terms of locomotion, feeding, and reproduction.

Liver Antioxidants in Relation to Beak Morphology, Gizzard Size and Diet in the Common Eider Somateria mollissima

Antioxidants in the liver are particularly abundant in capital breeders that rely on stored resources for egg production. Capital breeders like eider (hereafter common eider) Somateria mollissima have disproportionately large livers with low levels of coenzyme Q₁₀ when compared to other bird species. Concentrations of total carotenoids and vitamin E in the livers of eiders were smaller than predicted for similarly sized bird species. Eiders with high body condition estimated as body mass relative to skeletal body size had high levels of total carotenoids and low levels of coenzyme Q₁₀. The concentration of total carotenoids per gram of liver increased with age, and vitamin E and total carotenoids accumulated during the winter onwards from February to peak at the start of incubation in April. Total vitamin E, total carotenoids, and coenzyme Q₁₀ per gram of liver decreased with increasing beak volume. The size of the empty gizzard increased with increasing liver mass but decreased with total carotenoids and coenzyme Q₁₀. The main components of the diet were blue mussels Mytilus edulis (40%), draft whelk Nassarius reticulatus (27%), and periwinkle Littorina littorea (10%). The concentration of vitamin E increased with the number of razor clams Ensis sp. and draft whelks in the gizzard and the concentration of total carotenoids increased with the number of beach crabs Carcinus maenas. These observations are consistent with the hypothesis that eiders are limited in their levels of antioxidants through food limitation. Furthermore, they imply that diet and morphological characters involved in food acquisition and processing are important determinants of the level of antioxidants in the liver.

Extreme and rapid bursts of functional adaptations shape bite force in amniotes

Adaptation is the fundamental driver of functional and biomechanical evolution. Accordingly, the states of biomechanical traits (absolute or relative trait values) have long been used as proxies for adaptations in response to direct selection. However, ignoring evolutionary history, in particular ancestry, passage of time and the rate of evolution, can be misleading. Here, we apply a recently developed phylogenetic statistical approach using significant rate shifts to detect instances of exceptional rates of adaptive changes in bite force in a large group of terrestrial vertebrates, the amniotes. Our results show that bite force in amniotes evolved through multiple bursts of exceptional rates of adaptive changes, whereby whole groups-including Darwin's finches, maniraptoran dinosaurs (group of non-avian dinosaurs including birds), anthropoids and hominins (fossil and modern humans)-experienced significant rate increases compared to the background rate. However, in most parts of the amniote tree of life, we find no exceptional rate increases, indicating that coevolution with body size was primarily responsible for the patterns observed in bite force. Our approach represents a template for future studies in functional morphology and biomechanics, where exceptional rates of adaptive changes can be quantified and potentially linked to specific ecological factors underpinning major evolutionary radiations.

Personality and performance are affected by age and early life parameters in a small primate

A whole suite of parameters is likely to influence the behavior and performance of individuals as adults, including correlations between phenotypic traits or an individual's developmental context. Here, we ask the question whether behavior and physical performance traits are correlated and how early life parameters such as birth weight, litter size, and growth can influence these traits as measured during adulthood. We studied 486 captive gray mouse lemurs (Microcebus murinus) and measured two behavioral traits and two performance traits potentially involved in two functions: exploration behavior with pull strength and agitation score with bite force. We checked for the existence of behavioral consistency in behaviors and explored correlations between behavior, performance, morphology. We analyzed the effect of birth weight, growth, and litter size, while controlling for age, sex, and body weight. Behavior and performance were not correlated with one another, but were both influenced by age. Growth rate had a positive effect on adult morphology, and birth weight significantly affected emergence latency and bite force. Grip strength was not directly affected by early life traits, but bite performance and exploration behavior were impacted by birth weight. This study shows how early life parameters impact personality and performance.

Run for your life, but bite for your rights? How interactions between natural and sexual selection shape functional morphology across habitats

A central issue in evolutionary biology is how morphology, performance, and habitat use coevolve. If morphological variation is tightly associated with habitat use, then differences in morphology should affect fitness through their effect on performance within specific habitats. In this study, we investigate how evolutionary forces mold morphological traits and performance differently given the surrounding environment, at the intraspecific level. For this purpose, we selected populations of the lizard Podarcis bocagei from two different habitat types, agricultural walls and dunes, which we expected to reflect saxicolous vs ground-dwelling habits. In the laboratory, we recorded morphological traits as well as performance traits by measuring sprint speed, climbing capacity, maneuverability, and bite force. Our results revealed fast-evolving ecomorphological variation among populations of P. bocagei, where a direct association existed between head morphology and bite performance. However, we could not establish links between limb morphology and locomotor performance at the individual level. Lizards from walls were better climbers than those from dunes, suggesting a very fast evolutionary response. Interestingly, a significant interaction between habitat and sex was detected in climbing performance. In addition, lizards from dunes bit harder than those from walls, although sexual differentiation was definitely the main factor driving variation in head functional morphology. Taking into account all the results, we found a complex interaction between natural and sexual selection on whole-organism performance, which are, in some cases, reflected in morphological variation.

The old and new faces of morphology: the legacy of D'Arcy Thompson's 'theory of transformations' and 'laws of growth'

In 1917, the publication of On Growth and Form by D'Arcy Wentworth Thompson challenged both mathematicians and naturalists to think about biological shapes and diversity as more than a confusion of chaotic forms generated at random, but rather as geometric shapes that could be described by principles of physics and mathematics. Thompson's work was based on the ideas of Galileo and Goethe on morphology and of Russell on functionalism, but he was first to postulate that physical forces and internal growth parameters regulate biological forms and could be revealed via geometric transformations in morphological space. Such precise mathematical structure suggested a unifying generative process, as reflected in the title of the book. To Thompson it was growth that could explain the generation of any particular biological form, and changes in ontogeny, rather than natural selection, could then explain the diversity of biological shapes. Whereas adaptationism, widely accepted in evolutionary biology, gives primacy to extrinsic factors in producing morphological variation, Thompson's 'laws of growth' provide intrinsic directives and constraints for the generation of individual shapes, helping to explain the 'profusion of forms, colours, and other modifications' observed in the living world.

Cranial shape evolution in adaptive radiations of birds: comparative morphometrics of Darwin's finches and Hawaiian honeycreepers

Adaptive radiation is the rapid evolution of morphologically and ecologically diverse species from a single ancestor. The two classic examples of adaptive radiation are Darwin's finches and the Hawaiian honeycreepers, which evolved remarkable levels of adaptive cranial morphological variation. To gain new insights into the nature of their diversification, we performed comparative three-dimensional geometric morphometric analyses based on X-ray microcomputed tomography (µCT) scanning of dried cranial skeletons. We show that cranial shapes in both Hawaiian honeycreepers and Coerebinae (Darwin's finches and their close relatives) are much more diverse than in their respective outgroups, but Hawaiian honeycreepers as a group display the highest diversity and disparity of all other bird groups studied. We also report a significant contribution of allometry to skull shape variation, and distinct patterns of evolutionary change in skull morphology in the two lineages of songbirds that underwent adaptive radiation on oceanic islands. These findings help to better understand the nature of adaptive radiations in general and provide a foundation for future investigations on the developmental and molecular mechanisms underlying diversification of these morphologically distinguished groups of birds.

This article is part of the themed issue ‘Evo-devo in the genomics era, and the origins of morphological diversity’.

Size, shape, and sex-dependent variation in force production by crayfish chelae

The ability to generate large closing forces is important for many animals. Several studies have demonstrated that bite or pinching force capacity is usually related to the linear dimensions of the closing apparatus. However, relatively few studies have applied geometric morphometrics to examine the effects of size-independent shape on force production, particularly in studies of crustacean pinching force. In this study, we utilized traditional and geometric morphometric techniques to compare the pinching force of Procambarus clarkii crayfish to their chela morphology. We found that males possessed larger chelae and pinched harder than females, but that their chela shape and size were weak predictors of strength. Female pinching force was significantly affected by both chela size and shape, with shape variation along the short axis of the claw contributing most to pinching force. We discuss our results in the context of reliable signaling of strength by males and females, and the different selective forces acting on chela shape in the two sexes.

Sexual dimorphism in jaw muscles of the Japanese sparrowhawk (Accipiter gularis)

Materials suitable for anatomical research of raptorial birds are rare. Bird-eating raptors show distinct inter-sexual differences in body size and parental roles. The large females catch larger prey and prepare small morsels to feed their young using their hooked beaks. Here, we investigated the architectural properties of different jaw muscles of the Japanese sparrowhawk (Accipiter gularis) and examined whether there is sexual dimorphism in their architectural design. The results showed that musculus depressor mandibulae, the opener of the lower jaw, was characterized by relatively long fascicle length, whereas musculus pterygoideus was characterized by its larger mass and physiological cross-sectional area (PCSA) in both sexes. Females have the potential capacity to produce rapid and strong bites by their significantly longer fascicle length of M. depressor mandibulae and larger mass and PCSA of M. pterygoideus. For body size-matched gender, jaw muscles of males had fibres of relatively longer length than females, enabling greater velocity and excursion. Architectural characteristics of jaw muscles, together with the absolute dimorphism (the fascicle length of M. depressor mandibulae, the muscle mass and PCSA of M. pterygoideus) and relative dimorphism in the muscle mass of M. pterygoideus, reflect dietary difference and asymmetric parental roles between the sexes.

Is Beak Morphology in Darwin's Finches Tuned to Loading Demands?

One of nature's premier illustrations of adaptive evolution concerns the tight correspondence in birds between beak morphology and feeding behavior. In seed-crushing birds, beaks have been suggested to evolve at least in part to avoid fracture. Yet, we know little about mechanical relationships between beak shape, stress dissipation, and fracture avoidance. This study tests these relationships for Darwin's finches, a clade of birds renowned for their diversity in beak form and function. We obtained anatomical data from micro-CT scans and dissections, which in turn informed the construction of finite element models of the bony beak and rhamphotheca. Our models offer two new insights. First, engineering safety factors are found to range between 1 and 2.5 under natural loading conditions, with the lowest safety factors being observed in species with the highest bite forces. Second, size-scaled finite element (FE) models reveal a correspondence between inferred beak loading profiles and observed feeding strategies (e.g. edge-crushing versus tip-biting), with safety factors decreasing for base-crushers biting at the beak tip. Additionally, we identify significant correlations between safety factors, keratin thickness at bite locations, and beak aspect ratio (depth versus length). These lines of evidence together suggest that beak shape indeed evolves to resist feeding forces.

Jaw myology and bite force of the monk parakeet (Aves, Psittaciformes)

Psittaciform birds exhibit novelties in jaw bone structure and musculature that are associated with strong bite forces. These features include an ossified arcus suborbitalis and the muscles ethmomandibularis and pseudomasseter. We analyse the jaw musculature of the monk parakeet (Myiopsitta monachus) to enable future studies aimed at understanding craniofacial development, morphology, function and evolution. We estimate bite force based on muscle dissections, physiological cross-sectional area and skull biomechanical modelling. We also compare our results with available data for other birds and traced the evolutionary origin of the three novel diagnostic traits. Our results indicate that, in Myiopsitta, (i) the arcus suborbitalis is absent and the orbit is ventrally closed by an elongate processus orbitalis and a short ligamentum suborbitale; (ii) the ethmomandibularis muscle is a conspicuous muscle with two bellies, with its origin on the anterior portion of the septum interorbitale and insertion on the medial aspect of the mandible; (iii) the pseudomasseter muscle consists of some fibers arising from the m. adductor mandibulae externus superficialis, covering the lateral surface of the arcus jugalis and attaches by an aponeurotic sheet on the processus orbitalis; (iv) a well-developed adductor mandibulae complex is present; (v) the bite force estimation relative to body mass is higher than that calculated for other non-psittaciform species; and (vi) character evolution analysis revealed that the absence of the arcus suborbitalis and the presence of the m. pseudomassseter are the ancestral conditions, and mapping is inconclusive about presence of one or two bellies of the m. ethmomandibularis.